GLG Map Server Tutorial
Introduction
This tutorial presents an overview of the GLG Map Server and explains the basics of the map server's GIS data setup. While the map server may be used as an integrated component in the GLG Toolkit or as a stand-alone product, the materials presented in this tutorial are relevant to both of these usage scenarios.
The GIS Object of the GLG Builder described in the first part of this tutorial is used as a convenient map server prototyping tool, for both the integrated and standalone use. It allows you to quickly test the map server setup, as well as experiment with various map server settings without issuing map server requests manually.
The second part of the tutorial will walk you through the basics of the map server data setup. The map server data setup is the same for either the integrated or standalone use of the map server.
The Demo of the GLG Toolkit available for download on the Generic Logic's website contains all files used in this tutorial, including the GLG Builder and the GIS data used by the map server. The evaluation and production versions of the Toolkit may also be used.
The Map Server FAQs located on the website provide examples of command-line options and map query requests for the stand-alone use of the map server.
For programming examples, refer to the demos and examples listed in Demos and examples of the tutorial. For the complete documentation of the GLG Map Server, refer to the GLG Map Server Reference Manual.
Map Server Overview
GLG Map Server is a powerful and compact GIS Map Server designed to add mapping functionality to the real-time and web-based applications. It provides a high-performance rendering engine for vector and raster GIS data, generating detailed real-time map images for requested areas from the supplied GIS data. The Map Server is self-contained, does not depend on any third-party software or windowing system at run-time and can operate on a head-less server on a variety of hardware and software platforms.
The Map Server also provides capabilities for querying the lat/lon and elevation values of a selected point on the map.
Integrated Map Server Use
When the Map Server is used in conjunction with the Toolkit, the GIS Object is created in the GLG Builder as a part of the GLG drawing to provide an interface to the Map Server. The GIS object displays map images and transparently handles all low-level details of the map generation, automatically issuing map generation requests to the Map Server when the map is zoomed or panned.
Since the GIS Object is a part of the GLG drawing, other dynamic or static GLG objects may be placed on top of the map to visualize positions of moving objects, such as airplanes, delivery vehicles, patrol cars and other points of interest. Dynamic icons may be animated and positioned using real-time data and provide interfaces for handling object selection and mouse over events.
When the drawing containing the GIS Object is deployed in a C/C++ application, it utilizes the Map Server in the form of a library, accessing GIS data from a local drive without a need for an Internet connection. For cases when it is desirable to keep all GIS data on a central server, the GIS Object provides an option for getting map images from a web-based map server via a URL, to avoid replicating GIS data on each local drive. Both options are also available for the ActiveX control on Windows as well.
When the drawing is deployed in a Java application, the GIS Object uses the web access option, obtaining map images from a web server-based Map Server via a URL.
Stand-Alone Map Server Use
If an application needs to generate map images, but does not need other functionality provided by the Toolkit, the Map Server can be deployed as a stand-alone product. Two deployment options are provided: the Map Server library and Map Server executable.
The Map Server library provides an API for generating map images. The library is self-contained and is available on a variety of platforms. The GLG Map Server Reference Manual provides source code examples for the stand-alone library use.
The Map Server executable supports the CGI and FastCGI mode and is used for web-based setups. It is available for a variety of Unix/Linux and Windows platforms. The Map Server executable may also be used to generate map images using command-line parameters. Refer to the GLG Map Server Reference Manual for the command-line parameters and web server setup instructions.
When the stand-alone version of the map server is used, the GIS Object in the GLG Builder may be used for fast prototyping, providing a unique and convenient way for rapid application development and testing.
GIS Data and Setup Files
Regardless of the usage mode, the Map Server accesses GIS data and setup files in order to generate map images. All setup files are usually kept in one directory, and the setup is the same for any of the usage modes. The setup files describe available map layers and define layers' rendering styles, while the GIS data provide lat/lon coordinates of the GIS features as well as optional attributes. For example, a city may have a population and name attribute.
Key Features
The following is a brief list of the GLG Map Server features:
Unlimited zooming and panning, map rotation support for the "pilot view" displays.
Dynamic activation/deactivation of map layers, switching to more detailed layers as the map is zoomed in.
Dynamic attribute thresholds to vary the feature's appearance depending on its attributes.
Elevation display and query capabilities, support for shadow relief images.
On-the-fly data projection, rectangular and orthographic.
Support for volatile data and semi-transparent weather overlays.
Support for rectangular and hierarchical directory-based tiling, with tiling and setup utilities for handling huge datasets.
Tile cache and automatic switching to fallback layers when allowed number of tiles is exceeded.
Adaptive grid with support for custom grid labels.
Fast prototyping in the GLG Builder, via the use of the GIS object
Easy data setup via the use of ASCII files.
Demos and examples
Several GLG demos and examples demonstrate how the Map Server may be used in a GLG application that requires mapping functionality. The GIS and Airtraffic demos available in the GLG's DEMOS directory and on-line, on the Generic Logic's website, provide examples of air traffic monitoring applications. Smaller GIS examples in the examples directories may be used as a starting point to familiarize yourself with a structure of a GLG mapping application.
In all demos and examples, an interactive map is displayed in the background using the GIS Object, which transparently handles map zooming and panning, as well as activation or deactivation of map layers depending on the zoom factor.
The map has dynamic icons moving on top of it, which are rendered using GLG graphical objects. The icons are positioned using lat/lon coordinates and updated in real-time, displaying a tooltip on the mouse event and allowing the user to select an icon with the mouse.
The following GIS demos and examples for various programming environments are provided:
C/C++ Demos and Examples
<glg_dir>/DEMOS/gis_demo <glg_dir>/DEMOS/airtraffic <glg_dir>/examples_c_cpp/misc/GIS <glg_dir>/examples_c_cpp/misc/GIS2Compilation and Map Server setup
GLG Map Server may be used either in the form of a library linked into an application, or in the form of a GCI-bin executable which serves maps from a web server. The library version of the Map Server is used for C/C++ applications, as well as C# and VB.NET applications that use ActiveX control.
The CGI-bin Map Server executable is used for Java applications, as well as C/C++, C# and VB.NET applications which request map images from a local or remote web server via a URL.
Compilation notes for C/C++
When linking a GLG C/C++ mapping application on the Unix/Linux platforms, the Map Server library has to be included in addition to the GLG library. A sample makefile is provided in the src directory. Refer to Linking with the Map Server Library of the GLG Map Server Reference Manual for more information.
On Windows, no extra action is required, as the Map Server library is included in both the GLG library and GLG ActiveX control.
Web Server Setup
When the Map Server is deployed as a library, no web server setup is required.
When the Map Server is used as a CGI-bin executable to serve map images over the web, the Map Server should be set up on a local or remote web server. Refer to Appendix A: Web Server Installation Notes of the GLG Map Server Reference Manual for detailed setup instructions.
GIS Object usage in the GLG Builder
Adding GIS Object to the drawing
Let's create a GLG drawing and add a GIS object to it to display a map.
Start the GLG Builder and use the File, New Widget option from the main menu to start a new drawing. Use the OK button to close the information dialog.
Click on the GIS Object 
button in the object palette to create a GIS object. Following the prompt at the bottom of the Builder, select two points to define a rectangular area for the object.
A File Dialog comes up, asking you to select a GIS object's datafile. For now, accept the default, i.e. the sample.sdf file from the <glg_dir>/map_data directory. A satellite image of the world with a vector layer of the United States boundaries will be displayed in the GIS object using rectangular projection.
The sample.sdf file is a server data file (SDF), which describes layout and setup of the GIS data used to render the map image. We will examine the content of the SDF file in Map Data Setup.
Likewise other GLG objects, such as a polygon or arc, a GIS object is a GLG graphical object with a set of properties that control its appearance. The object's type, GIS , is displayed in the status area at the bottom of the drawing area. Same as other GLG objects, it is created as a child of the drawing area (viewport object) and positioned inside the viewport using the object's control points. The GIS object's properties and resources are used to configure the object, as well as control it at run-time.
GIS object's Properties
Make sure that the GIS object is selected, otherwise select it by clicking on it with the mouse.
Bring the object's Properties dialog, using the Object, Properties option from the main menu.
The Properties dialog for the GIS object is similar to the Properties dialog of other GLG objects. The properties that are common for all GLG objects, such as the object's name, type, visibility, are displayed at the top of the dialog. The Type field displays the object's type: GIS .
Enter "GISObject" in the Name field to name the object.
The FillColor property controls the background color of the map. Right now the background is not visible, because it is completely covered by the raster earth layer. The background will be visible when raster layers are used, or when the map image doesn't fill the whole area of the GIS object, for example when the whole globe is displayed in orthographic projection.
The rest of the properties are specific to the GIS Object .
GISProjection
This property defines the projection of the rendered map; possible settings are RECTANGULAR or ORTHOGRAPHIC.
Change GISProjection to ORTHOGRAPHIC and view the map being displayed in the form of a globe.
Set the
FillColor
property to black, by clicking on the ellipses button 
and selecting the black color in the color palette. Notice that the background surrounding the globe changes its color to black.
GISCenter
This property defines the center of the displayed map in the lat/lon coordinates, it is also used to pan the map. This property contains a triplet of the X, Y and Z coordinates: the X coordinate specifying the longitude, and Y specifying the latitude of the map center. The Z coordinate is reserved and must be 0. The property's data type is G (geometrical).
The default value of "0 0 0" requests the map to be centered around the Greenwich Meridian in longitudinal direction (lon=0), and around equator in latitudinal direction (lat=0).
Change the value of the GISCenter property to "-100 40 0" and press Enter . Notice that the map has moved and is now centered at the middle of North America (lon=-100, lat=40).
GISExtent
While the control points of the GIS object define the size of the map in screen coordinates, the GISExtent property defines the extent of the longitude and latitude shown in the map area. This is also a G (geometrical) property containing a triplet of the X, Y and Z coordinates, specifying the longitudinal and latitudinal extent of the map, respectively. The Z coordinate is reserved and must be 0.
For the RECTANGULAR projection, GISExtent is specified in degrees. The default value of "360 180 0" requests the map of the whole world (360 degrees of longitude and 180 degrees of latitude) to be displayed.
Change GISExtent to the value "100 70 0" . Notice that only North America is being displayed now, instead of the whole world.
The combination of the GISCenter and GISExtent properties define the exact mapping region being displayed in the GIS object. In the above example, the area of (100,70) lon/lat degrees, with the lon/lat center (-100,40) covers the region of North America.
The units of GISExtent depend on the GISProjection . In accordance with the OpenGIS standard, the GISExtent is specified in degrees for the rectangular projection and in meters for the orthographic projection.
Let's try using this property in the orthographic projection.
Change the GISProjection property to ORTHOGRAPHIC.
Notice that the value of GISExtent has changed to "1.4e+07 1.4e+07 0". Since the value of 1.4e+07 is slightly bigger than the length of the Earth's diameter in meters, the whole globe will be displayed.
Set the value of GISExtent to "7000000 7000000 0". It will display only the North America area, in the orthographic projection.
Note: For the orthographic projection, the X and Y values of the GISExtent property must be the same.
GISStretch
This property determines whether or not the map is stretched when the X/Y ratio of the GIS Object does not match the X/Y ratio of the requested GISExtent.
Currently, GISStretch =ON, and the displayed map image may be stretched.
Resize the GIS object by dragging its control points with the mouse a few times and notice that the map gets stretched as the object's size changes.
In the Properties dialog, set GISStretch to OFF.
Resize the GIS object again and notice that the aspect ratio of the map gets preserved. As a side effect, the map image may include an area which is slightly bigger than the area defined by the GISExtent attribute.
GISDataFile
This property points to an SDF file that describes the GIS data setup, including layers and fonts used to render the map. Currently it points to <glg_dir>/map_data/sample.sdf , a sample SDF file included in the GLG release.
Let's take a quick look at the content of a typical SDF file:
LAYER DIR=layers FONT DIR=fonts LAYER=earth earth.lif LAYER=political polit_bound.lif LAYER=states states.lif LAYER=states_fill states_fill.lif LAYER=grid grid.lif LAYER=us_cities us_cities.lif VECTOR FONT=roman romans.data VECTOR FONT=script scriptc.data IMAGE FONT=moderna MgOpenModernaRegular.ttf IMAGE FONT=moderna_b MgOpenModernaBold.ttfYou can see that the format of the SDF file is very simple. It lists the layers and fonts that may be used to render the map. The following layers are defined in the SDF file and may be used for rendering the map: earth, political, states, states_fill, grid and us_cities.
Each layer line specifies a layer name used to reference the layer, such as " earth " or " states ", and the layer information file (LIF) containing layer information, earth.lif and states.lif respectively.
An absolute or relative path may be used for LIF files; if a relative path is used, the LIF file is found in the directory specified by the LAYER DIR attribute. If the LAYER DIR is not defined, the filename is relative to the location of the SDF file.
In our example, all LIF files are using relative paths and are located in the layers directory of the dataset. The layers directory is located in the same directory as the SDF file.
The layer information file ( LIF ) specifies the GIS data used to render the layer; it also specifies the values of layer attributes used to render the layer. The format and parameters of the SDF and LIF files are described in greater detail in Map Data Setup.
The SDF and LIF files are ASCII text files, which provides great flexibility and makes the map server setup easy to customize.
GISMapServerURL
While the GISDataFile property points to an SDF file on a local filesystem and assumes that GIS data files are also located on a local filesystem, GISMapServerURL is used for mapping applications where map images are supplied over the web via a URL. The property points to a URL used for map query requests for the web-based Map Server. In this case, all Map Server data files, including the SDF and GIS data files are located on a web server.
If the web access option is used, the GIS Object transparently handles all interaction with the remote map server, issuing map server requests as needed. The local or remote web access mode is determined by the environment in which the GIS Object is used, and may be controlled by an application.
GISLayers
This is a string property that specifies a comma separated list of layers to be included in the map. While the sample.sdf file lists all layers, GISLayers string defines which layers should actually be rendered on the map. For example, the following GISLayers string will display the earth, states and grid layers:
The default setting of the GISLayers property is " default ", which means that only the layers that are marked as being ` default ' are included. The layer is marked as default by setting the DEFAULT attribute of the LIF file to 1.
The earth layer's LIF file, earth.lif , contains the following attribute setting:
This makes the earth layer a default layer, automatically displayed when GISLayers contains ` default ' as a layer name.
As you can see on the map, only two layers, earth and states, are included by default. Other layers need to be added explicitly.
Try changing the GISLayers property to
and notice that the grid layer has been added to the map in addition to earth and states.
Try changing GISLayers string to:
default,-states,gridIt would exclude the states layer from the default layer list, so that you will see only earth and grid layers on the map.
Note: Comma is the only separator that may be used in GISLayers; space is not allowed between the layer names. For example, the following setting would cause an error due to the space before the grid layer: "earth,states, grid".
GISVerbosity
This property is used for testing purposes, to display detailed information about the current map request. It may have a value in the range [-3,10]. When it is not equal zero , the information about the generated map image is displayed on top of the map.
The negative values of the property activate display of timing information, which may be used for performance testing and benchmarks. The positive values activate display of information about layers, zoom factor, extent, used tiles and so on.
Set the GISVerbosity property to 1. The map needs to be regenerated to display the verbose output.
Click on the
Reset
toolbar button to reset the drawing and regenerate the map image.
The information about current zoom factor and total map rendering time will be displayed on top of the map.
Note: If verbose output is merging with the map colors making it hard to read, try changing the GIS Object's FillColor to black or white colors. This will cause the output to be displayed in a different color as well, making it easier to read. On Unix/Linux systems, the output is also printed to stderr and will appear on the terminal from which the Builder was started.
Set the GISVerbosity property to 4 and click on the Reset toolbar button again to reset the drawing.
More detailed information will be displayed, including the center and extent of the map, zoom factor, layer and tiling information.
Set the
GISVerbosity
property to -3 and click on the
Reset
toolbar button again.
The timing information will be displayed, including rendering time for each layer.
Set
GISVerbosity
property back to 0 and click on the
Reset
toolbar button.
Positioning of the GIS object inside the viewport
When the GIS Object is used in a mapping application, it usually covers the whole area of the viewport that contains the map. This is achieved by positioning the GIS object's control points in the opposite corners of the viewport. If the viewport has Stretch=YES (default setting) and there is no zooming, the extent of the visible area of the viewport is [-1000;+1000] coordinates in both X and Y directions.
The GIS Object's control points can be positioned at these exact coordinates by either resizing the object with the mouse using the Snap To Grid feature, or entering the control point coordinates directly, as shown below.
If not yet selected, click on the GIS object with the mouse to select it. Notice the two control points shown in the opposite corners of the object. Dragging the control points with the mouse changes the object's size.
Press the
Hierarchy Up
button to go to the viewport level to see how the map image will look in an application. Notice that the map image covers only a portion of the viewport window, according to the GIS object size that we previously defined.
Click on the
Hierarchy Down
button to go down into the viewport again. Click on the GIS object to select it.
Drag one of the object's control points with the mouse and position it in a vicinity of the lower left corner of the drawing area. Drag the second control point and position it near the upper right corner of the drawing area.
Shift-click on the lower left control point with the left mouse button to bring its Control Point dialog. and set its coordinates to "-1000 -1000 0" in the Value text box.
Shift-click on the upper right control point and set its the coordinate values to "1000 1000 0".
Close the ControlPoint dialog. The map now covers the entire drawing area.
Press
Hierarchy Up
button to go to the top viewport level. Notice that the map image now covers the entire viewport window.
Try resizing the $Widget viewport by dragging its control points with the mouse, and notice that the map inside it is resized with the viewport, always covering the whole area of the viewport.
Zooming and prototyping in the Builder
A typical mapping application needs to support map zooming and panning functionality, including zooming in and out, as well as zooming to a region selected by the user with the mouse.
This functionality is transparently integrated in the GIS object, which automatically issues a map request to display a new map image when the map is zoomed, panned or resized. In an application, a single API call is used to zoom and pan the map when user requests it.
Zooming and panning may be easily prototyped in the Builder using zoom and pan accelerators in the Run mode. The Run mode may be also used to select a map area to be displayed at the application startup. Default key bindings for zoom and pan accelerators are documented on page 56 of Viewport of the GLG Builder User's Guide and Reference Manual .
To activate viewport's zoom and pan accelerators in the Builder, the ZoomEnabled property of the viewport containing the GIS object must be set to YES. The default setting of ZoomEnabled is NO, since mapping applications usually use buttons for zooming and panning, instead of the zoom and pan accelerators.
In our drawing, the GIS object is placed inside of a top level viewport named $Widget . Assuming that $Widget viewport is currently selected, display its Properties dialog by selecting Object, Properties from the main menu, or using the right mouse button and selecting Properties .
In the Properties dialog, set ZoomEnabled=YES , then close the dialog.
Note: In a real application, the GIS object may not be a direct child of a top level viewport. If the $Widget viewport contains interface widgets, such as buttons, a user would like zoom requests to be applied only to the map and not to the interface widgets. To achieve this, a separate child viewport may be created inside the main $Widget viewport to hold the map. The GIS object would be added to this child viewport, and to activate map zooming and panning for the Run mode, ZoomEnabled=YES would be set for this child viewport, as opposed to the $Widget viewport.
Click on the
Start
toolbar button, or select Run, Start from the main menu to start the Run mode.
A dialog will appear prompting for an animation command. Erase the animation command, as we are not going to animate any resources.
Note: Instead of erasing the animation command, you may accept it and then ignore the error messages pertaining to resources not being found.
Click on the OK button to close the dialog and start prototyping .
Click on the map to set the focus to the map window. Setting the focus is necessary, since we are going to use keyboard events for zooming and panning. The viewport that has the focus will be zoomed when zoom accelerators are used.
Press the `o' key a few times to zoom out .
Notice that the GIS object (the map image) is getting smaller in size, occupying a smaller area inside the $Widget viewport. However, the mapping region being displayed in the GIS object didn't change - the same map is still being displayed, in spite of the zoom out operation. This is the default, graphical zoom mode, but it is not what the user expects when zooming a map.
A more appropriate zoom mode, called the GIS Zoom mode, zooms the map by changing the map extent without changing the size of the GIS object, so that the map image would still occupy the whole $Widget viewport. Let's try it.
Press the `n' key to reset the drawing's zoom factor back to 1.
Position the mouse on top of the map and press the `g' key to start the GIS Zoom mode for it.
Press the `o' key to zoom out.
The map image is zoomed differently now, displaying more of the globe, i.e. displaying a map with a new extent. The GIS object's size has not changed, as it still covers the whole viewport.
Try using the following keys a few times to experiment with the integrated map zooming:
`p' to display the screen and lat/lon coordinates of a point on the map
`n' to reset the map to view the whole world.
The GIS object also supports the ZoomTo operation, so that the user may select a rectangular area on the map to zoom to.
To start the ZoomTo operation, press and hold the Shift key, then click and drag a rectangle on the map to select an area to zoom to. Alternatively, pressing the `t' key, then clicking and dragging may be used.
When the mouse button is released, the map image will display the selected map region.
The ZoomTo operation in the prototyping mode may be used to define the initial map region to be displayed at the application start-up. When the drawing is saved, the GISExtent and GISCenter properties of the GIS object are stored in the drawing. When the drawing is loaded in the application, it will appear in the exact map zooming state that had been saved.
Click on the Stop 
toolbar button to stop the
prototyping
mode.
Other uses of the map prototyping mode include testing automatic layer switching for layers that utilize zoom and number of times limits to switch to enable/disable the layer or switch to a fallback layer. It may also be used for testing setups of tiled datasets as well as querying lat/lon coordinates and elevations of points on the map.
The `c' and `C' keys may also be used in the GIS Zoom mode to rotate the map (only in the rectangular projection).
GIS Object Resources
The properties of the GIS object may be accessed as resources of the drawing without navigating down to the object level and selecting it. The resource mechanism is also used at application run-time to access GIS object and its properties programmatically. Let's use the Builder's Resource Browser to familiarize ourselves with drawing resources.
Make sure that the top $Widget viewport is selected and select
Object, Object Resources
from the main menu (or press the
Object Resources
toolbar button) to bring the
Resource Browser
.
The resource browser will list all resources of the $Widget viewport. Since we named our GIS object "GISObject", the GIS object will appear in the resource browser as the GISObject resource.
Double-click on the GISObject resource in the Resource Browser to open it up and view a list of the GIS object's resources. Most of the properties of the GIS object accessible through its Properties dialog are also accessible as resources through the Resource Browser.
Click on the GISExtent resource in the Resource Browser to bring its Resource Object dialog.
Notice that the value of the GISExtent resource was changed by the zooming operations performed in the prototyping mode. This value is defined in meters , since the current projection of the GIS object is set to ORTHOGRAPHIC.
Let's change the object' projection to rectangular. Select the GISProjection resource and change the projection to RECTANGULAR in the Resource Object dialog.
Since the rectangular projection uses different units for GISExtent, the value of the GISExtent property will be reset to the default value "360 180 0" and the whole globe will be displayed in the rectangular projection.
Let's try zooming the map again and see how it affects the GIS object's properties.
Press the Start
toolbar button to start the prototyping mode, then click on
OK
to dismiss the animation command dialog.
Click on the map with the mouse to set focus into the map window. position the mouse over the GIS object and press the `g' key to set GIS Zoom mode.
Press and hold the Shift key, then click and drag a rectangle for the ZoomTo operation, selecting the United States area to be displayed.
Click on the Stop
toolbar button to stop
Prototyping
mode.
Click on the map to select the
$Widget
viewport, then press the
Object Resources
toolbar button to display resources of the $Widget viewport.
Double-click on GISObject resource in the Resource Browser to open it up, then click on the GISCenter resource to display its value.
Notice that the GISCenter value in the Resource Object dialog is defined in the lat/lon degrees and corresponds to the center of the map region we zoomed to. Keep in mind that the first coordinate of the value corresponds to the longitudinal value, and the second to the latitudinal value.
Click on the GISExtent resource in the Resource Browser .
Notice that the value of this resource is defined in the lat/lon coordinates as well, specifying the extent of the longitude and latitude in degrees of the displayed map region.
The zoom and pan operations change GISExtent and GISCenter attributes to match the map region being zoomed to when prototyping GIS Zoom in the Run mode .
The GISCenter attribute is always specified in degrees ; the units of GISExtent depend on the projection -- it is specified in lat/lon degrees for the rectangular projection, and in meters for orthographic projection.
When the lat/lon value is entered as a triplet of X, Y and Z coordinates, the X coordinate specifies longitude, the Y coordinate specifies latitude and Z coordinate must be set to 0.
The current zoom factor may be viewed as part of the verbosity output, if the value of the GISVerbosity resource is greater than zero .
Assuming that the Resource Browser dialog for the GISObject is still open , click on the GISVerbosity resource in the Resource Browser and set the resource's value to 1.
Press the Reset
toolbar button to reset the drawing and notice the diagnostic output with the value of the zoom factor displayed on top of the map image.
Map Data Setup
The GIS data and setup files used to generate map images are organized in a hierarchical directory structure, which is kept on a local filesystem for the library-based deployment of the Map Server. For the web-based deployment, where the map images are served over the web via a URL, the GIS data and setup files reside in a map data directory on the web server. The content of the map data directory is identical for either deployment option.
The following sections describe configuration files used by the Map Server.
Server Data File (SDF)
The top level directory of the GIS data contains a server data file (SDF), the main configuration file that defines the layers and fonts used to render the map. The name of this file is specified as a GISDataFile property of the GIS object , or as the -dataset command-line option when the Map Server executable is used on a web server.
Sample map data provided with the GLG Toolkit is located in the GLG's map_data directory. Let's take a look at the content of this directory.
It contains a sample SDF file named sample.sdf, as well as fonts and layers directories. It also contains a few sample scripts that show the Map Server's command-line options needed to generate and save map images.
The following lists a partial content of the sample.sdf file:
#GLM ROOT DIR=/usr/local/glg/map_data LAYER DIR=layers FONT DIR=fonts # Layer string aliases used by gis and airtraffic control demos ALIAS=default_gis "earth,states" ALIAS=default_air "political_filled,states_dcw,grid50,outline" # Layers LAYER=earth earth.lif LAYER=earth_fallback earth_fallback.lif LAYER=states states.lif # Sample DCW layers LAYER=political polit_bound.lif LAYER=political_filled polbnd_polbnd.aft.gvf.lif LAYER=states_dcw states_dcw.lif LAYER=states_fill states_fill.lif # Sample grid layers with different colors LAYER=grid grid.lif LAYER=grid30 grid30.lif LAYER=grid50 grid50.lif LAYER=grid70 grid70.lif # Outline of the whole globe LAYER=outline outline.lif # Sample us cities layer LAYER=us_cities us_cities.lif # Fonts VECTOR FONT=roman romans.data VECTOR FONT=script scriptc.data IMAGE FONT=moderna MgOpenModernaRegular.ttf IMAGE FONT=moderna_b MgOpenModernaBold.ttf IMAGE FONT=moderna_i MgOpenModernaOblique.ttf IMAGE FONT=moderna_ib MgOpenModernaBoldOblique.ttfThe LAYER DIR and FONT DIR parameters point to the directories where the layer information files (LIFs) and font files are located, layers and fonts respectively.
The GLM ROOT DIR parameter defines where to look for the data files that are defined using relative paths. All files needed for the Map Server are relative to this directory, unless an absolute path is used. If this attribute is omitted, all files are relative to the location of the SDF file itself. For the example above, the layers directory defined by the LAYER DIR parameter is relative to the directory where the SDF file is located, since GLM ROOT DIR is commented out.
The sample.sdf file also lists all available layers that may be included in the map image. The LAYER token is used to describe each layer as follows:
LAYER=<layer_name> <.lif file><layer_name> is an arbitrary name given to a layer. This name is used to identify the layer when supplying the list of layers to be displayed, via the GISLayers property of the GIS object or the layer string of a map server request.
<.lif> file is a layer information file (LIF) , a configuration file that contains layer attributes, such as colors, zoom factor, transparency and so on. It also contains a name of the file with the GIS data used to render the layer.
The ALIAS parameter defines layer name aliases that may be used for convenience. An alias associates a single name, such as default_gis or default_air, with a list of several layers. When an alias name is included into the list of layers to be displayed, all layers listed in the alias will be displayed on the map. The alias also provides a logical layer name for an application, making it possible to change the layers displayed on the map by changing the layer names in the alias, without a need to change the application source code.
The SDF file also lists available fonts provided in the fonts directory. The map server may also use any True Type font available on the system.
Layer Information File (LIF)
Let's take a look at the content of the states.slf LIF file for the states layer, located in the map_data/layers directory:
TYPE=VECTOR TRANS TYPE=OPAQUE ALPHA=0.5 FILENAME=states.gvf EDGE COLOR=0.89 0.89 0.89 DEFAULT ON=1It includes the following layer attributes:
TYPE=VECTOR defines the layer as a vector layer, rendered using vector data. For raster layers, such as the earth layer , TYPE attribute is set to IMAGE .
TRANS TYPE=OPAQUE defines layer blending type to be OPAQUE. The layer will be fully opaque only if the layer's transparency is turned off by setting the layer's ALPHA attribute to 1 (default). If the value of the ALPHA attribute is between 0 and 1, the layer will be semi-transparent. For example, ALPHA=0.5 makes the layer 50% transparent. If ALPHA is set 0, the layer will be completely transparent. OPAQUE is the most often used blending type, other blending types are used only for special types of rendering, such as rendering shaded relief images or transparent weather layers.
FILENAME=states.gvf specifies the file containing GIS data for rendering the layer. The data file is located in the same directory as states.lif . The format of the file (GVF) is detected from its extension, but it may also be specified explicitly using the VECTOR FORMAT=GVF directive. Refer to GIS Data Formats for Vector Layers for more information on the GVF and other supported vector formats.
EDGE COLOR=0.89 0.89 0.89 specifies the color for rendering the states layer outline - light grey. The color is defined using the OpenGL style, with RGB values in the range from 0 to 1.Since the FILL TYPE attribute is not specified, the default EDGE value is used, and the layer is rendered as an outline with no fill.
DEFAULT ON=1 defines the layer as a default layer, which will be automatically activated if the `default' layer name is included in the layers string of the map request. If this attribute is equal to zero or omitted, the layer is not turned on by default, and the layer's name should be explicitly included into a layers string of a map request to display the layer.
Let's change the states layer to add fill, define the fill color and change the line width of the outline.
Edit the states.lif file, located in the GLG's map_data/layers directory and add the following three lines:
FILL TYPE="FILL AND EDGE" FILL COLOR=0.89 0.89 0.63 LINE WIDTH=3
In the GLG Builder, click on the
Reset
toolbar button to reset the drawing. This will regenerate the map image using the new
states.lif
file. The states layer is now rendered with the brown fill color, light grey edge color and wider line width, reflecting the new settings in the
states.lif
file.
Edit states.lif file again to undo the changes. You may just comment out the lines we just have added using the `#' character as follows:
TYPE=VECTOR TRANS TYPE=OPAQUE ALPHA=0.5 FILENAME=states.gvf EDGE COLOR=0.89 0.89 0.89 DEFAULT ON=1 #FILL TYPE="FILL AND EDGE" #FILL COLOR=0.89 0.89 0.63 #LINE WIDTH=1
Save the changes. In the GLG Builder, click on the
Reset
toolbar button again to redraw the map. The
states
layer should appear as edge-only layer again.
The states.lif file contains only a subset of all available layer attributes. Refer to Configuration Variables on page 33 of the GLG Map Server Reference Manual for a complete list of layer attributes.
Using Zoom Factor Limits for Automatic Layer Switching
Individual map layers may be switched on/off automatically by specifying zoom factor limits in the layer's .lif file.
Let's enable the us_cities layer to display major US cities on the map.
In the GLG Builder, select the $Widget viewport by clicking on it.
Display viewport's resources by clicking on the
Object Resources
toolbar button.
Double-click on the GISObject resource in the Resource Browser to open it up and view its resources.
Click on the GISLayers resource and change its value to `default,us_cities' .
Click on the GISVerbosity resource and change its value to 1.
Click on the
Reset
toolbar button to redraw the map.
Verbosity information on top of the map image will display the current value of the Zoom factor .
Notice that although the us_cities layer is included in the GISLayers resource, the city markers are not displayed on the map.
Start the
prototyping
mode by clicking on the
Start
toolbar button.
Click on the map with the mouse to set the keyboard focus into that window.
Press the `g' key to enable the GIS Zoom mode .
Press and hold the Shift key, then click and drag a rectangle on the map to define a map region to zoom to. Zoom on, let's say, New England or California. Notice the value of the Zoom factor displayed as part of the verbosity information on top of the map image.
If Zoom factor is less than 10., repeat the ZoomTo operation to zoom further, until the Zoom factor becomes greater than 10. You should see large US cities appearing on the map when the Zoom factor value exceeds 10.
Press the `o' key to zoom out -- city markers will disappear once ZoomFactor becomes less than 10.
The us_cities layer is enabled or disabled automatically, based on the map zoom level, which is achieved by the following setting in us_cities.lif file:
MIN ZOOM=10. MAX ZOOM=-1.A combination of MIN ZOOM and MAX ZOOM attributes determine the zoom factor range within which the layer is enabled. In our case, the us_cities layer will be enabled when zoom factor > 10 (zoom factor > MIN ZOOM). Since the maximum zoom is set to -1, there is no limit to the maximum zoom level.
This feature is very convenient for controlling the level of details to be displayed on the map. More detailed layers may be turned on at high zoom levels, while other layers may be turned off.
The us_cities layers also uses attribute thresholds attached to the MIN ZOOM attribute to control which cities and towns are displayed as the zoom factor increases. As the map is zoomed in further and further, smaller and smaller cities and towns get displayed, as directed by the popul_to_zoom.tt threshold table attached MIN ZOOM. While the global MIN ZOOM setting controls the zoom limit for the whole layer, the threshold table defines the minimum zoom limit for each city and town marker depending on it population attribute (defined in the GIS data file as an attribute with index 1).
Click on the Stop
toolbar button to exit the prototyping mode when you are done experimenting with various zoom factors.
GIS Data Formats for Vector Layers
For vector data, the Map Server uses an open specification GVF vector data format, which is a simple vector format optimized for run-time performance. Files in this format usually have the .gvf extension. The Map Server also supports shapefile format, which may be used directly or converted to the GVF format for efficiency. A shapefile conversion utility is provided as the -shp2gvf option of the map server executable. Refer to the Tools and Utilities of the GLG Map Server Reference Manual for information on command-line options.
Other vector formats are supported using filters, which are invoked by the Map Server at run time to read vector data in custom formats. While the vector data filters can be used by the Map Server at run-time, the data conversion may be performed at the setup time to optimize the run-time performance. Filters for most common GIS vector formats - shorelines, CIA World DataBank (WDB), etc. - are provided in the map_server/convert directory. Please contact Generic Logic if you need assistance.
The rest of the vector formats may be imported using gdal converters, by converting them to the shapefile format first.
Data sets for the commonly used GIS data sets, such as the Digital Chart of the World (DCW / VMAP0) and US Census Tiger Data , are available in the GVF format from Generic Logic, in the ready-to-use form that includes all setup files.
Image Layers and Tiling
So far, we have been examining vector layers, such as states and us_cities , which used vector GIS data for rendering.
There are also raster, or image layers which are rendered using raster images, in either TIFF or JPEG format. This layers are defined with TYPE=IMAGE in the LIF file.
The earth layer is a good example of an image layer. Its LIF file, named earth.lif and located in the map_data/layers directory, has the following content:
# The earth layer TYPE=IMAGE IMAGE FORMAT=JPEG TRANS TYPE=OPAQUE # this is the filename or template for tiling FILENAME=earth_tiff/earth_%_%.jpg # A fallback layer in case the number of tiles exceeds # memory requirements FALLBACK LAYER=earth_fallback MAX TILES=40 WIDTH=8192 HEIGHT=4096 NUM TILES X=20 NUM TILES Y=20 MIN LON=-180 MAX LON=180 MIN LAT=-90 MAX LAT=90 MIN ZOOM=-1. MAX ZOOM=-1. DEFAULT ON=1IMAGE FORMAT provides information about the format of the image data file. This parameter is optional, if it is omitted, the format is determined by the filename extension. JPEG and TIFF formats are natively supported for the image data.
FILENAME specifies the name of the image file that is used to render the layer. If the layer's image is tiled, which is the case for the earth layer, the FILENAME provides a filename pattern for layer's tiles. The pattern must have two `%' characters which are replaced with the tile row and column to construct the tile name. In our case, FILENAME=earth_tiff/earth_%_%.jpg specifies that the tiles are located in the earth_tiff subdirectory of the layers directory and named earth_0_1.jpg, earth_0_2.jpg, earth_0_3.jpg, and so on.
The tiling mechanism is used to optimize performance when rendering layers with large amount of data. It would be very inefficient to process all layer data when only a small area of the layer is displayed. For example, the earth image has resolution of 8192 x 4096 pixels, which would require in excess of 100 MB of RAM to load it. Loading the entire image when only one country is shown on the map would waste a lot of memory and make Map Server unnecessarily slow. With tiling, only the tiles needed to render a requested map area are loaded, minimizing memory and CPU consumption and enhancing performance.
The tiles for the earth image were created using a tiling utility, which is provided as a part of the Map Server executable and is invoked using the -tile option. For example, the following command will split the earth.tif file into 20 x 20 rectangular tiles:
GlmMap -tile -image -filename earth.tif-template earth_tiff/earth_%_%.jpg
-num-x-tiles 20 -num-y-tiles 20
GlmMap in the above command is the Map Server executable located in the GLG's bin directory; when used with the -tile option, it serves as a tiling utility .
The -image option specifies that image tiling is performed, as opposed to tiling vector data.
The -template option specifies a pattern for tile filenames. The used pattern, earth_tiff/earth_%_%.jpg, specifies that the generated tile files will be saved in the earth_tiff subdirectory using the earth_%_%.jpg pattern for filenames. The tile filenames will be formed by replacing the `%' characters in the pattern with the zero-based row and column indices of each tile.
The num-x-tiles and num-y-tiles options define the number columns and rows of tiles in the X and Y direction.
Note: In some datasets GIS data are grouped in non-rectangular categories. For example, the US Census Tiger dataset organizes GIS data by state and county using a tree-like directory structure. For such datasets the rectangular tiling becomes unusable, and hierarchical directory-based tiling option should be used. The hierarchical tiling is supported for both image and vector layers, and utilities for automatic generation of setup files for such datasets are provided as a part of the Map Server executable activated using the -lif option. Refer to Tools and Utilities of the GLG Map Server Reference Manual for more information.
The MAX TILES attribute specifies the maximum number of tiles (40 in this case) the layer can use for rendering. If the number of tiles needed to display a requested map region exceeds MAX TILES, the layers will not be drawn, and a fallback layer specified by the FALLBACK LAYER token will be drawn instead to avoid excessive CPU and memory consumption. If the fallback layer is not specified, the layer will not be drawn.
FALLBACK LAYER=earth_fallback defines the earth_ fallback as a fallback layer to be drawn instead of the layer when the number of tiles needed to draw the layer exceeds the maximum number of tiles specified with MAX TILES. The earth_fallback is just another map server layer that uses an earth image of a smaller resolution , earth_smaller.jpg. The smaller resolution image is displayed when large area of the earth needs to be rendered, and using the high-resolution tiled image would require too many tiles to be loaded.
In some cases, one fallback layer may not be enough, and FALLBACK LAYER attribute may be used recursively, to create a chain of multiple layers, either tiled or untiled.
For example, the original earth.tif file uses a high resolution image with about 8000x4000 pixels. It was split into 20*20=400 rectangular tiles, with 400x200 pixels each. These tiles are used for the main earth layer, with the earth.lif LIF file.
We could also setup an intermediate fallback layer, called earth_fallback_medium, that would use a medium resolution image with 4000x2000 pixels split into 10*10=100 tiles, with 400x200 pixels each.
Finally, the earth_fallback_small layer would use a single untiled image of a smaller resolution, 2000x1000 pixels.
The LIF files for these layers would have the following content:
earth.lif for the earth layer:
#The main earth layer, high resolution. FILENAME=earth_tiff/earth_%_%.jpg FALLBACK LAYER=earth_fallback_medium MAX TILES=15earth_fallback_medium.lif for the earth_fallback_medium layer:
#The first fallback, medium resolution FILENAME=earth_fallback_medium/earth_%_%.jpg FALLBACK LAYER=earth_fallback_small MAX TILES=20earth_fallback_small.lif for the earh_fallback_small layer:
#The last fallback in the chain, no further fallbacks, #smaller resoultion image. FILENAME=earth_smaller.jpgOnce the number of requested tiles for each layer exceeds the number defined by MAX TILES attribute, its fallback layer gets activated. In the above example, when the number of requested tiles for the earth layer exceeds 15, its earth_fallback_medium fallback layer gets activated; when the number of requested tiles for this layer exceeds 20, the last fallback layer, earth_fallback2, will be used.
Transparency and Blending Types
Simple Transparent Layers
The TRANS TYPE token of the LIF file defines blending function (blending type) used for rendering the layer. The simplest blending type is OPAQUE. This name is slightly misleading, because it does not mean that the layer is necessarily fully opaque. What it does mean is that if its transparency is turned off by setting the layer's APLHA attribute to 1 (default value), it will be fully opaque.
To make the whole layer transparent, the ALPHA attribute may be set to a value in the range from 0 to 1 as follows:
ALPHA=<value>For layers with the OPAQUE blending type, the alpha value of 0 makes the layer completely transparent and 1 makes it completely opaque. Other values in the range make the layer semi-transparent.
Layers with Transparent Background
In cases when an image has a background of a particular color which needs to be rendered as transparent, the TRANSPARENT COLOR OPAQUE blending type may be used.
The following parameters in the LIF file would remove green background from the image, while rendering pixels that have other colors:
TRANS TYPE="TRANSPARENT COLOR OPAQUE" #Define transparent color as green TRANS COLOR=0. 1. 0. #Define the color precision for JPEG images TRANS COLOR PRECISION=0.1The TRANS COLOR attribute defines which color to render as transparent in the TRANSPARENT COLOR OPAQUE rendering mode. In our example, the transparent color is green.
TRANS COLOR PRECISION defines a maximum distance in RGB color space (in the range from 0 to 1) used to determine a color's transparency. This parameter is used with JPEG images to overcome artifacts of JPEG's lossy compression, which slightly changes pixel colors. A typical parameter value is in the range from 0 to 0.5. If the value of 0 is used (default), only pixels with the exact transparent color value will be transparent.
When possible, we recommend using TIF format for transparent images, so that color precision would not be an issue. For TIF images, TRANS COLOR PRECISION parameter may be omitted.
Special Blending Types
There are cases when more complex blending is required. For example, for transparent cloud layers the brightness of the pixel indicates the thickness of the cloud cover. A black pixel in the cloud image specifies no cloud cover, while a white pixel specifies full cloud cover. The ALPHA WHITE OPAQUE and ALPHA BLACK OPAQUE blending type may be used to render such layers with transparency that depends on the pixel value.
For the ALPHA WHITE OPAQUE blending type, the brighter the color, the more opaque it is. For the ALPHA BLACK OPAQUE blending type, the darker the color, the more opaque it is. The alpha parameter can still be used with these two transparency types to alter the transparency of completely opaque pixels.
If the blending type is set to XOR, the color will be XOR'ed with the pixel in the image. This causes lines to stand out, and may be useful for grid layers, where it is desirable to have grid lines that always contrast with the background.
The BUMP MAP blending type may be used for rendering shaded relief images.
Markers and Labels
Vector map layers may contain point features that need to be rendered using icons, or markers . The markers may also have labels displayed next to them.
The MARKER ICON TYPE parameter of the layer defines the icon type, which may include a combination of predefined marker types, such as CIRCLE_EDGE, CIRCLE_FILL, BOX_EDGE, BOX_FILL, CROSS, etc.
For example, the us_cities layer displays markers that represent US cities and towns using the following parameters in its LIF file ( us_cities.lif ):
MARKER ICON TYPE="CIRCLE_FILL,CIRCLE_EDGE,DOT" MARKER ICON SIZE=10 MARKER MARGIN=3 MARKER EDGE COLOR=1. 1. 1. MARKER FILL COLOR=1. 0. 0.The marker type is defined as a combination of predefined types and will be rendered as a round icon with a fill, edge and a dot in the center.
If predefined types are not sufficient to display an icon, a custom icon may be used in a form of a TIF or JPEG image. In this case, the following marker parameters may be used:
MARKER ICON TYPE=CUSTOM_ICON ICON FILE=<icon filename>For example, airports may be displayed on the map as custom symbols, using a custom jpeg file:
MARKER ICON TYPE=CUSTOM_ICON ICON FILE=airport_symbol.jpgThe TRANSPARENT COLOR OPAQUE blending type described above may be used for layers that need to use icons with transparent background.
The us_cities layer also displays labels next to each city or town marker. It is achieved by using the following lines in the layer's LIF file:
# TrueType font sample FONT=moderna_b FONT SCALE=10 FONT TYPE="MAPPED FLAT" FONT ANGLE=0. FONT ANCHOR=HLEFT VCENTER LABEL EDGE COLOR=1. 1. 0. # Outline around the text is used to make the label more readable. LABEL STYLE=OUTLINE LABEL OUTLINE COLOR=1. 1. 1. # Simple label using place name (attr index=0): LABEL FORMAT=<0> # Enable layout negotiation LAYOUT TYPE=GLOBAL LABEL PRIORITY=0The FONT and FONT SCALE parameters define the type and size of the font used to render the labels.
The FONT TYPE parameter specifies that the labels have to be rendered using a fixed size font that does not scale or move with the map when the map is zoomed in or zoomed out.
FONT ANGLE may be used to specify an optional text rotation angle.
The FONT ANCHOR parameter specifies anchoring of the text label relatively to the marker.
The LABEL STYLE parameter requests an outline to be drawn around each character to improve text readability on top of a complex map background. LABEL OUTLINE COLOR specifies the color of the outline.
The LABEL FORMAT parameter is used to specify what needs to be rendered in the label. The LABEL FORMAT=<0> setting requests the name of the city or town (the attribute of the point feature with index=0) to be displayed in the label. The attribute index is specified inside angle brackets.
To use attributes of GIS features, one has to know what attributes are provided in the GIS data file. Since the population attribute is stored in the data file as an attribute with index=1, the following setting may be used to display a place's name and population:
LABEL FORMAT="<0>, population: <1%.0lf>"
The place's population will be displayed using the "%.0lf" C format, which is appended to the attribute index with no separator characters.
LAYOUT TYPE=GLOBAL requests the label layout negotiation to be performed on labels to make sure the labels do not obscure each other. The label layout negotiation will be performed globally for labels in all layers.
LABEL PRIORITY=0 defines the highest label priority for the labels in this layer. This makes sure that these labels will never be overwritten by labels from a different layer with lower priority. In the actual us_cities.lif LIF file, an attribute threshold is also attached to the label priority resource of the layer to vary the label's priority depending on the city or town population. The next chapter shows how to attach attribute thresholds.
Dynamic Attribute Thresholds
In some cases, a finer control of the attribute value in LIF file is required. For example, the MARKER ICON SIZE for the us_cities layer is set to 10, which means that all city and town markers will be 10 pixels in diameter. It would be nice, however, to vary the marker size depending on the city's population, displaying large markers for large cities, smaller markers for medium cities, and so on. In addition, markers could use different colors to visually differentiate large cities from small towns.
To accomplish this, a threshold table may be attached to the marker size attribute of the layer to change the city's marker size depending on the population attribute of a city in the vector data file. Another threshold may be attached to the layer's fill color attribute to vary colors for cities and towns with different populations.
This could be accomplished in a different way, by splitting the layer data into several layers based of values of their attributes using provided vector data splitting utility (the -split option of the Map Server executable). For example, large cities may be extracted into one layer, medium cities into a second layer, and small towns into the third layer. Each of the layers may use different rendering attributes (color, marker size, etc.) and may be independently activated. This option will also yield better run-time performance than the attribute thresholds. However, it may not be practical when the thresholds for splitting data may change, in which case it is more convenient to use the threshold tables which can be easily modified without a need to re-run the splitting utility.
In the us_cities.gvf data file that is used for the us_cities layer, the population is stored as an attribute with index 1. The following LIF directives may be used to attach threshold tables to vary marker color and size depending on the place's population attribute:
ATTR MAP= 1 MarkerFillColor popul_to_color.tt ABS ATTR MAP= 1 MarkerSize popul_to_size.tt RELThe first parameter of the ATTR MAP directive is the index of the population attribute in the data file, which is 1.
The second parameter, MarkerFillColor, is the name of the layer resource, which resembles the name of the corresponding MARKER FILL COLOR LIF parameter, but uses a different naming convention to differentiate LIF directives from layer resource names used at run time. A list of all layer resource names may be found in the Layer Resources of the GLG Map Server Reference Manual.
The third parameter, popul_to_color.tt, provides a filename of the threshold table.
The last parameter specifies a mode for applying thresholds: absolute (ABS) or relative (REL). In the absolute mode, the value from the threshold table replaces the value of the layer's resource. In the relative mode, the layer's resource value is multiplied by the threshold value, allowing the layer attribute to control the size of all city markers, while the threshold value controls the size of each city marker relative to the value defined in the LIF file.
Let's take a look at the content of the popul_to_color.tt file located in the GLG's map_data/layers directory :
INPUT DATA TYPE= GLM_D OUTPUT DATA TYPE= GLM_G 0 1. 1. 1. 50000 1. 1. 0. 100000 0. 1. 0. 500000 1. 0. 0.The INPUT DATA TYPE specifies the data type of the input attribute used to control the threshold table mapping. Since the population attribute uses double value, the matching GLM_D (double) type is used.
The OUTPUT DATA TYPE specifies the data type of the output. Since the threshold table maps the value of the population attribute to a marker color, the GLM_G (RGB triplet) type is used.
The remaining lines of the threshold table list the table's thresholds. For each threshold, an input value of INPUT DATA TYPE is listed first, following the output value of OUTPUT DATA TYPE.
According to this table, large cities with a population of 500K or more will be rendered using the red color, cities with population less than 500K but more or equal 100K will be displayed in green color, cities and towns with size from 100K to 50K will be displayed using yellow markers and the rest of the towns with population of 50K or less will be rendered using white markers.
The output value in this table, the value of MarkerFillColor resource, is specified as an absolute value of the marker's fill color.
Now let's take a look at the popul_to_size.tt file , located in the GLG's map_data/layers directory:
INPUT DATA TYPE= GLM_D OUTPUT DATA TYPE= GLM_D 0 0.35 10000 0.45 50000 0.62 100000 0.8 500000 1.0In this threshold table, the input value which is the city's population of type GLM_D (double), is mapped to the output value, a marker size of the marker representing a city (GLM_D as well). The output value will be used as a relative value of the marker size, since the REL parameter was used in the LIF's ATTR MAP directive.
According to this threshold table, the resulting value of the MarkerSize resource for towns with population less than 10K will be calculated as MARKER ICON SIZE * 0.35. Since MARKER ICON SIZE=10 in the us_cities.lif LIF file, the towns with population less than 10K will be represented by small markers with the marker size of 10 * 0.35 = 3.5 pixels, which will be rounded to the integer value of 3.
The us_cities layer also attaches a threshold table to the MinZoom resource of the layer to display more cities and towns as the map is zoomed in. This threshold table is located in the popul_to_zoom.tt file located in the map_data/layers directory and is similar to the marker size threshold table described above.