|Preparing Spatial Data|
Before you can use your spatial data as a basis for exploring your attribute data, you must link the attribute data to the spatial data. As explained in Overview of SAS/GIS Software, one way to use the attribute data after you have linked it to the spatial data is by creating a theme to control the appearance of features in the spatial data.
In the layer bar, right-click the COUNTY layer name to open the pop-up menu for the COUNTY layer. Select Edit to open the GIS Layer window. In the definition for the COUNTY layer, select Thematic. The GIS Attribute Data Sets window opens for you to define the link to the theme data set.
In the GIS Attribute Data Sets window, select New to define a new link. In the resulting Select a Member window, select MAPS.USAAC. You must next specify the values that are common to both the attribute and spatial data, since the common values provide the connection between the spatial data and the attribute data. The spatial database and the MAPS.USAAC data set share compatible state and county codes, so first select STATE in both the Data Set Vars and Composites lists, and then select COUNTY in both lists. Select Save to save the link definition to the Links list. Finally, select Continue to close the GIS Attribute Data Sets window.
After the GIS Attribute Data Sets window closes, the Var window automatically opens for you. Select which variable in the attribute data provides the theme data for your theme. Select the CHANGE variable to have the counties colored according to the level of change in the county population. Select OK to close the Var window.
Note: The theme ranges in the COUNTY layer reflect the range of values in the MAPS.USAAC data set, which contains data for the entire United States. See Chapter 5, "Customizing Maps," in SAS/GIS Software: Usage and Reference, Version 6 for details on how you can select different theme ranges that are more appropriate for the displayed counties.