The GMAP Procedure

Overview

The GMAP procedure produces two-dimensional (choropleth) or three-dimensional (block, prism, and surface) color maps that show variations of a variable value with respect to an area. A wide assortment of geographic maps are available with SAS/GRAPH software, and you also can create your own geographic or spatial maps.

Use the GMAP procedure to

summarize data that vary by physical area
show trends and variations of data between geographic areas
highlight regional differences or extremes
produce maps.

About Block Maps

Block maps display a block at the approximate center of each map area to convey information about response variable values. The height of each block represents a response level. The height of the blocks is not directly proportional to the value of the response variable. Instead, the block heights increase in order of the response levels.

Block Map (GR19N01) shows a simple block map of hazardous waste sites that are installed in each state. Each state is a midpoint. The number of sites in each state (the response value) is represented by the height of the block.

Block Map (GR19N01)

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The program for this map is in Producing a Simple Block Map. For more information on producing block maps, see BLOCK Statement.

About Choropleth Maps

Two-dimensional (choropleth) maps indicate levels of magnitude or response levels of the corresponding response variable by filling map areas with different colors and patterns.

Two-dimensional (Choropleth) Map (GR19N04) shows a choropleth map of hazardous waste sites that are installed in each state. Each state is a midpoint. The number of sites in each state (the response value) is represented by the pattern that is assigned to the state.

Two-dimensional (Choropleth) Map (GR19N04)

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The program for this map is in Producing a Simple Choropleth Map.

You can also produce a simple choropleth map that shows an outline of a map's areas by specifying your map data set as both the map data set and the response data set in a GMAP statement. For more information on producing choropleth maps, see CHORO Statement.

About Prism Maps

Prism maps use polyhedrons (raised polygons) in the shape of each map area to convey information about response variable values. The height of each polyhedron, or prism, represents an ordinal level of the response variable. Prism heights increase in order of response levels. That is, the lowest prisms correspond to the first level, and the tallest prisms correspond to the last level.

You can alter the perspective of the map by selecting a viewing position (the point in space from which you view the map). You can also change the position of the light source so that the shadowing on the prisms enhances the illusion of height.

Prism Map (GR19N07) shows a prism map of hazardous waste sites installed in each state. Each state is a midpoint. The number of sites in each state (the response value) is represented by the height of the state.

Prism Map (GR19N07)

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The program for this map is in Producing a Simple Prism Map. For more information on producing prism maps, see PRISM Statement.

About Surface Maps

Surface maps display a spike at the approximate center of each map area to convey information about response variable values. The height of the spike corresponds to the relative value of the response variable, not to the actual value of the response variable. Thus, a spike that represents a value of 100 may not be exactly 10 times higher than a spike that represents a value of 10. Map area boundaries are not drawn.

Surface maps provide no clear map area boundaries and no legend. Thus, surface maps provide a simple way to judge relative trends in the response data but are an inappropriate way to represent specific response values.

Surface Map (GR19N09) shows a surface map of hazardous waste sites that are installed in each state. Each state is a midpoint. The number of sites in each state (the response value) is represented by the height of the spike.

Surface Map (GR19N09)

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The program for this map is in Producing a Simple Surface Map. For more information on producing surface maps, see SURFACE Statement.

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