Concepts

About the Input Map Data Set

The input map data set must be a traditional map data set and contain these variables :

• a numeric variable named X that contains the horizontal coordinates of the map boundary points.

• a numeric variable named Y that contains the vertical coordinates of the map boundary points.

• one or more identification variables that uniquely identify the unit areas in the map. These variables are listed in the ID statement.

It also can contain

• one or more variables that identify groups of unit areas (for BY- group processing)

• the variable SEGMENT, which distinguishes nonconterminous segments of the unit areas.

Any other variables in the input map data set do not affect the GREDUCE procedure.

About Unmatched Area Boundaries

If you are using map data sets in which area boundaries do not match precisely (for example, if the boundaries were digitized with a different set of points), PROC GREDUCE will not be able to identify common boundaries properly, and this results in abnormalities in your maps. These abnormalities include mismatched borders, missing vertex points, stray lines, gaps, and distorted polygons.

If the points in the area boundaries match up except for precision differences, round each X and Y value in your map data set accordingly , using the DATA step function ROUND before using PROC GREDUCE. (See SAS Language Reference: Dictionary for information on the ROUND function.)

For example, if the map data set APPROX has horizontal and vertical coordinate values for interior boundaries of unit areas that are exactly equal only to three decimal places, then this DATA step creates a new map data set, EXACT, that will be better suited for use with PROC GREDUCE:

 data exact;        set approx;        if x ne . then x=round(x,.001);        if y ne . then y=round(y,.001);     run; 

See About Map Data Sets on page 999 for additional information on map data sets.