In the following example, the shapefile is a ZCTA file from the US Census Bureau that contain polygons that are based on ZIP codes. The ZCTA field is the identifier that you want to use, but the polygons in the shapefile do not have unique values for ZCTA. If you do not specify the ID statement, then the GMAP procedure draws extra lines between the map areas for ZCTA.

By identifying the ZCTA field in the ID statement, you ensure that the polygons for each value of ZCTA are grouped together and assigned different SEGMENT values in the output map data set. The GMAP procedure can now draw the map areas for ZCTA correctly.

proc mapimport out=myzcta datafile="c:\zt06_d00.shp";
  id zcta;
run;