For map data
sets and response data sets,
id-variables identify
the map areas (for example, counties, states, or provinces) that make
up the map. A
unit area or
map
area is a group of observations with the same
ID value. The GMAP procedure matches the value of the response variables
for each map area in the response data set to the corresponding map
area in the map data set. It does this in order
to create the output graphs.