The GREDUCE Procedure

Overview

The GREDUCE procedure processes map data sets so that they can draw simpler maps with fewer boundary points. It creates an output map data set that contains all of the variables in the input map data set plus a new variable named DENSITY. For each observation in the input map data set, the procedure determines the significance of that point for maintaining a semblance of the original shape and gives the observation a corresponding DENSITY value.

You can then use the value of the DENSITY variable to create a subset of the original map data set. The observations in the subset can draw a map that retains the overall appearance of the original map but contains fewer points, requires considerably less storage space, and can be drawn much more quickly.

GREDUCE does not produce any graphics output. Instead, it produces an output map data set that can become either

the input map data set for the GMAP procedure
the input map data set for a DATA step that removes points from the map.

CANADA2 Map before Reduction (GRDCANAD(a)) and CANADA2 Map after Reduction (GRDCANAD(b)) illustrate the effect of reduction on a typical map data set. CANADA2 Map before Reduction (GRDCANAD(a)) uses observations with all DENSITY values as input to the GMAP procedure.

CANADA2 Map before Reduction (GRDCANAD(a))

[CANADA2 Map before Reduction (GRDCANAD)]

CANADA2 Map after Reduction (GRDCANAD(b)) uses only those observations with a DENSITY value of 0 to 2 as input to the GMAP procedure.

CANADA2 Map after Reduction (GRDCANAD(b))

[CANADA2 Map after Reduction (GRDCANAD(b))]

The program for these maps is in Reducing the Map of Canada.

The reduced map shown in CANADA2 Map after Reduction (GRDCANAD(b)) retains the overall shape of the original but requires only 463 observations compared to the 4302 observations that are needed to produce the map in CANADA2 Map before Reduction (GRDCANAD(a)).

Note: Many of the map data sets that are supplied by SAS Institute already have been processed by GREDUCE. If the map data set contains a DENSITY variable, you do not need to process the data set using GREDUCE. [cautionend]

See also The GREMOVE Procedure for more information on how to

combine groups of unit areas into larger unit areas to create regional maps
remove some of the boundaries in a map and create a subset of a map that combines the original areas.

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