The GPROJECT procedure processes GfK and traditionalmap data sets by converting spherical coordinates (longitude and latitude) into Cartesian coordinates for use by the GMAP procedure. The process of converting coordinates from spherical to Cartesian is called projecting. All of the GfK and many of the traditional map data sets that are available with SAS/GRAPH contain unprojected longitude and latitude coordinates. When these coordinates are plotted by the GMAP procedure, which is designed to plot points on a two-dimensional plane, the resulting map is often reversed and distorted as a result of forcing the spherical map coordinates onto a flat plane.

The GPROJECT procedure enables you to use one of several map projection techniques to project the latitude and longitude coordinates onto a two-dimensional plane while attempting to minimize the distortion of area, distance, direction, and shape properties of the original sphere. The output map data set that is produced by the GPROJECT procedure contains Cartesian coordinates that can be displayed correctly using the GMAP procedure.

The GPROJECT procedure can also create a rectangular subset of the input map data set by excluding all points with longitude and latitude values that fall outside of a specified range. This provides a simple way to reduce the size of the map data set if you need only a portion of a larger map.

The GPROJECT procedure does not produce any graphics output. Instead, it produces an output map data set, which can be used as the input map data set for the GMAP procedure (see GMAP Procedure).

Map before Projection (GPJDEFLT(a)) and Map after Projection (GPJDEFLT(b)) illustrate the effect of using GPROJECT defaults (Albers projection with standard parallels that are calculated by the procedure) to project a typical map data set with coordinates that are stored as longitude and latitude.

The program for the following maps can be seen in Using Default Projection Specifications.