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13 GEMPAK Projections
Understanding the properties of map projections is useful when deciding how to display the output of various types of data. The USGS Map Projections poster provides useful information on the basic properties of common map projections available in GEMPAK.

When using pre-defined geographic area names in GEMPAK as defined in $GEMTBL/stns/geog.tbl, a default PROJ projection and GAREA is defined. The default projection can be used by specifying DEF with the desired GAREA value. For example, using the North America geographic name NAM with the default projection: 

   GAREA = nam
   PROJ  = def
produces the same plot as: 
   GAREA = -4.0;-134.0;39.0;3.0
   PROJ  = str/90;-95;0

The same GAREA which specifies the lower left and upper right corner points can produce significantly different areal coverage depending on the specified projection. For example, when using the default GAREA and projection name for the contiguous 48 United States (US), the polar stereographic projection captures the full CONUS region, while the Mercator projection with the same bounds clips the western and northern portion of the region:

   

To properly display the CONUS region using the Mercator region above, the lower left longitude value must be moved further west, and the upper right latitude value must be mored further north!

When using gridded data, it is often useful to know the projection of the grid points in order to prevent unexpected clipping in the data set.

13.0  GEMPAK Projection Examples

 


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