Previous: GEMPAK File Templates and Aliases Next: GEMPAK Projection Examples Table of contents Frames User Manual GEMPAK Online Tutorial

5.6| 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.

Projections and Interpolation

We've already seen that GEMPAK can use multiple files for grid diagnostics. GEMPAK can perform grid calculations which combine grids with differing projection and grid point spacing (introduced in GEMPAK5.8.1). If the grids are not of the same projection and grid point spacing, the first grid file in the list defines the internal navigation used for the computation of the diagnostic. So, an entry of "GDFILE = eta + gfs" is remapped to the eta file navigation, while "gfs + eta" is remapped to the gfs file navigation.

Many model data sets are transmitted on widely used grids. Some examples are:

Grid Name

Region of Coverage

Resolution

Models Commonly Available
Grid 211 CONUS 80 Km NAM, NGM, GFS, RUC
Grid 212 CONUS 40 Km NAM, GFS
Grid 202 CONUS 190.5 Km GFS, NGM
Grid 213 CONUS 95.25 Km GFS, NGM
WAFS/Thinned Global 1.25 degree GFS, UKMet

If we want to use model data with objective analyses of surface or upperair data sets, we can grid our data onto the same grid as the model data we have, although this is not required. Luckily, this is quite easy using the CPYFIL parameter with GDCFIL.

If we have two gridded data sets that are not of the same projection or resolution, we can interpolate one grid to the projection used by the other data set. The program GDBIINT can interpolate gridded data from one projection to another.

Projection Exercises
Map projections can be chosen based on the desired map properties. For example:

Orthographic projection is used to repesent the Earth as viewed from infinity. 

   GAREA = 0;-105;0;-105
   PROJ = ort/50;-105;0

The cylindrical projection can be rotated to view both poles at the same time. 

   GAREA = 0;0;0;0
   PROJ = ced/0;-105;45

Any straight line drawn on a Gnomic map is on a great circle, and can be used to find the shortest path between two points. Gnomic is often useful for ocean and seismic waves which tend to travel along great circles. 

   GAREA = -10;145;60;-80
   PROJ = gno/21;-157;0

 


Previous: GEMPAK File Templates and Aliases Next: GEMPAK Projection Examples Table of contents Frames User Manual GEMPAK Online Tutorial