3.4. Specifying Grids, Ellipsoids, and Map Projections

3.4.1 Specifying Grids

The map projection information in the GRIDDESC file consists of five descriptive parameters: PROJ_ALPHA, PROJ_BETA, PROJ_GAMMA, X-CENT, and Y-CENT. These parameters are dependent upon Projection Types in GRIDDESC File.  The projection section of the GRIDDESC file must start with the following line of text:

! coords --line: name; type, P-alpha, P-beta, P-gamma, xcent, ycent

The grid description section of the GRIDDESC file follows the projection section.  The following line of text must be included to show the separation between the two sections:

' ' ! end coords. grids: name; xorig,yorig,xcell,ycell,ncols,nrows,nthik

Definitions of regular grids require four descriptive parameters and three dimensioning parameters in addition to the specification of a map projection:

• X_ORIG - x coordinate of the grid origin (lower left corner of the cell at column=row=1), given in map projection units (meters, except in lat-lon coordinate systems)
• Y_ORIG - y coordinate of the grid origin (lower left corner of the cell at column=row=1), given in map projection units (meters, except in lat-lon coordinate systems)
• X_CELL - Grid cell size for cells parallel to the x-coordinate axis, given in map projection units (meters, except for lat-lon coordinate systems)
• Y_CELL - Grid cell size for cells parallel to the y-coordinate axis, given in map projection units (meters, except for lat-lon coordinate systems)
• NCOLS - Number of columns (dimensionality in the x direction)
• NROWS - Number of rows (dimensionality in the y direction)
• NTHIK - External boundary thickness of the grid (i.e., the number of grid cells to extend the grid beyond each boundary [and around the corners] in a direction towards the exterior)

The grid information section ends with the following text:

' ' ! end grids

3.4.2 Using PROJ4.5.0 Specifications for Projections

The Proj4.5.0 library (http://www.remotesensing.org/proj) is used by the srgcreate and allocator programs to specify map projections and ellipsoids. NOTE: this new library supports datum transformation, but the old map projection called LATLON is no longer accepted, nor is the ellipsoid caled SPHERE. The library is stored under the source code directory (src). In order for the datum transformation functions to find the proper transformation files, users must set the environmental variable PROJ_LIB to /YOUR_INSTALL_DIR/src/PROJ4.5/local/share/proj in their .cshrc files on unix or otherwise set PROJ_LIB to the directory as an environment variable. PROJ_LIB lets the system know where the PROJ4.5.0 share installed, so that it can find all files needed for datum transformation.

Map projections can be specified independently for data, weight, input, and output polygons using the DATA/WEIGHT/INPUT/OUTPUT_FILE_MAP_PRJN environment variables. A map projection must be specified for any spatial data. Information on specifying map projection parameters for PROJ4.5.0 can be found at the bottom of the web pages linked from (http://www.remotesensing.org/geotiff/proj_list). The projection parameters should start with "+proj=" and the parameters should be in a comma-separated list without white space. For example, to use a Lambert Conformal conic projection with standard parallels of 33 and 45 degrees, a central latitude of 40 degrees, and a central longitude of -97 degrees as the projection for data polygons, use the following syntax:

   setenv INPUT_FILE_MAP_PRJN "+proj=lcc,+lat_1=33,+lat_2=45,+lat_0=40,+lon_0=-97"

To specify a UTM projection for zone 17 for the weight polygons, use the following syntax:

   setenv WEIGHT_FILE_MAP_PRJN "+proj=utm,+zone=17"

To specify a geographic latitude and longitude data for the data polygons, use the following syntax:

   setenv DATA_FILE_MAP_PRJN "+proj=latlong"

Note: In the UNIX C-shell, quotes are needed to specify the map projection line above. For consistency, all examples use quotes.

The Spatial Allocator allows the user to set the shape of the Earth independently for data, weight, and output polygons with the INPUT/WEIGHT/OUTPUT_FILE_ELLIPSOID environment variables. The ellipsoid for the spatial coordinate system has to be specified correctly when datum transformation is involked in projection coversion for two different ellipsoids. To specify an ellipsoid, you must specify either +datum= in order to have datum transformation in projection conversion, +ellps= followed by the name of a standard ellipsoid, or the values that traditionally describe the shape of the Earth, including +a=, +b=, +rf= or +es=. A plus sign must precede either the name of the Earth shape or each ellipsoidal parameter.  When supplying multiple parameters, the list must be comma separated without any white space between the parameters. For example, you could specify the MERIT spheroid using the following syntax:

   setenv WEIGHT_FILE_ELLIPSOID=+ellps=MERIT 

A list of Datum Specifications which can be used in datum transformation and a list of Named Ellipses are available. You may specify the ellipsoid parameters as shown in the following examples:

   setenv WEIGHT_FILE_ELLIPSOID=+a=6370993.0,+b=6370993.0

   setenv WEIGHT_FILE_ELLIPSOID=+datum=NAD83

   setenv WEIGHT_FILE_ELLIPSOID=+ellps=GRS80

   setenv WEIGHT_FILE_ELLIPSOID=+a=6378137.0,+rf=298.257

NOTE: SPHERE is no longer an acceptable value for an allipsoid. It is important to know that there are different ways to specify an ellipsoid. But, a datum has to be specify directly or indirectly in order to do any datum transformation correctly. See PROJ4.5.0 data transformation for details.

To Section 4: Surrogate Computation and Quality Assurance Summary