Instructions for Reading Data 
                   on the TerrainBase CD-ROM
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This CD-ROM contains 27 digital terrain models that cover various
regions of the world.  The models are stored in separate files
in the "\DATA" directory and are named according to the specific
geographic areas that they cover.  

The models consist of unprojected gridded data.  The grid interval
varies from model to model and ranges from 30-arc-seconds (highest
detail) to 10-arc-minutes (lowest detail).  

Data files contain only Earth surface height values stored as 2-byte 
(16 bit) binary integers.  The data can be displayed and converted 
into ASCII format by using the GeoVu software that is supplied with 
these data, or the data can be read directly from the CD-ROM.  
Header (.HDR) files in the "\DATA" directory contain all of the 
parameters needed for interpreting the data and assigning reference 
coordinates to the cell values. Refer to the GeoVu User Manual found 
in the GEOVU\TUTORIAL directory for definitions of the header file elements.  

The following pseudocode can be used to generate an XYZ file from 
the source data files.  Variables in the code refer to parameters 
in the data header files.  The code can be converted to any suitable 
programming language for convenience.  

       
     ******************************************************
     latitude = upper_map_y
     if (grid_cell_registration = "center of cell") then
       latitude = upper_map_y - 0.5 * grid_size(y)
     end

     while (rows <= number_of_rows)
       longitude = left_map_x
       if (grid_cell_registration = "center of cell") then
         longitude = left_map_x + 0.5 * grid_size(x)
       end
       while (columns <= number_of_columns)
         read (elevation)
         write (longitude,latitude,elevation)
         longitude = longitude + grid_size(x)
       end
       latitude = latitude - grid_size(y)
     end
     ******************************************************


The parameter "grid_cell_registration" is used to differentiate 
between two type of grids that are inclued in this release --
center of cell and grid intersection.  The two terms are defined
in the user manual.  

Note, that the boundary coordinates given in header files
for center of cell grids is offset by one-half cell to the north 
and west.  The user should compensate for this offset while 
assigning latitude and longitude values to the elevation data as
is done in the above sample code.  Here, the conditional 
statements are used to adjust the latitude and longitude values 
by one-half cell for center of cell grids. 

The user should note that there are no other differences between
the center of cell and grid intersection registrations.  

Refer to the User Manual for more information on data format.
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