SVF is a standard file format consisting of two header lines followed by an integer array. The SVF files used for VEMAP data have three additional header lines but are otherwise in standard SVF format.
Each file contains an array of values for the variable described by the filename. For the VEMAP project the conterminous US was divided into 0.5 degree by 0.5 degree grid cells. There are 115 grid cells across the US from west to east and 48 from north to south, for a total of 5520 cells. All grid cells are referenced by the latitude and longitude of the cell center, beginning with the northwest corner of the grid. The SVF files are presented in an array consisting of 48 rows of data (latitude), with 115 columns per line (longitude).
SVF files and site files contain the same information, however, the data are stored differently. The information from several SVF files is combined within each site file.
Note: The grid cells are large enough (0.5 degree X 0.5 degree) to contain significant terrain variation within the cell. Grid cells with more than half their area covered by water are assigned a background value (-9999).
row_number = 1 + [(49.0 - 43.03)/0.5] = 1 + (11.94) = 12.94 = 12Similarly for the east-west position:
column_number = 1 + [(124.5 - 87.92)/0.5] = 1 + (73.16) = 74.16 = 74The data for this site are stored in the 12th row as the 74th value (column) of that row.
column_number = 1 + [(124.5 - 88.42)/0.5] = 1 + 72.16 = 73.16 = 73
The southern border of the VEMAP grid is 25N and the eastern border is 67W. Selecting a latitude of 25.0N will cause the algorithm given above to produce a spurious row number of 49. Similarly, a longitude of 67.0W will result in an incorrect column number of 116. These values should be 48 and 115, respectively.
The grid cells are numbered consecutively from 1 to 5520, beginning in the NW corner of the grid and increasing first to the east and then to the south. The grid point value for a specific cell can be derived from the row and column numbers calculated above:
grid point = [(row_number - 1) * 115] + column_number
grid point = [(12 - 1) * 115] + 73 = 1338
The file position for a specific grid point can be easily calculated.
row_number = 1 + [(grid point - 1)/115]
Truncate the decimal portion of row_number without rounding (i.e., 5.6 = 5). Then use row_number to calculate the column number.
column_number = grid point - [115 * (row_number - 1)]
Example for grid point 1338:
row_number = 1 + [(1338 - 1)/115] = 1 + 11.63 = 12.63 = 12 column_number = 1338 - [115 * (12 - 1)] = 1338 - 1265 = 73
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