SEARCHING THE SITE FILES TO FIND DATA FOR A SPECIFIC LOCATION

Site files list the 12 monthly values for a single variable in one file. This time-sequential format was developed to facilitate data extraction for individual grid cells.

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. The 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. All grid cells are referenced by the latitude and longitude of the cell center.

Each variable with monthly data has an associated site file. The site files list all non-background grid points sequentially, with one line per grid point (3261 records). Each record contains the grid point and 6 columns of data describing the general physical characteristics of the grid cell (e.g., latitude, longitude, elevation). These 7 columns are followed by 12 columns of monthly values for the variable described by the filename.

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 less than half their area covered by land have been omitted. This occurs both near the ocean and near inland lakes.

FIND THE NEAREST GRIDCELL

In order to find the specific grid cell containing the site you are interested in, first determine the latitude and longitude, in degrees and hundredths of a degree, of your site. For example, if you are looking for Milwaukee, Wisconsin, the latitude is 43 degrees 2 minutes N and the longitude is 87 degrees 55 minutes W. This corresponds to a latitude of 43.03N and a longitude of 87.92W when converted to degrees and hundredths of a degree.

Determine the grid cell coordinates nearest to your site. Because of the 0.5 degree grid spacing, all cell coordinates in the dataset end in either [25] or [75]. The latitude and longitude of each cell are first multiplied by 100 and then stored as 6 digit integers:
```
       xx25  xx75  xx25 ....
```
In our example the nearest grid cell coordinates are latitude 43.25N and longitude 87.75W, which delineates a cell centered slightly to the northeast of Milwaukee.

To find the data quickly, search the site file for the string:
```
      	4325  8775
```

It is always important to check:
- whether the grid cell is representative of the site you are considering (e.g., mountains versus plains),
- whether the grid cell is classified as a background cell.
For example, it might not be appropriate to choose the numerically closest cell to the site you are interested in. For our example we find that the coordinates we have chosen are not listed in the site file because this grid cell is classified as a background cell (more than half of the cell lies in Lake Michigan). Background cells are not listed in the site files.

Choose a slightly different grid cell. For example, you might choose the adjacent cell to the west. Since the cells are 0.5 degrees wide the coordinates of the adjacent cell are 43.25N and 88.25W. When we search the file for these coordinates, we find that they correspond to grid cell 1338.

You will find the data for this cell on the line corresponding to 1338. To extract additional variables for this cell from the other site files search for the same grid cell or record number.

FIND SVF FILE POSITION FROM THE GRIDPOINT

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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