Geographic Files

Geographic files contain gridded information about each grid cell, including cell latitude, longitude, elevation, land area, and vegetation.

USING SVF FILES

We present a description of SVF files and a tutorial on how to use them. The tutorial includes instructions on how to translate latitude/longitude into a VEMAP grid cell number and SVF file position.

To download the tutorial directly from our ftpsite:

FTP SVF-TUTORIAL

THE VEMAP GRID

The grid used for the VEMAP coverage is a 0.5 degree latitude by 0.5 degree longitude grid covering the conterminous U.S. Grid edges are aligned with 1.0 degree and 0.5 degree lat/lon lines (centers at 0.25 degrees and 0.75 degrees).

All the data given are values for the central point of the grid cell. Cells with less than half their area composed of land within the borders of the conterminous US are background values (-9999) except in the lat and lon files which still contain data values for those cells.

The grid's minimum bounding rectangle (MBR) is defined by the following corners:

Lower Left Corner => -124.5 degrees lon, 25.0 degrees lat
Upper Right Corner => -67.0 degrees lon, 49.0 degrees lat

where negative longitudes are degrees West.

Gridpoint number starts at 1 in the far north west corner of the bounding rectangle and finishes at 5520 in the south east. There are 115 grid points across the domain and 48 down. Gridpoint position is shown in the diagram below.

---------------------------------------
|1   2   3   4   5   6...       ...114|
|115 116 117...                 ...229|
|230...                               |
|                                     |
|                                     |
|                              ...5520|
---------------------------------------

SCALING FACTORS

The scaling factor is the factor by which the data were multiplied before they were put into the files. To get the values back into the units described in the file header, you need to divide the values in the files by the scaling factor.

For example, latitude is given in units of degrees and hundredths of a degree and has a scaling factor of 100. This means that a value in the file of 4875 is actually 48.75 degrees, which when converted to degrees and minutes, is 48 degrees 45 minutes.

FILES IN THE DIRECTORY

AREA

Absolute area of a grid cell [km**2].
Scaling factor: 1

AREAP

Percent of the area within a grid cell that is covered by land within the VEMAP domain (the conterminous US) [%].
Scaling factor: 1

VAREA

Absolute area of a grid cell within the VEMAP domain [km**2]. This is determined as (area) x (areap).
Scaling factor: 1

ELEV

The elevation of the center of the grid cell [m].
Scaling factor: 1

LAT

The latitude at the center of the grid cell [degrees and hundredths of a degree].
Scaling factor: 100

LON

The longitude at the center of the grid cell [degrees and hundredths of a degree]. Negative longitudes are degrees West.
Scaling factor: -100

VVEG

VEMAP classifications aggregated from the TEM (Kicklighter and McGuire) Küchler (1964,1975) potential natural vegetation map. For a description of the VEMAP vegetation classifications click here.

There are two versions of this file, vveg.v1 and vveg.v2:

vveg.v1

- The vegetation file used for all standard VEMAP simulations.

vveg.v2

- Contains slight variations in vegetation classification and was used for optional on-the-side VEMAP simulations. Classification codes are the same but some cell allocations differ.

FILES AVAILABLE

To copy the files using Mosaic you need to select the option 'Load to local disk' and then click on the file you wish to transfer. The files are all 33.6 kbytes except the README which is 3.7 kbytes, and SVF_FORMAT which is 6.4 kbytes.

README.GEO
SVF_FORMAT
area
areap
elev
lat
lon
varea
vveg.v1
vveg.v2

All these files are also contained in one tarred and compressed file which is available from here.