General Considerations:Coordinate System

GSSHA performs calculations on raster grids and data can be input with GRASS ASCII raster data. While any logical, and consistent, coordinate system can be used, the preferred coordinate system for raster based data is the UTM map projection. The UTM System breaks the entire earth into zones 6 degrees of longitude wide. The zones that cover the continental United States are:

ZONE	WEST LIMIT	EAST LIMIT
10	126° W	120° W
11	120° W	114° W
12	114° W	108° W
13	108° W	102° W
14	102° W	96°W
15	96° W	90° W
16	90° W	84° W
17	84° W	78° W
18	78° W	72° W
19	72° W	66° W

The standard specification of the UTM system includes (Davis et al., 1981):

The reference ellipsoid is Clarke 1866 in North America.
The origin of longitude is the central meridian.
The origin of latitude is the equator.
The unit of measure is the meter.
A false easting of 500,000 m is used for the central meridian of each zone.
The scale factor at the central meridian is 0.9996.
The zones are numbered beginning with 1 for the zone between 180° W and 174° W meridians and increasing to 60 for the zone between meridians 174° E and 180° E.
The latitude for the system varies from 80° N to 80° S.
In the southern hemisphere, a false northing of 10,000,000 m is used.
The scale error is 1/2500 on the central meridian.

The UTM coordinate system was chosen because of its global applicability and widespread acceptance. Note that watershed data that lie in two zones must be merged into one zone. The data should be merged into the zone that contains the majority of the watershed area. This is accomplished using a GIS.