MSI PLANET Terrain Data UTM Projection
Universal Transverse Mercator (UTM) Projection
The UTMprojection divides the world into 60 zones or segments. Each zone covers 6 degrees of longitude.
The first zone starts at the international date line and the numbers increase eastwards. At the center of each
zone is a central meridian, which is the measurement reference for that zone.
East-west grid lines are attached to the central meridian at right angles and follow great circle paths away
from it. These are lines of constant northing and do not follow parallels of latitude, since parallels of latitude
are not great circles. Finally, north-south grid lines are draw as lines of constant easting.
UTM coordinates are relative to a particular zone number and are expressed as an easting or X coordinate
and a northing or Y coordinate. The easting is the distance from the central meridian along the east west
grid lines using a positive east negative west sign convention. To avoid negative numbers, a false easting
of 500,000 meters is assigned to the central meridian.
The northing is the distance from the equator along the north-south grid lines using a positive north - negative
south sign convention. To avoid negative numbers in the southern hemisphere, a false northing of
10,000,000 meters is assigned to the equator. This false northing creates an ambiguity in this coordinate
system. Consider the UTM coordinates:
zone = 12, easting = 575 kilometers, northing = 5600 kilometers.
Using the WGS84 datum, this converts to:
latitude 50 32 49.64 N and longitude 109 56 29.12 W in the northern hemisphere
latitude 39 44 47.78 S and longitude 110 07 28.56 W in the southern hemisphere
To resolve this ambiguity, the Pathloss program adds the suffix N or S to the UTM zone. e.g. 12N or 12S.
This false northing introduces an additional complication when a terrain data base spans the equator. Northings
of 20 and 9980 kilometers represent points 20 kilometers north and south of the equator respectively.
Planet Terrain Database Description
The Planet terrain database format consists of regular array of elevations. Each elevation represents the average
elevation in a square cell. The file is arranged in rows running from west to east starting at either the
SW or NW corner of the file. Normally the elevations are stored as 2 byte integers. The byte order could
be INTEL (little endian) or SPARC (big endian)
The geo-referencing information is contained in two external files: an index file and a projection file.
An example of a projection file is given below. The format will vary depending on the projection.
denotes a UTM projection in zone 18 using the WGS84 datum.