UTM stands for Universal Transverse Mercator. It is a map projection system developed (or adopted) by NATO on 1947. It was originally based on the Clarke 1866 ellipsoid model of the earth for the conterminous United States and the "international ellipsoid" elsewhere. (Can anyone smell a committee at work here?) Now it uses the WGS84 ellipsoid everywhere. Hang on regarding this ellipsoid business, there are a whole flock of ellipsoids that can be used to describe the shape of the earth and the issues involving them is worth a whole discussion of its own. Ellipsoids are often also refered to as "datums" and another datum that pops up fairly often is NAD83. Differences between NAD83 and WGS84 and GRS80 are probably unimportant for what I want to do.
To produce a Mercator Projection what you do is to wrap a cylinder around the earth, but with the axis of the cylinder in the plane of the equator. If the earth was a sphere, this would put the cylinder in contact with the earth all along some chosen meridian (which is the point of the whole thing). As long as you wanted a map near that meridian, you would be in good shape anywhere from the equator to the poles. This projection, as it stands is not conformal, so what is done is to apply some non-linear scaling to both eastings and northings to produce a conformal map.
What UTM does is to switch meridians every now and then. In particular, the earth gets divided into 60 zones, each 6 degrees wide. Zone 1 covers 180 to 174 West, and is centered on the 177 West meridian. Zones then count to the east from there. Northwest California is in zone 10. Most of Arizona is in zone 12 (between 108 and 114 West Longitude). Maine is entirely in zone 19.
A scale factor of 0.9996 is applied to hold overall distortion to less than 1 part in 1000 over the zone. This yeilds two lines of true scale 180 km on either side of the central meridian.
Each longitude zone is then divided into 20 latitude zones, each 8 degrees tall. The "C" zone starts at 80 south latitude, and then zones count through the english alphabet, skipping "I" and "O" to avoid confusion with the digits "0" and "1". Zones A, B, Y, and Z are special and used for the antarctic (A and B) and arctic (Y and Z) regions. Note that "N" is the first zone in the northern hemisphere. There are some weird exceptions near Norway.
Toronto, Canada is in zone 17T.
The CN Tower is at 79:23:13.7 West longitude, 43:38:33.24 North Latitude.
This is grid position 630,084 meters east, 4,833,438 meters north.
UTM coordinates are in meters! The central meridian of each zone is given a false easting of 500,000 meters to avoid any negative eastings. At the equator, eastings range from 167,000 to 833,000; the range narrows to the north in each zone. In the northern hemisphere, northings range from 0 at the equator to around 9,328,000 meters at the latitude of 84 North. In the southern hemisphere, the equator is given a false northing of 10,000,000 and northings decrease from there to the south.
Uncle Tom's Computer Info / tom@mmto.org