Print
A plane coordinate system that is convenient for GIS work over large areas is the Universal Transverse Mercator (UTM) system. UTM with the Universal Polar Stereographic system covers the world in one consistent system. It is four times less precise than typical State Plane Coordinate systems, with a scale factor that reaches 0.9996. A Universal Transverse Mercator zone embraces a much larger portion of the earth than does a state plane coordinate zone. When you get a larger bite, a larger portion of the earth, the scale factor is less attractive. Yet, the ease of using UTM and its worldwide coverage makes it very attractive for work that would otherwise have to cross many different SPCS zones. For example, nearly all National Geospatial-Intelligence Agency (NGA) topographic maps, U.S. Geological Survey (USGS) quad sheets, and many aeronautical charts show the UTM grid lines. It is often said that UTM is a military system created by the U.S. Army; but several nations, and the North Atlantic Treaty Organization (NATO), played roles in its creation after World War II. At that time, the goal was to design a consistent coordinate system that could promote cooperation between the military organizations of several nations. Before the introduction of UTM, allies found that their differing systems hindered the synchronization of military operations. Conferences were held on the subject from 1945 to 1951, with representatives from Belgium, Portugal, France, and Britain, and the outlines of the present UTM system were developed. By 1951, the U.S. Army introduced a system that was very similar to that currently used. The UTM projection divides the world into 60 zones that begin at longitude 180º, the International Date Line. Zone 1 is from 180° to the 174° W longitude. The conterminous United States is within UTM zones 10 to 19. Here is a convenient way to find the zone number for a particular longitude. Consider west longitude negative and east longitude positive, add 180° and divide by 6. Any answer greater than an integer is rounded to the next highest integer, and you have the zone. For example, Denver, Colorado is near 105° W. Longitude, –105°.
–105° + 180° = 75°
75 °/ 6 = 12.50<
Round up to 13
Therefore, Denver, Colorado, is in UTM Zone 13.
All UTM zones have a width of 6° of longitude. From north to south, the zones extend from 84° N latitude to 80° S latitude. Originally the northern limit was at 80° N latitude and the southern 80° S latitude. On the south, the latitude is a small circle that conveniently traverses the ocean well south of Africa, Australia, and South America. However, 80° N latitude was found to exclude parts of Russia and Greenland and was extended to 84° N latitude. You will notice letters of the alphabet along the right edge of the illustration. There are a few letters missing. You see C,D,E, F, G,H —but there is no I, because it's too close to the number 1 and could be confused— J,K, L, M, N —but there is no O, it could be confused for the number zero— P,Q, R, S,T U, V, W, X, there is no Y, — and finally there is Z. I point this out because there are GPS/GNSS systems that use these letter designations along with the number of the UTM zone to identify a particular quadrangle. For example, C21 would be the square that you see just immediately above 21 in the Southern Hemisphere. This is a useful method of referring to a particular quadrangle in a particular UTM zone. The designations are also used by the military.
Unlike any of the systems previously discussed, every coordinate in a UTM zone occurs twice, once in the Northern Hemisphere and once in the Southern Hemisphere. This is a consequence of the fact that there are two origins in each UTM zone. The origin for the portion of the zone north of the equator is moved 500 km west of the intersection of the zone’s central meridian and the equator. This arrangement ensures that all of the coordinates for that zone in the Northern Hemisphere will be positive. The origin for the coordinates in the Southern Hemisphere for the same zone is 500 km west of the central meridian, as well. But in the Southern Hemisphere, the origin is not on the equator, it is 10,000 km south of it, close to the South Pole. This orientation of the origin guarantees that all of the coordinates in the Southern Hemisphere are in the first quadrant and are positive. In other words, the intersection of each zone’s central meridian with the equator defines its origin of coordinates. In the southern hemisphere, each origin is given the coordinates:
easting = X0 = 500, 000 meters, and northing = Y0 = 10,000,000 meters
In the northern hemisphere, the values are:
easting = X0 = 500, 000 meters, and northing = Y0 = 0 meters, at the origin
The UTM zones nearly cover the earth, except the Polar Regions, which are covered by two azimuthal polar zones called the Universal Polar Stereographic (UPS) projection.
The foundation of the 60 UTM zones is a secant Transverse Mercator projection, very similar to those used in some State Plane Coordinate systems. The central meridian of the zones is exactly in the middle. For example, in Zone 1, from 180° W to the 174° W longitude, the central meridian is 177° W longitude, so each zone extends 3 degrees east and west from its central meridian. The UTM secant projection gives approximately 180 kilometers between the lines of exact scale where the cylinder intersects the ellipsoid. The scale factor grows from 0.9996 along the central meridian of a UTM zone to 1.00000 at 180 km to the east and west. Please recall that SPCS zones are usually more limited in width, ~158 miles, and, therefore, have a smaller range of scale factors than do the UTM zones. In State Plane coordinates, the scale factor is usually no more than 1 part in 10,000. In UTM coordinates, it can be as large as 1 part in 2500. The reference ellipsoids for UTM coordinates vary.