WORKING WITH COORDINATES AND UNITS

(...continued)

UTM

Let's suppose we took and orange and cut the peeling vertically around the orange in several places and then peeled it off and laid the pieces side by side. Then we took a hammer and flattened the whole thing out. We might have something that looked a little like the image below:

This image is similar to the projection system we get to create the UTM grid system. The UTM, Universal Tansverse Mercator, system does a similar thing with the earth by slicing vertically every 6 degrees thus creating 60 such slices around the world. All the slices touch at the equator and get further apart as we get closer to the poles. Another interesting thing to notice is that the longitude lines at the center of each slice are straight while at the edges of each slice they are curved. The amount of curvature increases as we get closer to the edge. A vertical line drawn near the edge would not point directly north.

Each slice, called a zone in UTM, is given a number from 00 to 59 starting at the international date line and progressing east. While not necessary to the measuring system UTM also divides each zone horizontally as well. These divisions start at the equator and are 8 degrees wide. The first half of the alphabet is used for the southern hemisphere while the second half is used for the northern. Thus a point just above the equator would be the letter N proceeding to the letter X while just below it would be an M and proceeding backwards to the letter C. In addition to the letter assignment UTM also measures the distance from the equator in meters along the central meridian (which is the only straight meridian line in the zone). Since every zone is measured directly from the equator the letter is not necessary to define the location except in some special cases described below. East-west measurements within a zone are always referenced to the central meridian.

The full UTM grid system is now defined. There is one latitude grid line, the equator, and 60 longitude lines in the centers of each of the zones. Further each zone is 3 degrees wide on each side of its central meridian. At this point we quite worrying about the angular measurement system and instead just measure direct distances in meters from these lines. Since measuring from a line implies positive and negative values the designers have tried to simplify this problem by defining the central meridian to arbitrarily always be 500,000 meters. This is called a false easting. This means that distances to the left of the central meridian will be subtracted from 500,000 and those to the right will be added to 500,000. In this way all measurements will always be a positive number of 6 digits. North/South distances are measured from the equator directly and can get as large as 7 digits. For this reason we usually add a leading 0 to the east/west distances to make them 7 digits long also. To avoid negative distances in the North/South direction in the southern hemisphere always add 10,000,000 meters to this measured negative distance.

Since each central meridian is always 500,000 meters there needs to be a way of designating which central meridian is being talked about. This is accomplished with the zone prefix (a number between 00 and 59) thus a full UTM specification consists of a two digit zone number, the zone letter and 14 digits of measurement data to measure down to the level of one meter anywhere on earth. The first half of the measurement data is the east/west number while the last half is the north/south component. If you don't need the full precision of 1 meter you can leave off pairs of digits. You could use 12 digits for 10 meter, 10 digits for 100 meter accuracy, etc. Sometimes you will see an odd number of digits by leaving off the leading 0 in the east/west number.

Since there are so many numbers there needs to be some method of separating them out. In standard usage a comma or a point is used to separate the numbers into groupings. Map makers have chosen to vary the font size. For example instead of 0392000E and 3382000N you would see something like 0392000E and 3382000N. This is particularly important alignment information if you leave off some of the digits.

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