Universal Transverse Mercator, or UTM, is a map projection used by cartographers to display Earth's curved surface on a sheet of paper. Any point on a map can be located from its X and Y UTM coordinates. The X-coordinate describes how far the point lies east or west of a specified longitude line, and a Y-coordinate describes how far the point lies from the equator. A map point can be precisely located on the ground using a GPS unit once that point's X-Y coordinates are known. The coordinates can be calculated using the UTM grid printed on a topographic map.
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Things you need
- Topographic map
- GPS unit
Locate the UTM grid on the topographic map. The UTM grid is printed in lines that form squares on the map face. Each square is 1,000 meters (1 kilometre) on a side. Each line is labelled where it reaches the edge of a map, usually with one or two small numbers followed by two large numbers. At least one label on each edge also has three zeroes followed by a small "m" and the letter "N," "S," "E," or "W."
Locate the point of interest on the map.
Find the first north-south (top to bottom) UTM grid line right of the target point and track it the map's edge. Record the number found there: if the number does not end in three zeroes and a letter, look right or left to find a number that does (example: 309000mE) and note the letter E or W.
Find the first east-west (left to right) grid line below the target point and track it to an edge. Record the number found there. If the number does not end in three zeroes and a letter, look up or down to find one that does (example: 4437000mN).
Use a ruler to measure the height and width of the grid square that surrounds the target point. If it is near the edge and not within a complete square, measure an adjacent square. Record both height and width.
Measure the distance from the target point to the north-south line located above. Divide this distance by the width of the square and multiply by 1,000 (example: 201/334 = 0.602, times 1000 = 602). This is the distance from the north-south line to the point in meters. Call this number A.
Measure the distance from the target point to the east-west line located above. Divide this number by the height of the square and multiply by 1000. This is the distance from the point to the east-west line in meters. Call this number B.
Calculate the UTM X (east-west) coordinate. If the north-south grid line is labelled with an E (309000mE) , subtract the number A from the line's labelled value (example: 309000-602 = 308398mE). If the line is labelled with a W (309000mW), add A to the number (example: 309000+602 = 309602mW).
Calculate the UTM Y (north-south) coordinate. Add B to the number (example: 4431000+602 = 4431602mN).
Tips and warnings
- When loading UTM coordinates to a GPS or mapping software, you must also provide the UTM zone. On USGS topographic maps, this information is printed near the lower left-hand corner of the map. For greater precision, also provide the datum, e.g., NAD 1983.
- UTM X-coordinates (east-west) are always six digits. UTM Y-coordinates are usually seven digits but may be six digits at low latitudes.
- Different vintage USGS maps may show a full UTM grid or only ticks (crosses) where the grid lines cross.
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