GPS units are handy for all sorts of outdoor activities.
For decades topographic maps have been the gold standard in location information for hiking and other outdoor sports. In the past decade, however, hand-held GPS (Global Positioning System) units have become far more widely available and have replaced paper maps for many users.
In an area without roads or other landmarks, however, finding specific points on the ground with GPS may require calculating coordinates from a paper map. Once you know the a map point's coordinates, finding that same point with a GPS becomes a matter of comparing the GPS coordinates of your current position with those of the target.
Instructions
1. Accurate location of the target point is essential.
Locate the target point on the topographic map.
2. Locate the latitude-longitude coordinates of the map, which are printed on the corners (example: 95°15'). Latitude is printed even with top and bottom edges of a map and longitude is printed centered on left and right edges.
3. Locate latitude-longitude ticks on the map edges. These are short lines projecting outward from the map border. Ticks are labeled with latitude (left and right edges) or longitude (top and bottom edges). The degrees value is commonly omitted, so only minutes and seconds values are used (example: 12'30'').
4. Locate internal latitude-longitude ticks, which are small crosses, within the body of the map. These mark points where latitude lines drawn between tick marks of equal latitude cross longitude lines. Make a box around the target point by connecting the four closest tick marks.
5. Determine box width in seconds of longitude. A degree of longitude has 60 minutes, and each minute has 60 seconds. Example: one edge of the box is at 94°12'30'' and the other is 94°15' so the box is 150 seconds wide. It is also 150 seconds of latitude tall.
6. Measure the width of the box with a ruler and record. Measure the distance from one edge of the box to the target point and record. Divide the second number by the first: the answer must be less than one. Example: box is five inches wide and target is 1.5 inches from edge; the answer is 0.30
7. Multiply the answer times the box's width in seconds. Convert the product to minutes and seconds to obtain longitude difference in seconds--e.g., 150 seconds times 0.30 = 45 seconds. If you measured from the right edge, add 45 seconds to the longitude of the right edge. If measurement was from the left edge, subtract 45 seconds from the longitude of the left edge. Record the point's longitude.
8. Repeat the process to obtain the point's latitude. If you measure from the bottom to the target, add the latitude difference to the latitude of the bottom edge; if measured from the top, subtract. Record the longitude.
9. Convert the minutes and seconds of each number to decimals. Divide the number of seconds by 3600 to do this. Example: for 95°5'17''divide 317 (5 minutes times 60 seconds plus 17 seconds) by 3600. The decimal equivalent is 95.088055° (record at least five decimal places).
10. Enter latitude and longitude of target location in GPS unit.
Tags: target point, minutes seconds, bottom edges, edges longitude, from edge, latitude-longitude ticks, left edge
