GPS for Dummies.pdf - Engineering Surveyor

24Part I: All About Digital MapsMost topographic maps show magnetic declination. Compass needles point tomagnetic north, but most maps are oriented to true north. Because the earth’smagnetic field varies from place to place, magnetic north usually isn’t the sameas true north; in the continental United States, the difference can be as muchas 20 degrees. If you don’t account for the magnetic declination, you can getfar off-course trying to navigate someplace with a compass. The declinationtells you how many degrees you need to adjust your compass: If the declinationis west, you subtract the degrees from 360 to get true north; if it’s east,you add the degrees.Magnetic declination changes over time, and older USGS maps can have incorrectdeclination information printed on them. Using the wrong declinationcan cause all sorts of navigation problems, so check the current declinationfor your area at the following Web site: www.ngdc.noaa.gov/cgi-bin/seg/gmag/declination1.pl.If your job or hobby takes you off the beaten path, you definitely need a topographicmap. If you’re staying in your car, driving on paved roads, you probablydon’t need a topographic map.Planimetric mapsPlanimetric maps don’t provide much information about the terrain. Lakes,rivers, and mountain pass elevations may be shown, but there isn’t anydetailed land information. A classic example of a planimetric map is a statehighway map or a road atlas. Planimetric maps are perfect in cities or on highways,but they’re not suited for backcountry use. Figure 2-2 is a planimetricmap of The Dalles, Oregon, area.Figure 2-2:A planimetricmaplacks terrainfeatures andcontours.