GPS for Dummies.pdf - Engineering Surveyor

Chapter 2: Dissecting Maps29comfortable using. For example, USGS topographic maps have latitude andlongitude, Universal Transverse Mercator (UTM), and township and rangemarks.Most coordinate systems are based on x and y; where x is a horizontal value,and y is a vertical value. A location’s coordinates are expressed by drawing astraight line down to x and across to y. Mathematician René Descartes devisedthis system in the 1600s.Letter-and-number coordinate systems are fine for highway maps, road atlases,and other simple maps where precise locations aren’t needed. However, if youwant to focus on a precise location on a map, you need a more sophisticatedgrid system. That’s where coordinate systems such as latitude and longitudeand UTM come in.When you’re figuring out a location’s coordinates on a paper map, you have afair amount of work to do, aligning the location with primary tick marks andthen adding and subtracting to get the exact coordinate. With digital maps ona computer, that’s usually just a matter of moving the cursor over a locationand watching with relief as the coordinates automatically appear. If you’reusing a paper map, you can make life easier with free overlay grids and rulersfrom www.maptools.com. With these, you can print grids and rulers for differentcoordinate systems on clear transparency sheets.Latitude/longitudeLatitude and longitude is the oldest map-coordinate system for plotting locationson the earth. The Roman scholar Ptolemy devised it almost 2,000 yearsago. Ptolemy wrote about the difficulties of accurately representing the earthon a flat piece of paper and created latitude and longitude as a way of solvingthe problem. That’s pretty impressive for a time way before computers andsatellites.Latitude and longitude are based on a little math, but they’re not really complicated.Angles are measured in degrees, and they’re used for measuring circlesand spheres. Spheres can be divided into 360 degrees; because the earthis basically a sphere, it can also be measured in degrees. This is the basis oflatitude and longitude, which use imaginary degree lines to divide the surfaceof the earth (see Figure 2-5).The equator is an imaginary circle around the earth; the circles are an equaldistance from the north and south poles and perpendicular to the earth’saxis of rotation. The equator divides the earth into the Northern Hemisphere(everything north of the equator) and the Southern Hemisphere (everythingsouth of the equator).