GPS for Dummies.pdf - Engineering Surveyor

28Part I: All About Digital MapsYou only need to be concerned with datums under a few circumstances, suchas these: A location is plotted on two different maps. A map and a Global Positioning System (GPS) receiver are being used. Two different GPS receivers are being used.In these instances, all the maps and GPS receivers must use the same datum.If the datums are different, the location ends up in two different physical placeseven though the map coordinates are exactly the same.This is a common mistake: GPS receivers use the WGS 84 datum by default,and USGS topographic maps use the NAD 27 datum. If you mix the datums,your location can be off by up to 200 meters (roughly 200 yards, if you’re metricallychallenged).Utilities can convert coordinates from one datum to another (some aredescribed in Chapter 11) but it’s easier just to get all the datums on thesame map.Working with Map Coordinate SystemsA coordinate system is a way to locate places on a map, usually some type ofgrid laid over the map. Grid systems are a whole lot easier to use and moreaccurate than “take the old dirt road by the oak tree for two miles, then turnleft at the rusted tractor, and you’ll be there when the road stops gettingbumpy.”A simple coordinate system can consist of a vertical row of letters (A, B, C)on the left side of the map and a horizontal row of numbers (1, 2, 3) at thebottom of the map. If you want to tell someone where the town of BiggsJunction is (for example), you put your finger on the city and then move it ina straight line to the left until you hit the row of letters. Then put your fingeron the city again, but this time move down until you reach the row of numbers.You now can say confidently that Biggs Junction is located at A12.I call this the Battleship Grid System because it reminds me of the game whereyou call out coordinates to find your opponent’s hidden aircraft carriers, submarines,and destroyers. “B-3. You sank my battleship!”A grid may be printed on the map or provide tick marks (representingthe grid boundaries) at the map’s margins. Often maps have multiple coordinatesystems so you can pick one that meets your needs or that you’re