GPS for Dummies.pdf - Engineering Surveyor

62Part II: All About GPSElectronic compassAll GPS receivers can tell you which direction you’re heading — that is, aslong as you’re moving. The minute you stop, the receiver stops acting as acompass. To address this limitation, some GPS receivers incorporate an electroniccompass that doesn’t rely on the GPS satellites.OperationLike with an old-fashioned compass, you can stand still and see which directionyour GPS receiver is pointing toward. The only difference is that you seea digital display onscreen instead of a floating needle.On some GPS receivers, you need to hold the unit flat and level for the compassto work correctly. Other models have a three-axis compass that allowsthe receiver to be tilted.Paying attention to these factors can improve the performance and convenienceof an electronic compass: Magnetic fields: Metal objects, cars, and other electronic devices reducethe accuracy of any electronic or magnetic compass. Battery life: Using an electronic compass can impact battery life. SomeGPS receivers have settings that turn off the compass or only use itwhen the receiver can’t determine a direction from satellite data.CalibrationElectronic compasses need to be calibrated whenever you change batteries.If your GPS unit has an electronic compass, follow your user guide’s instructionsto calibrate it. Usually, this requires being outside, holding the GPS unitflat and level, and slowly turning in a circle twice.AltimeterThe elevation or altitude calculated by a GPS receiver from satellite data isn’tvery accurate. Because of this, some GPS units have altimeters, which providethe elevation, ascent/descent rates, change in elevation over distance or time,and the change of barometric pressure over time. (The rough-and-ready ruleis that if barometric pressure is falling, bad weather is on the way; if it’s rising,clear weather is coming.) Calibrated and used correctly, barometric altimeterscan be accurate within 10 feet of the actual elevation. Knowing your altitudeis useful if you have something to reference it to, such as a topographic map.Altimeters are useful for hiking or in the mountains.