GPS for Dummies.pdf - Engineering Surveyor

Chapter 4: Grasping Important GPS Concepts73Here’s a quick story from my Forest Service firefightingdays that illustrates the importance ofbeing aware of datums. A fire was reported in amountainous area of eastern Oregon, and mypartner and I helicopter-rappelled in to put outthe fire. Because the fire was bigger thanexpected, we requested some smokejumpers toassist. They were down from Alaska, helpingout during the lower 48’s fire season, and theyall had new handheld GPS receivers, whichwere pretty state of the art back then (1998). Thefire continued to grow, and we called in a smallair tanker to stop the fire’s spread. One of theDatum lessons learnedAlaska jumpers pulled out his GPS unit, and wecalled in an exact set of coordinates for the pilotto hit. As the tanker approached, the pilotradioed us, asking whether we were sure thatwas where we wanted the retardant to go.Turns out that the jumper still had his GPS unitset with an Alaska map datum (that didn’t matchthe local datum on the pilot’s GPS receiver), andthe coordinates that the Bureau of LandManagement (BLM) jumper gave were on theother side of the ridge, nowhere near the fire.Fortunately, the pilot used his own initiative anddumped his load right where it needed to go.The moral of the story is to make sure that the datums on your GPS receiverand maps are the same. Or, if you’re with a group of people using GPSreceivers, make sure that all your datums match.Not having the map datum match the GPS receiver datum is one of thebiggest errors that new users of GPS receivers make. I can’t emphasize thispoint strongly enough: Make sure that the two match!Setting WaypointsA waypoint is GPS lingo for a location or point that you store in your GPSreceiver. Some manufacturers also call them marks or landmarks. A waypointconsists of the following information: Location: The location of the waypoint in whichever coordinate systemthe GPS receiver is currently using. Some receivers also store the elevationof the location. Name: The name of the waypoint. You get to choose your own name; thelength varies between GPS receiver models from six characters on up. Date and time: The date and time the waypoint was created. Optional icon or symbol: An optional icon or symbol associated withthe waypoint that appears on the GPS receiver’s map page when thearea around the waypoint is displayed. This could be a tent for a campground,a boat for a boat launch, or a fish for a favorite fishing spot.