JARS v49n2 - The Rapidly Changing World Of GPS

The Rapidly Changing World Of GPS
Jerry Broadus
Puyallup, Washington

GPS technology is constantly becoming more accessible to general users. GPS receivers are potentially capable of determining positions much more precisely than 100 meters. The 100 meter limitation that we had to work with in China is imposed by the Department of Defense as a security measure. Services are, however, coming on the civilian market to provide precise positioning by broadcasting information from receivers set up at local radio stations. The user buys a small radio receiver-computer that resembles a pager, which plugs into the GPS receiver. By subscribing to such a service, you can get up to one meter accuracy from some GPS units. Such services are presently available all across the United States and Canada and in many foreign countries. As of this writing, negotiations are underway to provide the service in the People's Republic of China. Eventually, it may be possible to use a GPS receiver so accurately that, even in the most remote regions, one can pinpoint the locations of individual plants in the wild.

Recommended Procedures for Using GPS Receivers
1.  Familiarize yourself with the features of the particular receiver you have selected. Check the options to see that it is using World Geodetic System 1984 (WGS84) datum, as this is the only latitude and longitude system with worldwide application. Local datum may be used if you identify it as the basis for your readings to avoid confusion.
2.  Take as many readings as your battery supply will allow. When you go into an area, take a fix on your way in and again on the way out, so that you have a check. If you are not retracing your steps, take repeat readings at campsites, and take one while you eat your lunch.
3.  Even with a three dimensional fix, your GPS elevation readings are not as accurate as a properly used altimeter. Calibrate your altimeter in the field several times during the trip by plotting your position on a base map and setting your instrument to match the contour lines. Recalibrate after sudden changes in weather.
4.  Ask your local guides for names of villages, passes and rivers. This information will be invaluable when you try to plot your readings on another map.
5.  Keep sketches of each area and orient yourself to landmarks that you can see. Record the direction of river drainages and take compass readings to nearby peaks. Take photographs of these same features from the sites, and keep notes for each exposure. ARS members working in the United States, Canada and Scotland are encouraged to note numbers of highways, and local landmarks such as taverns with good beer.

Jerry Broadus, a member of the Tacoma Chapter, is a licensed land surveyor and vice president of Geometrix Surveying, Inc., of Puyallup, Wash.