by Cass Lewart
What Is It Good For
A Global Positioning System (GPS) receiver, when properly used, will help to guide you to your destination, and will also take you back to where you started. Still, if you really want to be sure that you will not get lost, then in addition to a GPS, you will also need a decent compass, a map and, if you hike in mountainous terrain, an altimeter.
Bearing, Heading and Distance
A GPS, when set to guide you to a specific waypoint displays navigational information derived from signals received from satellites, the most important being Bearing - direction from the current location to a waypoint, Heading - direction in which you are traveling, both in degrees, and Distance in km or miles to a waypoint. Remember that though one of the GPS displays includes a compass rose, a GPS is not a compass. Unless you move at 10 miles per hour or faster, the Heading and speed indication given by the "compass" display on the GPS are not reliable. However, the BEARING to your next waypoint as reported by the GPS is accurate independent of speed of travel. This is the reason why you need a compass when hiking. However, some GPS units, in particular those made by Magellan, in addition to the Sun/Moon rise and set computations, superimpose on the compass rose the current position of the sun and the moon. If the sun or the moon are visible, and they are not too high above the horizon, then the Magellan type GPS will also provide you with a convenient and a fairly accurate compass display when you match the compass rose "sun/moon" with the real sun/moon..
Most GPS units let you choose between True and Magnetic North. The magnetic deviation computed by your GPS is of course an approximation based on your current position. It may vary by a few degrees from the actual magnetic deviation. However, unless you know the exact magnetic deviation for your location, use the one provided by the GPS to coordinate the GPS with the compass readings.
Marking Starting and Intermediate Locations
When traveling or hiking a GPS lets you easily enter/mark coordinates of your current location usually by pressing once or twice a certain key. Always mark your starting location so that you can find your way back after the hike and also mark intermediate points.
Hiking in the Woods
When you hike under dense foliage your GPS may have difficulty locking on to satellites. In general, once a lock is achieved, it is easier to keep it, even under heavy foliage, then to reacquire the satellites under the same conditions. Thus try to keep the GPS on while hiking in dense woods. If the satellite lock is lost try to find a clearing to reacquire them.
Traveling or Hiking to a Specific Waypoint
You can either follow a route consisting of several waypoints or you can set the GPS to guide you to a specific waypoint. In either case the GPS display will show the distance to the next waypoint and the Bearing to that waypoint. Set your compass to that Bearing and follow the compass. You will not need the compass if you travel at 10 miles/hour or more - the Heading indicator on the GPS is then reliable. As you walk or travel, the distance to the next waypoint should be getting smaller - if it does not, than you better check your equipment. Sometimes as you follow a trail, or drive on a city street, you know that to get to a specific waypoint you will have to make, e.g., a 90 degree turn. The problem is that you don't know exactly when to turn. To find out when to turn watch the Bearing to that waypoint on your GPS and watch your current Heading (on the compass if you walk, on the GPS if you drive). As you approach your turning point the Bearing should be approximately 90 degree less than the Heading for the left turn or 90 degree more for the right turn. Of course if you straddle 360 degree mark, you will have to correct for it by adding or subtracting 360. E.g., if your Heading is 10 degrees and you expect to make a 90 degree left turn, then proceed till the Bearing indicator shows 10 - 90 + 360 = 280 degrees.
Due to the Selective Availability, a fudge factor introduced by the Defense Department, signal reflections, atmospheric phenomena, number and spatial geometry of visible satellites and the inherent precision of the GPS satellite clocks, there will always be some error in determining your location, speed and altitude from GPS readings. I find my GPS to be accurate to within 0.1 miles most of the time. However, occasionally the error can go up to 0.25 (or more) miles. Take this error into account when hiking. The altitude accuracy is specified as +/- 600 feet (95% confidence) - that is why you should also carry an altimeter.
Communicating With Your GPS
You plan your trip by entering a series of waypoints. The coordinates of these waypoints can be obtained from paper or computerized maps, mapping programs or from various databases accessible via Internet. A specific street address or a landmark will return the latitude and the longitude. Mapping programs and a cable connecting your GPS to a serial port of the computer will let you transfer automatically this information to the GPS. Otherwise you have to enter the coordinates manually one by one. Make sure that, when using manual entry, the angle units are consistent, e.g., ddd.mm.sss (Deg/Min/Sec), ddd.mm.mmm (Deg/Min) or ddd.ddddd (Deg). If necessary, perform the conversion before entering data manually into the GPS (e.g., 72.530 deg = 72 deg 31 min 48 sec). Make also sure that you have the correct latitude (North or South) and the correct longitude (West or East). Connecting a GPS to a serial port on a laptop will let you trace your progress on a moving map display on the computer as you travel. The communication and handshaking between the GPS and the computer is controlled by the NMEA 0183 (National Marine Electronics Association) protocols. In general the setup menu on the GPS will give you some choices in selecting the appropriate NMEA protocol and the Baud (bits/sec) transmission rate. The choice is in general between 1,200, 4,800, 9,600 and 19,200 bits/sec. Most mapping programs recommend 4,800 bits/sec. Other serial transmission parameters on the computer should be set to 8 data bits, 1 stop bit, no parity and no flow control (or software ack/nak flow control depending on your GPS/map program settings). Make sure that both the GPS and your computer serial communication parameters are set to the same values and that the cable is connected to the COM port used by the mapping software. Most GPS give a choice of the following NMEA 0183 protocols:
Questions? Corrections? Comments?
Email to: Cass Lewart