GPS Data Logging

Most recreational GPS receivers can store the locations that interest you as "waypoints", and many can record your "track" - the path you take to get there.

Both are shown here on a gpsMap. The named boxes are waypoints, and the dashed line is a GPS track, The direction of travel is toward the colored end of the dashes.

How you record waypoints and tracks depends on the receiver, so study the manual, and practice a little before launching your first mapping project.

Clover Fern

Acquiring Satellites

When first turned on, a GPS receiver starts listening for satellite signals. On the "satellite display" here, the numbered satellites appear somewhere in the sky. The middle of the diagram is directly overhead, and the smaller circle is halfway up (45 degrees). The horizon is the large circle with four compass points: N, S, E, W. Signal strength is also shown for each satellite.

Your receiver needs a 30-second message from each satellite to use it for navigation. During this time, the signal strength bar is hollow, and the satellite is not usable. Once a complete message is received, the signal bar turns solid and the satellite is "locked". Your receiver may remember a locked satellite for several hours, even while turned off, making acquisition quicker the next time.

More satellites means less error. For an accurate GPS location, wait for a lock on most or all of the available satellites. Always try to maximize your coverage, even beyond the four satellites needed for navigation.

Even with a good view of the sky, acquisition can sometimes be difficult and slow. GPS signals shift frequency a lot - predictably, in a programmed sequence, and unpredictably, from speed and temperature variations.

Searching for all 24 satellites could take 10 minutes or more. Instead, your receiver stores an "almanac" of satellite locations and listens for overhead ones first. If the receiver has moved a long way, those satellites aren't there. You can then either suggest a location, or "auto-locate", which may take several minutes.

Satellite Coverage

Trees are often found outside, as are mountains, canyons, tunnels, and buildings. All of these can block satellite signals, making it difficult to keep the four needed for navigation. You may see "poor coverage" when your receiver can no longer navigate.

If you're standing still, try adjusting your position or the aim of the antenna. In a forest, you can catch satellites between the trees. Orient the satellite display with a compass, then move perpendicular to a missing satellite - about a foot at a time. GPS satellites are always moving slowly across the sky, so a coverage problem can clear up in a few minutes.

If you're moving, and an obstruction is brief, a good GPS receiver should have no problem keeping track of where you are. Rarely, loss or re-acquisition of satellites can cause position errors of several hundred miles.

Clover Blue

Marking Waypoints

Before marking your favorite locations as GPS "waypoints", make sure the receiver has a good "3D" navigation lock. For permanent marks, pick a name you'll remember. Temporary marks can usually be numbered automatically. To restart waypoint numbering, erase any numbered waypoints and save a new one called "001".

To mark a place you can't go, like a freeway interchange, a cave, or building where satellites are blocked, start by marking a point nearby, then offset it by distance and bearing. Just another reason it's handy - and safer - to carry a compass when you venture out with GPS.


Average Position

Some receivers allow you to accumulate an average position over several minutes or hours. Place the receiver with a good view of the sky, start the average, then take a break. When you're ready to move on, save the result as a waypoint.

Averaging can drastically reduce errors from Selective Availability (SA) and other sources, depending on how long you wait. For good accuracy (about 40 meters), wait at least 30 minutes. Averaging for 10 hours can put you close to the receiver's native accuracy (usually about 15 meters), significantly reducing most errors.

If your receiver doesn't average, just mark a point every 5 minutes or so, then average them mathematically, or on the map screen.

For points that have to be correct, you can record a "static track" while standing still, and we'll average it into a waypoint. You can also describe the point, or draw a map of it. The GPS Accuracy section covers other ways to reduce error, including Differential GPS (DGPS) corrections.

NEXT: Trail Mapping, Tracks, Tree Cover

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