Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Saturday, July 16, 2011

Understanding GPS Accuracy

When you hear that your GPS is accurate to 15 meters, just what does that mean to you in the backcountry?

This is a frequent topic in my GPS land navigation class.

Today, a GPS receiver is accurate to the design specifications of the model.  Generally, a GPS is accurate to plus or minus 15 meters (a circle with a15 meter radius or 30 meters in diameter.)  Many models offer more accuracy through the technology of the Wide Area Augmentation System (WAAS); 3 meters is possible.  (Note that to take advantage of WAAS that option must be "enabled" in the setup function of the main menu.)

There are several factors that contribute to the accuracy of the signal data received and geographic coordinates displayed.  Some of these factors include:

  • Number of satellites signals being received
  • Multi-path interference (signals reflected off a surface yet being received)
  • The age of the unit and antenna model/type
  • Receiver sensitivity
  • Atmospherics (such as solar flares and sun spot activity)
To counter the factors listed above the wilderness traveler has only a few options and these include: 
  • Buying a new unit
  • Relocate to an area with a clear sky view (get away from trees)
  • Give the receiver more time to capture and process the satellite signal
Giving a receiver the time to capture and process data received is especially true with the older models.  For example,  I have students attending my classes bringing in the older Garmin 12, Garmin Etrek Summit, Magellan 315 and Magellan Sport Track.  These are old but  functional, usable models that will definitely get one back to the truck or trail head.  My recommendation is to give the receiver the time to do the job; this could be five minutes.  (If it takes longer than 5 minutes you may have a problem with the receiver.)  Remember, with the older models, if your travel over 50 miles from the location where the unit was last used, it will take more than 5 minutes to "re-initialize;" check the owner's manual.

No matter what the age of the GPS receiver is, my recommendation is to consider that the traveler is moving down a lane in the backcountry.  See below:

In the example above, the hiker will be somewhere in the lane traveling from "start" to "elk camp."  At one moment in time the hiker may very well be in the center of the lane but a short time later his actual position will shift ahead, to the side and so on.

I understand that this may not be as "spot on" as some would like but overall the GPS will provide adequate information to reach the destination.

Depending on the model, I've found GPS compass information to be variable.  Rather than use the GPS compass/electronic compass information exclusively I'll use my magnetic compass too. 

First, I will take the bearing info located in the data field first (see diagram to the right.)

In the display to the right, the destination is at a bearing of 298T (T for degrees true.)  I take that information and apply the bearing information on my declination adjusted Silva Ranger compass.  I'll adjust and sight the compass to the new bearing.  Next, I'll take a few steps keeping an eye on the GPS (older models and those with out an electronic compass need movement and distance to level out and provide accurate information.)  I'll then compare the sighted bearing with that of the GPS receiver's compass. 

Don't rush navigation.  Navigation deserves the time, patience and attention to detail as does any other skill of the backcountry. 

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