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Blake Miller: Understanding GPS Accuracy

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How accurate is your GPS? Really?

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

by Blake Miller

This is a frequent question 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 a 15 meter radius or 30 meters in diameter.)

Many models offer more accuracy through the technology of the Wide Area Augmentation System (WAAS); three meters is possible.  (Note: 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.  These include:

  • Number of satellite signals being received
  • Multi-path interference (signals reflected off a surface, yet still 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 five 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.

A GPS provides a lane of accuracy.

In the example to the left, 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.

GPS compass

First, I will take the bearing info located in the data field first.
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 without 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.

Blake Miller has made a career out of staying found and knowing where he is at all times. His formal navigation training began when he joined the U.S. Navy in 1973. He served as an officer aboard several Navy ships over his

Blake Miller

twenty-year career; many of those tours included the duty of Navigator. Blake began working with satellite navigation systems at sea in 1976, culminating with the then-new satellite positioning systems aboard the Battleship WISCONSIN in early 1990.

In 1998 Blake started Outdoor Quest, a business dedicated to backcountry navigation and wilderness survival. Blake has taught classes to wild land firefighters, state agency staffs, Search and Rescue team members, hunters, hikers, skiers, fishermen and equestrians. He regularly teaches classes through the Community Education programs at Central Oregon (Bend) and Chemeketa (Salem, OR) Community Colleges.

As a volunteer, Blake teaches navigation and survival classes to students in the local

school districts, and conservation groups. He is a member of a Search and Rescue team.

 

If you have any questions about land navigation or wilderness survival, you can contact Blake through SurvivalCommonSense.com@gmail.com, or you can go to his website.

Contact Information:

Website: www.outdoorquest.biz

Blog: outdoorquest.blogspot.com

Phone: 541-280-0573

Email: outdrquest@aol.com

To hear the Blake Miller interview about choosing a magnetic compass and GPS on SurvivalCommonSense.com Radio, click here.

 

 

 

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