Map, Compass & GPS

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

Monday, May 23, 2016

GPS Accuracy

Are you comfortable with the accuracy of your GPS receiver?  
The package says that your Global Positioning System (GPS) receiver is accurate to +/- 15 meters and some advertise +/- 3 meters.  Just what does that mean to you?
Accuracy depends on several things, most of which are beyond your control.  For example, it is reasonable to expect a new GPS with the latest antenna, circuitry, processor capability and memory technology will perform better than one made in 2005.  The number of satellites signals a receiver acquires helps too; you’ll need at least four.
The graphic below tells an interesting story.  Through the center of the topographic map, marked with dashed lines is the Pacific Crest Trail (PCT) in Oregon’s Cascades mountain range.  Next to the trail is my track log (in red) downloaded from my GPS to my Terrain Navigator software.  The track log is my electronic path calculated by the receiver.
I walked on the PCT the entire time.

  My GPS receiver was in a holster attached to the shoulder straps of my backpack.  The receiver’s antenna was exposed but only received data from my front and straight up, my chest blocked signals from behind my back.

As the green of the map indicates I was in a forested area. Tree canopy was moderately thick and may have interfered with signal reception.

Further, I was on the move the entire time, stopping only occasionally.
Obviously, there is a distinct difference between the map and the track log. 

To improve the accuracy of my track information I could do three things.  First, I would have removed the receiver from the holster.  Second, I could have moved into an area clear of forest canopy. Third, I would give the GPS time to develop good satellite tracking information.

I have found that with older receivers moving out from underneath canopy and giving the unit time to calculate position data is extremely important.  A new Garmin 62 might have accurate position information in less than 20 seconds while an older Garmin Map60CS might take a minute.

Many people tend to think that a GPS receiver pinpoints their position exactly where they are standing all the time.  A hiker’s position is within the diameter of the specifications of the model.  If a receiver is accurate to +/- 15 meters (a radius) the unit’s calculated position will be somewhere inside the 30 meter diameter of a circle.  The accuracy could improve. 
In my GPS classes I recommend to my students to consider that they are traveling down a lane in the backcountry.  The size and width of the lane might grow or shrink depending on the number of satellites received and if terrain is blocking signals.

Test your receiver’s accuracy at home.

Find a quiet street or side walk to view how position accuracy changes.  Ideally it will be oriented true north and south.  Using your GPS receiver and compass, walk true north and look at your latitude and longitude.  Because lines of longitude are also oriented true north and south that data shouldn’t change.  Walk about 100 yards; again going true north.  Observe how much the longitude coordinates change.  Maneuver to stay on the original longitude and see how far off you’ll have to wander from your planned track.

Read more about GPS receivers. and their accuracy.accuracyl

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