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GPS Accuracy

GPS Accuracy - On It or Close Enough?

Some people do have concerns over the accuracy of a hand-held GPS units. With WAAS enable GPS units, accuracy can be down to 7 feet, so it says. GPS units which also support averaging (or pinning) seem to make better measurements. GPS units that support an external antenna make the best measurements when the external antenna is used.

It all depends on what you are using the GPS to locate, a large structure like a house, a tree, or an underground items like a shut-off valve. 

  • House - Finding a home based on the Lat-Long associated to a street address may only get you to the beginning of a road, or to the driveway, or to the front door of the house. If you are with a 100 feet or so, you should be able to see it.  A Microsoft USB GPS receiver should get you close enough.

  • Tree - Even though tree can be very large, they may only be spaced 10 to 30 feet apart.  Most GPS units do not even zoom in enough so you can tell the difference between trees. An Over-Zoom feature does help when trees are spaced close together. With direction and distance to target, you know which way to look and can better determine which tree it is.  But, get close and just look up.  A good hand held GPS will provide good enough results.

  • Underground Items - Now even 7 feet may not be close enough, other markers or signs on the ground may help, but you still may need a detector of some sort.  Best to have a good GPS with a good external antenna.


GPS Accuracy - Test Using a Microsoft USB and Garmin eTrex Handheld

Measurements were made for one hour with the two units side by side.  The orange dots are from the Microsoft USB GPS receiver and the green lines are from the Garmin eTrex.  Both were connected to similar computer via USB ports running the same program acquiring data via standard NMEA messages.

The Microsoft USB GPS had a spread of about 90 feet while the Garmin eTrex some 7 feet.  The X_PD point was located using Google Earth and varied based on the image being used.  The Garmin eTrex drifted between 7 to 12 feet from the point defined using Google Earth.