How Good is your GPS at Measuring Trail Distances?

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very so often we get an email or a comment on the accuracy of our trail distances, and when we track down the cause of the discrepancy find that the problem relates to incorrect odometer (distance) displays on the users GPS receiver and/or poor track logs that have not been “cleaned up”. Let’s look at these two different sources of errors.

Snack stop

Our snack stop resulted in the eTrex adding 0.14 km to the track log.

Inaccurate odometer (distance displays)
The odometer display is usually the running total of the distance between individual track points. While  our eTrex does this very poorly and we can never trust the odometer reading, the iPhone 6s using the GPS Kit app gives a reading very close to the raw track log distance calculated by MacGPS Pro software. The difference, I believe, is due to how each unit decides if you are moving or standing still. The iPhone has an accelerometer, and knows for sure that you are not moving. The eTrex does not know for sure and only adds points to the running total if you are moving faster than about 1 km/h. So the more you dawdle or slow down on very steep hills, the more inaccurate the odometer becomes. Also, the eTrex continues to record track points when you are not moving which adds to the raw track log distance. See also an earlier post.

Snack stop compare

When we stopped for a snack the eTrex recorded multiple track points. The iPhone (orange) only recorded our move off the trail. Note the errant track point bottom centre.

Track log inaccuracies
There are several factors that affect the accuracy of an individual track point over which we have no control. They include a delay as the signal passes through the atmosphere, the reflection of the signal off rock faces or in heavy tree cover, receiver clock errors, and orbital errors. Factors where we do have some control are the number of satellites that are visible—the more satellites a receiver can see, the better the accuracy, and the geometry of the satellites—the more spread out the better. Accuracy is much worse in a narrow valley than on a ridge top.

These inaccuracies often result in errant track points many metres off your line of travel. If you stop for breaks on the trail some receivers continue to add points (again some of them out by many metres). This can result in a raw track log that is much longer than the actual trail distance. On one short loop, where we had stopped for lunch, the eTrex raw track was 4.61 km and the odometer reading 4.11. A difference of 0.5 km! By comparison, the iPhone readings were both 4.59 km.  The average cleaned-up track distance was 4.42.

In order to record an accurate distance:

  • Don’t believe a Garmin odometer reading.
  • Use a receiver capable of receiving signals from both the US GPS and the Russian GLONASS satellites. Both our eTrex and iPhone do so.
  • Hike at a good pace and clean up your track logs using software such as  GPX Editor on a PC or MacGPS Pro.
  • Mountain bikers should set their unit to record more frequent track points if there are lots of S-bends.
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