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How does the Garmin GPS62s calculate distance and why is it wrong?

I own both a Garmin GPS62s and a MemoryMap TX4. I regularly use both on my Sunday Ramblers' walks which are typically about 12 to 14 miles. The Garmin consistently reports a longer walk, probably about 5%? It is the norm on these walks that we have a couple of "comfort stops", a coffee stop and a lunch stop near a Pub where we use the Loo . . . and perhaps have a drink!

I also recently left both devices on a table in the garden for about eight hours. The Garmin created a large .GPX file detailing movement, the MemoryMap created a very small file. The MemoryMap is based on an industrial Smartphone (Defender from Toughphone) and may include an accelerometer? On the other hand, it may be slightly less sensitive and to ignore minor "movements"?

I also once used both devices on a 300 mile car journey from the Lake District to London and the reported distance covered was almost identical on both devices and pretty much agreed with the car odometer and TomTom SatNav.

Incidentally, the Ascent/Descent figures reported by the MemoryMap are quite ridiculous - the Garmin is probably not far off given changes in barometric pressure as one goes up a mountain and during the day - I often check and recalibrate at Trig Points.

My belief is that the MemoryMap is more accurate on distance and the Garmin is reasonably accurate on Ascent/Descent.

On a recent local walk,
the Garmin reported:
Points = 1,178
Distance = 21.3 km
Ascent = 239m
Descent = 236m

the MemoryMap reported:
Points = 2,459
Distance = 20.0 km
Ascent = 582m
Descent = 595m

What puzzles me is why the Garmin is less accurate than the MemoryMap when recording distance covered?


  • sussamb 621 Points
    edited January 12
    You're making an assumption there that it's the Garmin that is less accurate when it could be Memory Map, unless you do a controlled test you can't be sure. I used my Garmin to walk the Pennine Way a few years ago, a walk stated in most if not all guide books to be 268 miles, my Garmin recorded 269 ... indicating a fairly good accuracy. Your car journey also indicates the same.

    Why any GPS records the distance it does is difficult to determine without knowing fully the algorithm used, and how often it's set to record. In general though errors can occur due to 'jitter', as you found often they will record movement when they're sitting still, and the recording method selected. For example, your GPS can be set to record based on time, distance or auto (I believe) and I suspect so can memory map. If they're set differently that could also account for differences.
  • Thanks for the reply. Actually, I'm not making the assumption that the Garmin is less accurate than the MemoryMap, I know that it is. Other people use other devices which give results closer to the MemoryMap. As it happens, the Leader of the walk prepares the walk on mapping software which gives a pretty clear indication of the distance. I have also used both devices on walks from books and on guided walks - the MemoryMap result for distance is ALWAYS closer to the claimed distance than the Garmin is.

    As you say, any GPS device will suffer 'jitter', - e.g. they will record movement when they're sitting still.

    I don't know how one changes the settings on either the Garmin or the MemoryMap - neither manufacturer is very helpful with either documentation or support - which is kinda why I came here :)
  • sussamb 621 Points
    On the Garmin you can change it in SetUp, Tracks where there should be options to set Record Method and Record Interval.

    I wouldn't be so sure about accuracy based on other peoples devices or mapping software either. Mapping software calculates the distance based on effectively a lot of straight lines, as you walk you rarely will follow the exact line used by the mapping software. You can check this by downloading your recorded track into Garmin's BaseCamp, having first plotted a route in it and transferred it to your device. You'll see where your track deviates from the route you plotted, that additional distance will mean your device will invariably read more than the suggested distance.
  • Once again, many thanks for that. I have just had a look and "Record Method" is set to "Auto", "Recording Interval" is set to "Normal" - I will have a play about with those settings.

    I take your point about "wandering on & off" a defined route - I still believe that the Garmin exaggerates and for sure, it seems to record movement more than the MemoryMap when it is not moving. I recently left it in the garden for a day and using BaseCamp I got the following results:
    Points = 1,196
    Distance = 712 m
    Ascent = 25 m
    Descent = 25 m

    the MemoryMap reported:
    Points = 5
    Distance = 88 m
    Ascent = 12 m
    Descent = 0 m

    I will try this exercise again because the Memory Map may have stopped logging when it realised I wasn't moving.
  • sussamb 621 Points
    Try that with the 62 set to distance, you should get less jitter.

    Personally I prefer mine set to auto but if I stop for lunch or any other reason I switch it off so that it doesn't add unnecessary distance.
  • privet01 142 Points
    I always thought the recording interval only affected how often data gets logged in the file. Not how often the device itself processes and accumulates data for it's display. But am not certain which totals the OP is questioning, the log totals as displayed by some other software, or the accumulated total on the device. They can and likely are calculated in different ways.

    Without having a surveyed course with a guaranteed accuracy I would be interested in seeing the results of two different courses. One straight as possible, the other as crooked as possible at the same velocity. If there is a difference in the straight then it will be much easier to tell which is less accurate. If the straight is reasonably agreed by the two and there is a difference in the crooked, then I'd believe the difference to be due to the number of position points the device determined and accumulated.

    Essentially the one with fewer points is missing the extreme point of curves and switchbacks which the other is getting.

    And again, the plot points in the log file likely have nothing to do with the plot points the device actually uses itself. But I admit that statement is an assumption.
  • privet01 142 Points
    Here is a link to an article that will also give some insight.......
  • I will have a go with the GPS62s set to distance on Sunday.
    I am not certain which totals the OP is questioning,
    I reset the displayed (and stored) data before starting a walk. I believe that the figures on the GPS62s at the end of the walk tend to be exaggerated.

    I then download the .GPX file to a PC. If I look at that file using BaseCamp (v4.6.2). the summary distance data is the same as displayed on the GPS62s at the end of the walk. This appears to be retained in the .GPX header and if I edit the file using WordPad in order to remove 'jitter', this seems not to be reflected in the summary.

    Sorry, I may not be making much sense here . . . I will try to compose a more meaningful analysis.
  • sussamb 621 Points
    Just FYI there is no need to use Wordpad, you can edit the track directly in BaseCamp.
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