Unit over-reading mileage

[reklawpete]

On a recent walk we stopped for coffee and my Active 10 read 4.05 miles. Another walker with the same GPS and 2 others with Garmins all read about the same 3.3 miles. Why is mine over-reading? I zeroed it before starting the walk.

[LostMike]

Hi reklawpete.

I do not know why your GPS read more than the others on this one occasion. What I find more odd it that three other GPS devices all read the same.

Almost every week a group of people I walk with compare the different distances that their GPS devices record for the distance walked. They are hardly ever the same. And interestingly I get the impression that they look to me (with my A10) as having the more reliable distance record..

Also, when I finish my walk the track show a certain length for the walk but when I save the file, Satmap rounds up the oddities and it always gives a shorter distance - not much but shorter all the same.

Reasons for differences, apart from a faulty unit can be many-fold.

The device may not have got a good fix when the track was started. I never start tracking until the device reports a good fix.

Different people have different walking styles and walk different routes some wander from side to side looking at things some walk a straighter route.

Where the GPS device is carried and the cover of trees etc can effect the signals reaching the device. This impacts on the accuracy and the fix.

I'm sure I could come up with more but I think I will leave it there for now. You might like to check on future walks and see if any trend appears. If you appear to be getting consistently high distance readings, or if your location does not seem to be accurate on a regular basis then it might be worth contacting SatMap.

Did that help?
_________________
LostMike
Satmap A10. Platform 21
Software version 1.5.9193
Satsync 1.525

[reklawpete]

Thanks LostMike for your thoughts. As soon as a had a fix I zeroed the unit and zeroed it again as we started the walk. Our path was single file in many places so not much chance of walkers wandering.

As this is not the first occasion when the mileage seemed odd I have decided to have it serviced to see if that solves the problem.

[LostMike]

Seems a reasonable course of action.

Do let SatMap know of your concerns when you send it back.

And do let us know if it helps.

Good luck.
_________________
LostMike
Satmap A10. Platform 21
Software version 1.5.9193
Satsync 1.525

[GPSwalker]

I have found that my A10 usually over-measures walk distances by 5-8%, thus a 10 mile walk reads nearer 11 miles. Other walkers with different GPSs on the same trail tend to do the same, so our mileages are similar.

It does improve things if you turn off and back on the device during a lunch or coffee stop, because I have noticed that small distances are still added when you are going nowhere.

Perhaps a software addition could allow for a 5% (and other percentage)error by user optional calibration, i.e., a devise's distance measurements could be reduced by say 5% to compensate for the typical over-measurement.

I often check my new walk distances using the premium Xpedition map and its measuring tool. This always gives a lower mileage than my GPS, usually in the range 5+%. Can anyone tell me if a GPS actually measures distance covered during up and down hills, which is going to be more than a straight line 'as the crow flies' as measured on a map?

[JimmyTheHand]

[davidor]

I've always been a bit wary of the distance recorded as I noted how much distance it records when just sitting on a table. My Garmin doesn't log any distance if it is not acutually moving.

The same goes for speed - the A10 shows a significant variation in speed when walking at a steady pace. Walk at 3mph say at it will show the moving speed as anything between 1.5 and 8 mph. Again, my Garmin shows a steady speed. My A10 has recently been "tuned" in a Satmap service as well.

As for height...

[dales]

Hi reklawpete,

0.7 miles in 3.3 miles is a pretty large discrepancy. If you plotted your track on the PC, you'd probably see it caused by a "spike" in the track, veering way off to one side and back again. You can get these errors in areas of difficult reception eg woodland cover or high ravines. I would treat it as a one-off for now.

Personally I wouldn't send the unit back, unless the fault is persistent.

As has been said, you can get small discrepancies a) because people measure the map in straight lines, whereas the path weaves and so do walkers; b) because the fix is not 100% precise, you do get "wander" of the fix during your rest break; c) because some people carry their device with the aerial facing the stars and the satellites, whereas others stick it in their pocket, shielding reception somewhat and reducing accuracy. Saving the track to computer reduces the number of trackpoints significantly, and so does serve to smooth out the recorded path and reduce tracking error.

Dales.
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nuvi 2599LMT-D, oregon 700, basecamp, memory-map.

[Cullin]

I was wondering if you were getting poor geometry, so your position was not being shown at the correct location, hence adding this distance to your mileage.

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