Odometer Inaccuracy in Handheld Garmin GPS

Have you ever noticed that the odometer reading on your Garmin GPS appears to be displaying a shorter than expected distance, especially on long, steep uphill sections of trail?

I have done some web searches and the consensus on forums seems to be that Garmin integrates velocity over time to calculate the distance.  It is suggested that the receiver is set up to assume that below a certain speed what it receives is due to inherent inaccuracies such as ionic interference, so it just assumes no movement at that point. The cut-off appears to be about 1.6 km/h (1 mph). So when hiking slowly, say up steep sections of trail, the odometer does not record any movement. 

I don’t know if other makes of handheld GPS behave the same way. If anyone has any feedback on this topic I will incorporate it into the blog.

The procedure I use to get accurate distances is:

  • Waypoint all trail junctions or other points I need distances for.
  • Using GPS mapping software (MacGPS Pro), correct my original track log for errant track points. 
  • Use MacGPSPro’s cumulative distance feature to read the distances at each waypoint.

garmin

In the picture, the odometer reading of 18.74 km will be reasonably accurate as I was moving at an average of 7.8 km/h (skiing) and there were no very steep uphills where I slowed down appreciably. Unfortunately the moving average is not always a good indication of the odometer accuracy as it will not indicate that on a particular uphill section of trail you slowed down to a crawl.

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6 comments… add one
  • jan Oct 8, 2010, 3:12 pm

    Most Garmins seem to accumulate distance only if you are moving above some threshold speed. All of my GPS units do it… so your correct distance is the 8.9km.

  • Pat Sep 13, 2010, 7:29 am

    I’m new to this GPS stuff and I’m trying to make sense of why the distance measured by my Dakota 20 seem incorrect. Last weekend I walked a forest orienteering event. The official distance was 7.2 km as measured by the course designer using mapping software.

    My Dakota 20 said I went 7.06 km, including an extra few hundred metres from the registration tent to the start area and then back from the finish area to registration.

    When I downloaded the track onto my computer and looked at it using Mapsource the software measured my track as 8.9 km.

    So, my Dakota 20 showed 7.06 km but the downloaded track measured 8.9 km using mapping software. The 8.9 km certainly makes more sense.

    Why the discrepancy?

  • Dan Hickstein Sep 22, 2009, 8:50 am

    I’ve found that when I’m mountain biking, setting my Garmin Dakota to more frequent recording reduces the underestimation of distance. I find that, compared to my bike computer, it underestimates distances by 5% on roads and straight trails, and 10% on trails with lots of curves and switchbacks.

  • Jan Nademlejnsky Mar 10, 2009, 6:46 pm

    Set your GPS to less frequent recording. This will significantly reduce error, provided you walk in straight line even up or down, otherwise rolling wheel with counter would be the best 🙂

  • admin Mar 10, 2009, 5:31 pm

    Thanks for your comment Jan. I am not concerned about the accuracy of my GPSMAP 60cx. It has good accuracy at any speed, and on days when I have good satellite coverage produces a very even track log with few errant track points.
    My concern is that at slow speeds the odometer reading on the screen of the GPS is inaccurate as explained in the blog. This is of concern to hikers trying to get an accurate distance for a trail. I discovered this when doing research for Popular Hikes in the Canadian Rockies and my GPS distances did not agree with parks signs, other guidebooks or the old method of using a piece of string on a map.

  • Jan Nademlejnsky Mar 9, 2009, 9:10 pm

    You have to understand how GPS works in order to understand its limitation:
    GPS takes position reading in predetermined intervals (selectable). GPS has to have line of site to at least 3 satellites (4 is better) which are as far apart as possible and not in single line. Deeper in valley less access to satellites.
    Many GPS display circle around your position with accuracy number. Larger number less accurate it is. It might show circle 50m in mountain or on bike trails, because of the trees, hills…
    Your GPS could be set to take reading every 30 sec of 1 min… If you ride bike at 30 km/h on winding trail you could drive 250 m in 30 sec, but to be practically only few meters from your original point, because you had to go around some rocks.
    You could imagine ride on multiple S curves and it just happens that the position reading will result straight line track record; like this $. You ride S but GPS records just in intersections between S and n I which looks like $. Your actual distance traveled would be 250 m and indicated could be somewhere between 0 and 250 m.
    The accuracy at your point is also very important. Two points taken at 30 sec. with accuracy 50 m at 30 km/h will give you the following results on straight line:
    30 km/h : 60 sec = 0.5 km : 2 = 250 m/30 sec. GPS will indicate that you traveled distance of 250 m but only if that accuracy is 100% or 0 m deviation.
    With 50 m accuracy your 250 m distance will be in extremes 250 m + 50 m or 250 m – 50 m. You will end up either with 300 m or 200 m traveled distance, which is 50% error.
    When you travel on steep hills then there is another inaccuracy in vertical direction. I do not know what it is, but I would guess that it is the same. Your location is somewhere inside of ball with radius of 50 m (in above example).
    Faster you go, more precise your speed and distance is calculated. Slower you go more difficult is to navigate even to your waypoint.
    GPS is much more accurate in the car than the car speedometer, because the error influence is diminishing with speed.
    This subject is difficult to explain without a drawing.
    I would like to say that there is nothing wrong with your GPS, just limitation of triangulation calculation. This is pure geometry. There is practically no difference between cheap and expensive GPS. All work by the same principle. The only difference is the software, which Garmin is still far superior.

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