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.
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.