Different Distances On Same Hike

I've noticed this before but it's become a problem now so I'm hoping someone can explain things to me. Yesterday I hiked to Silver Lake in WA. This is an out-and-back hike so the distance to the lake and the distance from the lake should be the same. When I got to the lake my Garmin GPSMap 64s said I had hiked 4.04 miles. I turned off the GPS to save the battery. I figured it would be an 8.08 mile roundtrip but when I got back to the car my GPS said I had hiked a total of 7.5 miles. How did I lose a half mile after lunch? This has happened before on out-and-back routes where the outbound distance when doubled is a lot loner than the total showing after a hike. And I was careful to make sure the GPS was on and showed several green bars of satellite connection before leaving the lake for the return trip.

Then I loaded the route into Basecamp. At the bottom of the screen it confirmed the 7.5 mile total that was on the GPS but the Adventure Viewer said it was a 7.1 mile hike. This has always been the case. The Adventure Viewer summary shows a shorter distance than the GPS or even the Basecamp total and the bottom of the screen. Now I've lost an additional 0.4 miles.

I posted the adventure to Garmin Adventures to share with friends and online it says the hike was 6.5 miles. This is also something that happens frequently. What the GPS says and Basecamp says differs (sometimes dramatically) from what the online Adventure says.

OK, so I hike to the lake and my GPSMap 64s says I've hiked 4.04 miles but upon returning to the car it says the total was 7.5 miles. Basecamp confirms the 7.5 mile total but the Adventure Viewer says it was a 7.1 mile hike. There's a discrepancy right there inside of Basecamp. And then the online Adventure says it was a 6.5 mile hike. From 8.08 miles (distance to the lake X 2) to the online Adventure total is a difference of a little over 1.5 miles. What's going on?

Any advice or explanation will be appreciated.

0 Former Member over 9 years ago

Here's my WAG: as far as the difference between the distance computed on line and the distance computed on either your device or in BaseCamp I suspect that it might be due to the algorithm used to compute the distance numbers with one taking into account the topography of your route and the other not doing so. In other words if you are starting a mile away from your destination and you walk across a flat plain you have traveled one mile. If there is a 500 foot high hill between you and your destination and your direct path goes over that hill you will have actually walked further than one mile to get there. Some map products have a digital elevation model incorporated and some don't.

As far as the difference between in and out that might be attributable to what I'm going to call the dithering of your position as calculated by the device and the speed it uses to decide you have stopped. If you stay in a single position your device keeps calculating that position usually about once per second. The position as calculated dithers or moves around a bit, it is never the same. Your device is always calculating the difference between the positions to see if you are moving. As long as it thinks you are moving it will be adding the distances between the points. When it decides you aren't moving it just throws away those points and the measurements associated with them. Also, depending on the satellite geometry and the signal propagation your calculated position will also jump around. Tree cover also affects position.

0 Richard_S. over 9 years ago

OK, what you've said makes sense. But do you think the data generated directly by the GPS and displayed on the GPS is the most accurate or would it be the Adventure Viewer data within Basecamp or the data displayed online with the Adventure Online posting? The GPS display is always the longest distance and most elevation change.

And thanks for getting back to me so quickly.

0 Former Member over 9 years ago

Its difficult for me to have a rational basis to answer your question since I don't have the same device nor do I have the track. But, after a little experimentation with devices I have handy I would suggest that you, if you have a google account, go to maps.google.com, click on the hamburger menu to the left of the map search bar at the top left, then select "Your Places" from the drop down, then select the "Maps" tab on the top right of the drop down and then click on "Create Map" at the bottom of the drop down then import the track into your new map and click on it on the left side and a pop up will give you statistics including the distance. Compare that distance to the various distances Garmin has given you and use the closest as the most accurate.

0 Richard_S. over 9 years ago

Its difficult for me to have a rational basis to answer your question since I don't have the same device nor do I have the track. But, after a little experimentation with devices I have handy I would suggest that you, if you have a google account, go to maps.google.com, click on the hamburger menu to the left of the map search bar at the top left, then select "Your Places" from the drop down, then select the "Maps" tab on the top right of the drop down and then click on "Create Map" at the bottom of the drop down then import the track into your new map and click on it on the left side and a pop up will give you statistics including the distance. Compare that distance to the various distances Garmin has given you and use the closest as the most accurate.

That's a great idea. Thanks.