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Distance Accuracy


SgtCacher
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I have a Vista and took it out on a 10 mile run last week. I had it attached to the shoulder strap of my Camelbak with a clear view of the open sky the entire time. After running 10 miles on the B&A Trail there was more than a 1 mile difference between the trail markers and the distance travelled shown on the Vista. The trail markers are placed every half mile and I assume were surveyed when the trail was built. Which is right?

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To answer this question I would have to know something about the B&A trail. Start with its full name. Is it a winding trail? Does it have significant elevation changes? Have you contacted the trail maintainers to learn how they measured the length of the trail? Which of the two distances measured was larger? If this is a straight line “Rail to Trail” have you calculated the distance between the endpoint?

 

You are talking about a 10% difference in the trail distance. As a trail maintainer of a section of the Appalachian Trail in Pennsylvania I have found a difference of well over 10% on sections. The GPS distance always being shorter. Most of the AT in PA is measured using a surveyors wheel. Many rocky sections are measured using the “extended string method”. This very closely follows the path walked, and is thus more accurate.

 

GPS measures the horizontal distance between sets of points. Thus it will read a lower number if you are turning or have a significant elevation change.

 

Two test you might try to show inaccuracies in your GPS are described below:

 

Test One – Mark two points on the ground, ten feet apart in an area with a clear view of the sky. Call them Point A and Point B. Start your GPS measured distance at Point A. Walk to Point B and back to Point A. Repeat the walk until you have done it 261 times. You have walked a mile, the GPS will read close to zero.

 

Test Two – Locate an outside quarter mile running track. Walk (or run) around it four times. You have covered 1.00 mile. The GPS will read slightly less than 1.00 mil

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Test One – Mark two points on the ground, ten feet apart in an area with a clear view of the sky. Call them Point A and Point B. Start your GPS measured distance at Point A. Walk to Point B and back to Point A. Repeat the walk until you have done it 261 times. You have walked a mile, the GPS will read close to zero.

Normally this would be extremely rude, but since GC is a sport for geeks (and even if it isn't, this topic is !), I just can't prevent my fingers typing this... that should read 264, I think. Assuming you lay out the ten feet to within about an inch, of course :D

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Depending on the accuracy of your GPS, and the number of visible satellites, the GPS position will walk over time even when you are not moving. On a good day, my GPS walks about a tenth of a mile every hour. On a bad day, or with tree cover it could be several times more. This will show up if you are looking at the track length, or the trip odometer.

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The thread appears to be only considering the inaccuracies of the GPS. I believe it is more likely with the inaccuracies with the GPS, there are still possibilities of problems with the listed distance. Some include:

1. If trail length was measured by use of a pedometer. This counts the number of steps made by a walker, and assumes all steps are of equal length and in the proper direction.

2. Calculation, copying, or entering data errors. Like the one I made above and nickbrown was kind enough to point it out.

 

Note – All methods of measurement have inaccuracies. The question is which has the least.

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Thanks for all the suggestions. The Baltimore-Annapolis Trail is a Rails to Trails, straight and flat, asphalt paved, trail. I had very good satelite reception throughout the run. It's interesting to note that most of you suspect the GPS is in error, especially since Garmin is now marketing a GPS based device to runners as a way to keep track distance, speed and pace. If I'm training for a marathon or longer distance and my measuring device is a mile or more off every ten miles I run, then it's not worth investing in. I'm new to geocaching and have had some fun with it, but the more I learn the more disappointed I become. I use my Vista for ultra trail running and back country camping as well as geocaching. So far I've been told not to be surprised if the altimeter is hundreds of feet off, the electronic compass may not be accurate and now the distance may be off by miles. So much for technology. Thanks for your help.

Rich

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