GPS accuracy experiences

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retired jerry
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Joined: May 28th, 2008, 10:03 pm

Re: GPS accuracy experiences

Post by retired jerry » October 11th, 2019, 6:38 am

My favorite place to do gps tests is the Deschutes River trail because it's fairly straight, no switchbacks. And there are mileage posts. Mostly it's wide open, no trees to block reception. Except there are places where there are steep canyon walls.

Another place is a city street. Straight, so the distance can be determined easily just by looking at the location of the endpoints.

With an oval track, it's possible to cut off the ends.

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aiwetir
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Re: GPS accuracy experiences

Post by aiwetir » October 11th, 2019, 1:51 pm

The auto pause is useful depending how it's implemented vs your particular activity. If you have a cycling computer set to auto pause, it usually pauses at 2 mph or so (I forget the exact number but it's configurable too) the problem is that on steep hills with bad signal, it can pause you and you lose much of your hill data. If you hike with it it may stay on auto pause most of the day.

If your device uses something like the iPhone where it uses a combo of accelerometer and GPS movement, it might be useful.

Where auto pause on a device also does not work is doing something like geocaching where you might be trying to find a particular point and need the device to keep updating location and since you aren't "moving" it doesn't update. Workaround is to tap the device in a walking rhythm and it'll usually stay "awake"

This is a bit off topic though.

Yesterday I hiked and my phone is the first line. The app I used doesn't have any auto pause feature so it is like raw uncorrected data. Likely the EG is high due to using unsmoothed data. It collected 7914 points which averages out to 1.3 meters between points which also happens to average out to 2.9 mph if it collected one per second.

Map Plus
6.83 mi / 3:24 moving / 2388 ft

Gaia GPS, probably the most accurate, they figure in their own stopped time mathematically
6.16 mi / 3:08 moving / 1226 ft

Ride with GPS, really just to show you how differing stopped times can screw things up.
6.40 mi / 1:40 moving / 1380 ft
- Michael

Lurch
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Re: GPS accuracy experiences

Post by Lurch » October 15th, 2019, 7:37 pm

kepPNW wrote:
October 10th, 2019, 10:02 am
johnspeth wrote:
October 10th, 2019, 9:47 am
measured distances that are about 20% longer
In my experience, it's easy for two identical devices to be off up to 10% simply due to different sampling rates. In general, the more data points there are the longer the line is. Phones don't tend to record points very often, and Garmin devices (eg, the "trip computer" on their GPS units in particular) recalculate results up to once per second.
There is a 'coastline paradox', where in trying to measure a coastline of a country, which clearly has a finite area, the smaller the ruler you use the longer the coastline measurement becomes, and arguably approaches infinity...

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BigBear
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Re: GPS accuracy experiences

Post by BigBear » October 16th, 2019, 9:15 am

I don't use a GPS, just a pedometer, when the route is not defined on a map. In comparing the three pedometers in our group with the one GPS, we have noticed that the GPS understates the distance hiked by 1-2 miles on a walk, depending on the number of switchbacks and turns. The GPS is worthless in Tryon Creek (snake-like trails), for example, where we know he exact difference by the trail map that is detailed to the 1/100th of a mile.

The GPS seems to be the most accurate when we hike in a nearly straight line. It is also best-used when dead-reckoning is not available (on a ship in the ocean, in a white-out, in a featureless desert).

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