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GPS Accuracy

Former Member over 9 years ago

Does anyone know if there is a way to ensure my distance is accurate? This may sound silly, but I feel like some of my miles are off on my 235 (a little shorter then they should be). It's hard to know for sure, it's possible that I'm a little faster and with the auto pause at traffic lights, etc it could be correct.

I have my watch set to auto lap for each mile, but maybe I messed something up???

Any feedback would be great! Thank you!
Bette

0 Former Member over 9 years ago

If you have access to a running track that would confirm your distance. When I programmed my steps I went to the local High School and used their 1/4 mile track to confirm my step total and to see how accurate my GPS signal was too.

0 Dr Phil over 9 years ago

Spend some time reading the myriad of forum threads about GPS distance and accuracy. There's heaps.

Probably the worst place to check on the accuracy of a wrist-worn GPS device is the track. Since data is collected every second that means between each data collection point a straight line is assumed. Going around a curve you are effectively cutting a corner each time.

If you want to check your accuracy find a straight distance with a clear view of the sky. Take several measurements with a properly calibrated wheel, then compare your GPS distance with that.

Here's a GPS primer from Garmin - http://www8.garmin.com/aboutGPS/

0 Former Member over 9 years ago

Spend some time reading the myriad of forum threads about GPS distance and accuracy. There's heaps.

Probably the worst place to check on the accuracy of a wrist-worn GPS device is the track. Since data is collected every second that means between each data collection point a straight line is assumed. Going around a curve you are effectively cutting a corner each time.

If you want to check your accuracy find a straight distance with a clear view of the sky. Take several measurements with a properly calibrated wheel, then compare your GPS distance with that.

Here's a GPS primer from Garmin - http://www8.garmin.com/aboutGPS/

I found no problem while using the GPS on the track, so you're comment intrigued me to do an Internet search myself. I wasn't able to find any information stating running curves would be one of the least accurate GPS readings. I'm not disputing you; I'm just saying I might not be looking in the right place. I did find this in an article (I apologize, I couldn't figure out how to attach the image): "The second (and bigger) problem is the accuracy of those dots. Garmin's web site itself states the following: "Garmin® GPS receivers are accurate to within 10 meters on average."
The image to the right shows what happens 95% of the time. So the result is that your GPS can read your path as weaving 10 meters in either direction, when you are actually moving in a straight line.
I agree, several measurements with a calibrated wheel while the most satellites are available is the best way to determine a more accurate distance, but unless I'm misunderstanding what I've read, using a straight line or an arc isn't going to change the degree of error.

0 Ea2ctb over 9 years ago

Hi,
WIth the intention of learning more about it, this is my view:

"Garmin® GPS receivers are accurate to within 10 meters on average."
The image to the right shows what happens 95% of the time. So the result is that your GPS can read your path as weaving 10 meters in either direction, when you are actually moving in a straight line.

This is correct, but when moving in straight line, the straight line plotted by the gps is basically maintained "straight", although it could be plotted 10 meters aside (parallel or about parallel) the actual path.
You can see your path in a map, and when you run straight it is always plotted "basically straight". There is no a "zig-zag" of 10m of diference between one point and the following one. The GPS "identifies" you are running straight and there is no much difference in distance from a theoretical straight line
Therefore the distance measured by the gps in straight lines is pretty accurate, and the "10m error" is not relevant when distances are of hundreds/thousands of meters.

using a straight line or an arc isn't going to change the degree of error.

The GPS gets points every second (or every number of seconds), and it always plots (and measures) straight segments. When running an arc, it unavoidably measures a poligon, so you will have an additional source of error compared to the straight line.

AN important source of error comes also when running along corners, because the GPS, by definition, always has a "cutting corner effect". The more frequent the sampling, the lower the error, but in a long distance it is cumulative, and this is one of the reasons why a half marathon can measure 20,9K in the GPS and things like that.

0 Former Member over 9 years ago

That makes sense. I would assume speed and distance travelled between GPS readings is a factor too? I was walking the 1/4 track to count steps and my graph shows very smooth lines for my path. It seems to me that if I were running or sprinting, those lines may have appeared jagged.

0 Ea2ctb over 9 years ago

Yes, it is a factor too.
However, the relation between speed and accuracy is not evident to me... It depends on the path.
If you are moving in a "straight line", the faster you do it, the more accurate readings will get, because the GPS error (let's assume 10m) will affect on a longer distance. Let's say you move 1Km in one second in straight line; if sampling is 1 per second, you get 2 readings joined by a perfect straight line, exactly as your actual path, with a max error 10 meters in a total length of 1K.
On the contrary, if you move along an arc or around a corner, the faster the worse, because the side of the polygon between readings of the GPS (or the cutting effect in a corner) will be very different from the actual arc or corner.

The GPS cannot change your speed but the choice of "Smart sampling" in the menu helps to avoid this problem by increasing the frequency of sampling when you turn, and reducing it when you move straight. (and at the same time saves memory when you are moving straight and there is no need to add points)

0 Ea2ctb over 9 years ago

What model of GPS Chipset is in 235?

BTW, to avoid a new thread only for this point; does anyone know if the GPS chipset in the FR 235 is "SIRF" or "MEDIATECH" (or other).
I have used SIRF (FR 610) with an excellent accuracy.
Then Fénix 2 with MEDIATECH chip, and really bad accuracy.
Now FR 235 (I was afraid after the bad experience with Fénix 2 and bad comments with FR220/620), but I do not see any problem at all. I wonder if it is a result of SIRF chipset again, or MEDIATECH improved version, or if simply it does not matter the chipset and it is a matter of luck of the specific device you get.
Regards

0 timgrose over 9 years ago

Garmin have used MediaTek for about the last 5 years now. The make of the chip is seldom the major factor but more the antennae design.

0 Ea2ctb over 9 years ago

Thanks TIMGROSE. Good to know (both things).
The Fenix series is giving a lot of troubles in GPS accuracy (see forum related). I do not want to be simplist, but if the chipset is not the main cause, GARMIN should be able to solve it easily, having in mind that other devices (Forerunners) are much better in this sense.
On the other hand, sometimes it seems that accuracy also depends on the specific device....
Well, this is another story for another post.