GPS Accuracy

Mr Haematocrit

msg me on kik for android
An interesting comparison regarding the GPS Acuracy of smart phones and dedicated sports GPS devices. Although the data is for running devices specifically, it is worth considering if the same is true for cycle GPS products.
Along the route the Garmin has approximately 58 data points, campared to 14 on the iPhone and 12 on the HTC. This suggests with a larger number of data points the Garmin is the most accurate.


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Is this good enough reason alone to buy a dedicated sports GPS?
Does GPS accuracy matter to you?

just curious about what cyclists think of such things
 

MrJamie

Oaf on a Bike
More data points doesn't mean more accurate unfortunately, every time a data sample is taken a little error is added to the distance due to inaccuracy. If you look at the Garmin in "Heath Cresent" if you were to take every 2nd/3rd data point on the straighter part, the distance measured by gps alone would be more accurate (like measuring with a slack tape measure). I know from races runners with Garmins often have quite varied distance estimates. You can get Android apps with higher sample rates and you can boost certain apps rates with an app called TrackerBooster but the increased sampling just overestimates your distances covered by around 10% even though the map pictures look better.

I often run and cycle with 2 or 3 smartphone tracking apps, which all consistently under/overestimate based on their sample rate and distance calculations, ignoring rogue values etc. For example Strava says im 4-5% faster than Endomondo and Sportypal, which sounds like nothing, but run a marathon and find an extra 2km at the end or finding out you're 3mins/hour slower than you thought sucks.

For cycling dont Garmin's have wheel sensors which sync up and keep the distance measured correct, if so I think the higher sampling is better in that instance :smile:
 

jdtate101

Ex-Fatman
The only reason I have a forerunner 110 for my running is I don't want a heavy smartphone bouncing around in my back pocket, it's rather annoying after a few miles. The same is also true for my 800 when cycling, but the primary reason for using it has got to be the amount of data available and, most importantly, the battery life.
 

RhythMick

Über Member
Location
Barnsley
Are you questioning the accuracy of the GPS location, or the accuracy of the distance logged by the tracks recorded by the software ?

I'm assuming the latter, in which case you can usually control how often the software records points. The closer together the points the more accurate the distance, it's a trade-off right ?
 

RhythMick

Über Member
Location
Barnsley
More data points doesn't mean more accurate unfortunately, every time a data sample is taken a little error is added to the distance due to inaccuracy.
Sorry, but surely this can't be correct ? While each data point will of course have a degree of error (however small), surely more data points means the error cancels itself out ? I feel I'm missing a point here?
 

trampyjoe

Senior Member
Location
South Shropshire
Such is the beauty of the GPS system and GPSr's that you can run two or more identical units alongside each other and they will all give different results.
As long as I can get a reasonable idea of where I've been, at roughly what speed and how long I was going then I'm happy (anyways, I've got a separate wired bike computer that tells me all the same stuff except the route)
 

Rob3rt

Man or Moose!
Location
Manchester
Sorry, but surely this can't be correct ? While each data point will of course have a degree of error (however small), surely more data points means the error cancels itself out ? I feel I'm missing a point here?
Depends on the nature of the error. Would depend on the method used to interpolate between points etc.

As for the runners example raised by MrJamie. Running the same race and getting different distances on their GPS, while it will in part be due to GPS error, it will also be due to people taking different lines round bends, trying to get around slower runners etc. The distance of a race, as stated on the certification of distance, is measured around shortest line possible, so unless you take the "racing line" you will always go a bit further.
 

srw

It's a bit more complicated than that...
I think it was on here (Andrew Culture?) where someone has recently posted a cycling comparison. The smartphone is hopeless while the dedicated GPS unit is spot-on.
 

MrJamie

Oaf on a Bike
Sorry, but surely this can't be correct ? While each data point will of course have a degree of error (however small), surely more data points means the error cancels itself out ? I feel I'm missing a point here?
It's probably more about my inability to convey it ;) There's an explanation from one of the SP programmers, trying to explain why more points doesn't necessarily mean more accuracy which might help: http://forum.sportypal.com/default.aspx?g=posts&m=5743#post5743 The Garmin being a dedicated purpose-built unit with better GPS is probably accurate enough to get away with a higher sample rate, but smartphones arent designed for it. Speed also matters as to how big the error data is to distance covered, hence most sports apps take more data points from faster exercise types like cycling than they do for running. Strava for example takes a lot of data points in order to catch the start and end of segments accurately, so it adds a small % of distance and speed depending on your GPS accuracy.

Depends on the nature of the error. Would depend on the method used to interpolate between points etc.

As for the runners example raised by MrJamie. Running the same race and getting different distances on their GPS, while it will in part be due to GPS error, it will also be due to people taking different lines round bends, trying to get around slower runners etc. The distance of a race, as stated on the certification of distance, is measured around shortest line possible, so unless you take the "racing line" you will always go a bit further.
It's quite common after a running race for lots of people to post online worried that their time doesn't count (for marathon entry etc) because their GPS device measured the course slightly under as well as complaints that the route was actually longer than it should be :smile: It's that being just 20cm out over 10 metres, becomes 200m over 10k and half a mile by marathon distance.

Now I want to buy a Forerunner :sad:
 
I use a Garmin E trex (old yellow one) on a handle bar mount and a Cateye Velo 8, they never agree. On Saturday the Velo showed 50.34 @ an average of 8.3, while the Garmin showed 49.2 @ an average of 8.3 as well. I like the Garmin for the Grid refference otherwise I wouldnt use it. I have thought of changing to one of the dedicated cycle GPS's but cost etc has put me off. Any body any thoughts on this?
 
I think the sportypal link is a good example of someone who doesn't know what they are talking about or at least doesn't understand the temporal characteristics of errors in GPS receivers. Short-term errors that can be characterised as random in nature will be reduced by increasing the number of independent samples. Other error sources will manifest themselves as longer-term biases and so won't be mitigated by simple averaging.
 

RhythMick

Über Member
Location
Barnsley
Yes I clearly missed a point there. I blame lack of sleep ... :-)

So, if the true track is a straight line then yes increased data samples will increase the distance recorded. If the random errors alternate to either side of the true track then the increase could be quite large. The example shown on sportypal makes sense: in that example the point error was 75% of the distance between samples resulting in an increase in distance recorded of 25% over the true distance. Increasing the distance between samples compared to the point error reduces the increase in distance. Ok I get it.

However, a couple of points.

Firstly, the error can only be positive. The distance recorded cannot be less than the true track.

Secondly, the software which is calculating the distances could well be smarter than the example given. It could be comparing multiple points and self-correcting.

Finally, does anyone know what the actual error point rate is on the various GPS chips ?

My SGS2 using either Strava or ViewRanger seems to produce tracks that fit very closely to the roads/tracks and it's certainly accurate enough for my purposes. Especially if it's giving me an extra mile in a hundred ... :-D

EDIT:
Of course the balancing issue is that if you take too few point samples then the GPS will 'cut the corners' and record less distance than the true track. This cannot increase the distance recorded though.

So increasing the sample rate can increase the distance recorded, but reducing the sample rate can reduce the distance recorded.

Finding the right sample rate to capture enough points but not introduce error will depend on knowing the point error in the GPS software I guess. With a point error of 0.3 metres (purely taken from the sportypal example) then taking data points every 0.7 metres limits the track increase to 8.5% (again taking the sportypal example, I think the maths is actually a little more complex, but it's illustrative).
 
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