GPS - not so accurate?

axisus

I always thought that GPS on smartphones was pretty accurate. Not so sure now.

Having been doing the 5k parkrun for 4 weeks now, I noticed that it measures the course as a different distance each time. My runs may be fractionally different, but as the run basically follows a footpath there isn't much actual variance. I checked out an online 'map' of my run, and the 3.5 laps are marked in very different ways, even though as I said, they are all on the same path. at one point it has me in the lake! Admittedly there is some tree cover at some points, but other big areas are open above. 

So, their measured course is 5k. One week my phone made it 4.88km, another week 4.68km!

Maybe it's the app? (Runkeeper), or maybe GPS just isn't as accurate as I thought.

exocet

GPS is very accurate - the consumer version that we access has a 10 metre resolution. 
I suspect that the differences that you are experiencing are down to the app or loss of GPS signal at various points on the route. 

ThorpyFX

gps has an accuracy of +/-10m at best for commercial usage. this depends on the time of day, satellites acquired, and the device that you have in your hands. that does however seem a bit too large a swing

paul_c2

Yeah it will be loss of signal which causes the phone to lose accuracy, not the underlying GPS system. Sometimes it is less accurate than other times, eg when the satellites are aligned badly (in a worst case scenario) but so long as it sees 3 or 4 satellites, will still give a location (better sat navs will also give an indication of where the satellites are and the accuracy its delivering). I suspect the GPS receivers in phones are quite poor at picking up signals.

And of course we've all heard stories of completely losing GPS signal in cities with tall buildings etc. Signals also won't go through trees, you need clear line-of-sight of the sky.

crunchman

Is it worth trying a different app to track it?  I've used Endomondo in the past (and Sports Tracker as well I think).

fretmeister

Runkeeper sometimes tells me my usual route has a total climb of 90 feet.

On a few occasions the same route has shown 700 feet. I also notice on the website map the route is sometimes right on the pavements and sometimes it looks like I'm running through houses.

So I fix it on the website.

Roland

crunchman said:

Is it worth trying a different app to track it?  I've used Endomondo in the past (and Sports Tracker as well I think).

Different apps use GPS in different ways. Some save battery by using mobile mast triangulation, which uses less power, and only check GPS infrequently.

BigMonka

Roland said:

crunchman said:

Is it worth trying a different app to track it?  I've used Endomondo in the past (and Sports Tracker as well I think).

Different apps use GPS in different ways. Some save battery by using mobile mast triangulation, which uses less power, and only check GPS infrequently.

I'd also imagine that most apps will only ping the satellites at fairly infrequent intervals so as to maximise battery life over GPS accuracy - fine for driving-distances but not for looking back at fairly short distances.

Sporky

BigMonka said:

I'd also imagine that most apps will only ping the satellites at fairly infrequent intervals so as to maximise battery life over GPS accuracy - fine for driving-distances but not for looking back at fairly short distances.

GPS is receive-only.

Car sat-nav has the advantage that the unit can be reasonably confident that you're on a road. Usually.

BigMonka

Sporky said:

BigMonka said:

I'd also imagine that most apps will only ping the satellites at fairly infrequent intervals so as to maximise battery life over GPS accuracy - fine for driving-distances but not for looking back at fairly short distances.

GPS is receive-only.

Car sat-nav has the advantage that the unit can be reasonably confident that you're on a road. Usually.

oh yeah, of course. But do they still not need to set a resolution/refresh rate within the app?
I'd always assumed that that was why on apps like Strava you tell it before starting the recording whether you're going to be running on cycling.

Sporky

BigMonka said:

oh yeah, of course. But do they still not need to set a resolution/refresh rate within the app?

Probably, yes.

It occurs to me that I was being very pedantic there. Sorry.

paul_c2

Doesn't the app simply call an underlying "location" function presented by Google in the Android middleware? So, the Android OS decides whether to use cell location, GPS, frequency of updates to save battery, etc etc?

crunchman

I never let Andoid use the cell location etc. to get my location.  Big Brother can probably track me anyway but I don't want Google using it to sell ads etc.  I only turn GPS on when I need it - e.g. to track a cycle ride.

menamestom

 

Sometimes when I go out for a 5 mile run, mine only measures a mile!  I don't get it, the time must be off as well as it only says 8 mins when I get back.

MtB

^^^ Timex and Ordnance Survey are your friends!

crunchman

MtB said:

^^^ Timex and Ordnance Survey are your friends!

Wasn't sure whether to LOL or Wis.