So how accurate is GPS?

[BOBTEE]

At the risk of sounding like a sore loser, how accurate is GPS and can different devices give different results? Add in Strava!

I ask because I did a ride yesterday and the same again today (kinda getting caught up chasing segments!). I'm 100% certain I pushed harder today and was sure in my mind that I'd gone quicker...two segments, both exactly the same times on two rides!

Secondly, I've been in the top ten for both these segments, I'm now equal to the KOM when I'd first used Strava. Some bloke is now KOM on both the segments I'm after, the first he has 7 seconds on me and I did it in 54, 100% flat knacker! If I look at the comparison, he doesn't move for 12 seconds and then at 15 seconds he's 9 seconds in front of me!

So, is my HTC Desire crap, is mystery KOM man's GPS crap, is Strava crap or should I just get faster?!

[Fluffsri]

Ive ridden home with 2 mates all using phones as GPS. one i phone, one HTC wildfire and my HTC desire HD. When we arrived at our split point we all had different data yet we all rode together. I would say they arent 100% accurate but better than nowt.

[itsnotarace]

Phone GPS is not as accurate as a dedicated GPS unit such as a Garmin, especially when there is interference from trees and buildings.

[BOBTEE]

Hmm, may have to see what he's using and get one of them!

On one segment, I'm in 2nd position with 54 seconds... joint 2nd with six others, this dude is 3 seconds faster! I'm just not buying it, you can't be that fast, you just can't!

[daz3210]

And don't forget that GPS is only accurate to something like 10m at the best, and if the American military see fit they can reduce this somewhat. I was told once that during the first Gulf War GPS accuracy was reduced to within 100m.

[TonyHetherington]

BOBTEE said:

On one segment, I'm in 2nd position with 54 seconds... joint 2nd with six others, this dude is 3 seconds faster! I'm just not buying it, you can't be that fast, you just can't!

I thought that about a friend of mine's time on a segment. We went and rode it together; he bloody shot up the hill

He can be that fast

[BOBTEE]

These two segments in question are both (largely) downhill and I'd wager I have more potential energy at the top than any of the other riders pushing for the lead!

Seems odd to me that seven people can be tied for 2nd place with the leader three seconds down the trail! Having looked at a trace of his ride, for starters he enters the forest at a different point so he'll be much fresher at the segments in question. Also, my trace looks massively more accurate than his, to the point where if his is to believed he cut a new, perfectly straight trail through the forest!

I guess the only way to settle it is call him out and get some proper timing gear... or one of them things for wazzing dog balls and a lot of bubble wrap!

[TonyHetherington]

Do remember that Strava only considers 2points for the overall time; the moment you cross the start and the moment you cross the finish, and that's your time.

So not only does it not consider auto-pauses and such, but it will also not matter if the the traces are not completely identical (though if they are too far apart it won't actually match it as a segment!)

[BOBTEE]

Yeah fair points

Gonna take the ipod and try that against my HTC, see if that throws up any different times!

[Rowan138]

GPS is only accurate to about 10m, while GLONASS, the russian version of satnav is accurate to about 8m and when used in conjunction with GPS a time to first fix is about half a second and accuracy is improved to about 5m.

[tank slapper]

Most modern GPS receivers can make use of the the EGNOS augmentation system (or WAAS in America) that increases accuracy to about 1m. It provides ionospheric correction data to reduce uncertainty in the timing information received from the GPS satellites. Most stand alone receivers can use it these days, but I don't know whether the integrated receivers in phones are can.

[Gizmoish]

tank slapper said:

Most modern GPS receivers can make use of the the EGNOS augmentation system (or WAAS in America) that increases accuracy to about 1m. It provides ionospheric correction data to reduce uncertainty in the timing information received from the GPS satellites. Most stand alone receivers can use it these days, but I don't know whether the integrated receivers in phones are can.

I was going to say this: the whole thing about GPS having a built in fuzz factor are out of date.

The problem with Strava GPS seems to be a sampling rate - I think it only picks up once every couple of seconds so gets thrown off sometimes.

[Silent1]

In europe you have Galileo as well which reduces the error using ground based stations and the new US satellites don't have the capability to turn on selective availability (Basically, increase the error) and all have had it turned off since 2001

[Watchman]

daz3210 said:

And don't forget that GPS is only accurate to something like 10m at the best, and if the American military see fit they can reduce this somewhat. I was told once that during the first Gulf War GPS accuracy was reduced to within 100m.

The US turned off Selective Availability in 2000 following President Clinton's directive that GPS be considered "dual-use" in 1996.

[daz3210]

Watchman said:

daz3210 said:

And don't forget that GPS is only accurate to something like 10m at the best, and if the American military see fit they can reduce this somewhat. I was told once that during the first Gulf War GPS accuracy was reduced to within 100m.

The US turned off Selective Availability in 2000 following President Clinton's directive that GPS be considered "dual-use" in 1996.

Does that mean the accuracy can no longer be diminished?

[fid]

I thought GPS accuracy was around 5 feet rather than 10 metres.

Anyhoo, I use GPS Logger on my Blackberry and upload the .gpx file to Strava, I can increase the sampling rate...can't that be done on the iPhone/Android apps?

Edited by fid on Thursday 9th August 00:31

[anonymous-user]

daz3210 said:

Does that mean the accuracy can no longer be diminished?

Aircraft are using it for flying approaches. Nobody is switching it on and off or reducing the accuracy. GPS is very accurate, it depends on the receiver you are using.

It's possible to buy receivers with accuracy down to 10cm,

Edited by el stovey on Wednesday 8th August 23:57

[2DDav]

I-phone accuracy is down to something like 16ft, I use my i-phone for another GPS based ap that gives you the accuracy and its at best 16ft, this can obviously vary and in some circumstances Ive seen accuracy as far out as 32ft not taking into account signal lose etc.

[Watchman]

daz3210 said:

Watchman said:

daz3210 said:

And don't forget that GPS is only accurate to something like 10m at the best, and if the American military see fit they can reduce this somewhat. I was told once that during the first Gulf War GPS accuracy was reduced to within 100m.

The US turned off Selective Availability in 2000 following President Clinton's directive that GPS be considered "dual-use" in 1996.

Does that mean the accuracy can no longer be diminished?

Not unless they reverse the directive and give ample warning to the commercial users, like aircraft, who are "critically" dependent upon it.

[crisisjez]

I don't get Strava, whats the point of snapshotting a ride to be the best. If everyone else is riding a whole route its easy to beast them over a short segment.
I recently got a KOM on a 27 mile ride I reguarlly do, just .8 of a mile's worth, the next day the previous rider(who had KOM'd it months ago) had gone back out and beaten it, what was the point unless he was riding the same route as me on the same day?
Also if I simply warmed up and rode out to the segment I would be a great deal quicker but what does that actually prove?
Not having a pop, just need to be educated I guess.