cycle computer vs gps accuracy

[GrasB]

A well set up computer is more actuate than a GPS. Unless you have a differential GPS system with a correctly set up base station, your ordinary GPS is, at best, only accurate to +or- 5m.

Maybe, maybe not. A well set up computer isn't going to take into consideration that when hammering through a set of tight corners at highish lean angles your 700x28c tyres are giving an equivalent rolling radius as a 23c tyre on the flat.

 

[andrew_s]

Another point is that to be "well set up", you would have to make checks on a regular basis.
Eg note the tyre pressure when you calibrated and check/adjust pressure before each ride, recalibrate on a change of tyre or if the load varied significantly (put front panniers on).

Furthermore, a front wheel travels further than the back wheel does - outside line on bends, wandering from side to side as you maintain balance. Which wheel do you pick?

 

[alecstilleyedye]

of course when i'm zig-zagging my way up a steep climb, the bike comp probably is more accurate than measuring straight lines…

 

[jimboalee]

Smokin Joe mentioned earlier the 'true reading'.

What is the 'true reading' and how is it established?


I did mine by riding alomg a main A road with km marker posts. I compared my computer distance with the 5km of marker posts and did a correction sum.

I reckon ( because no-one can be totally sure ) I'm less than 0.5% away.

 

[Alien8]

A well set up computer is more actuate than a GPS. Unless you have a differential GPS system with a correctly set up base station, your ordinary GPS is, at best, only accurate to +or- 5m.

But we're not talking about absolute position - it's about the accuracy in determining change in position.

Generally the longer-term bias will be mitigated by differencing successive fixes and a high sampling rate and appropriate dynamic model will smooth out short-term variation.

But unless you happen to know the inward workings and algorithms in your GPS it's hard to determine what it's doing.

 

[andy_wrx]

I've seen GPS units wandering so much that you can go into the caff and come-out later to find you've covered an extra couple of km while you were having a coffee...

 

[ColinJ]

I've seen GPS units wandering so much that you can go into the caff and come-out later to find you've covered an extra couple of km while you were having a coffee...

That's probably because the bike with the GPS on was shoved up against a wall with a pile of other bikes leaning against it and the device was not getting a clear view of enough satellites OR (more sensibly) the owner of the GPS took it with him/her into the cafe to avoid it getting stolen and it couldn't get a proper signal indoors.

I've only had a couple of hiccups with my cheapo Garmin Etrex and that is usually due to setting off before the GPS has locked on to enough satellites. It needs 3 for an accurate positional fix and 4 to get the elevation right too. It did lose satellite lock once in a local valley with very steep sides (at Mill Bank near Ripponden) but normally it is extremely reliable.