hi all ,
i'm using my gt31 to log mountain biking trails , when i put it through real speed there is at least two km difference , between doppler distance and distance .
is that because of rules real speed uses ??

Too complicated for mere mortals to understand but some discussions here if you really want to know.
www.seabreeze.com.au/forums/Windsurfing/Gps/How-do-Realspeed-GPSResults-calc-TP-Distance/?whichpage=-1&REPLY_ID=343519


decrepit's response

Posted 04/05/2009, 8:04 pm
There's a different calculation method.
I assume something like this.
"Total Dist" uses trackpoints and joins a straight line between all the recorded points, so you don't get the full circumference of any turns.
Doppler checks the speed (once a second for the GTs) then calculates the distance from that.

And yes I've noticed that if there are no spikes, doppler is a slightly greater distance, but if there are any spikes trackpoints can give a very big extra distance error.

thanks mark , still not sure . this is true in this case ,
cycle computer agrees with total distance , within 200m over 12 km roughly .maybe its because of the switch backing in cyclying.
and to clarify to mark it is Doppler that is less than track this time , never seen it before only mountain biking .two kms over twelve a massive fault

bigdog,

What does the cycle computer agree with? Doppler or Trackpoints?

Providing the cycle computer is calibrated accurately, I'd be inclined to trust it, but accurate calibration for distance is tricky with a cycle computer, especially over rough ground.

Which reminds me, I have a feeling that 3D distance may come into play with doppler too. Can't remember.

Dylan ,
i'm anal and set my cycle computer with the use of three different gps , on the road .
the cycle computer supports the track points.
road today on same bike on road , with same computers and gps , doppler/track point out by 200m , excepted , and only out to doppler/computer 400m over a 44k ride .
it has to be the switch backs and the rules that apply to the windsurfing software.
thanks