Curves analysis Delta time

[Cyrille71]

Hi everybody,
How works the curve ? how to understand the time lost (or win) ?
See file enclosed.
I don't understand why I lost 2 seconds until I drive faster (White curve / Red curve)
Thank in advance

[Harry]

That's an interesting one

Any time made good or lost is calculated from *time from start* and *position on track*. So for your situation, this means you needed more time to advance your position for the current lap, compared to the reference lap. Speed is not derived from position changes but the so called Doppler effect. This in turn means position change and speed reported by modern GPSes are not necessarily consistent (kind of two sources of information). The delta shown makes transparent that the internal GPS sensor (I assume this has been recorded with the internal sensor) is not as accurate as in depth data analysis would require. But in general, speed measurements using GPS is precise for constant speeds only.

- Harry

[Harry]

P.S. The other way around is possible too: your GPS had bad position accuracy and calculated the aggregated win/loss wrong. Check the Map view and switch to Accuracy display (blue bubbles) to check your data for the reported accuracy you got.

[Cyrille71]

Thanks Harry for your answers.

You are right there is an trouble with the GPS in this area.
Nevertheless, is it possible to calculate the delta time with the ODB speed instead of the GPC from the GPS ?
Thank

[Harry]

No, not currently. I'd suggest an external GPS in case you want to work seriously with the analytic tools., or at least work on the mounting position?

Harry

[MatthewS]

Harry, sorry to revive an old post, but have a further question about time difference calculation between laps. How do you deal with laps that are a slightly different length? (E.g <15m) Do you use the average of them? Or do you use "time from start" and "percentage of lap complete"? Or something else?

I'm using HLT for autocross, and sometimes am slightly baffled when I see near identical speed traces through a corner, and HLT shows one lap gaining time on the other. My conclusion thus far is that I must have drove a tighter line on the faster lap. Can you comment on this as well?

Thanks,
Matt

[Harry]

Gap analysis works different. It takes your current position (and lap time) and searches for nearby positions recored for the reference lap. Next, the lap time at your current position is calculated from these reference lap positions and lap times (using interpolation - similar to what is done when passing triggers). Finally, the gap in time is calculated from your current lap time and this interpolated lap time. As long as position information is of high accuracy, the time gap calculated will be of high accuracy too. The driving history or line driven ahead of the current position (in terms of distance from start) has no influence on this.

- Harry