Internal GPS vs BT OBD accuracy

Mateusz
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Internal GPS vs BT OBD accuracy

Postby Mateusz » Sun Oct 15, 2017 8:34 am

In a number of “drag” runs 0-100 readings differ by about 1s between internal iPhone gps based and BTLE 5hz OBD readings. GPS is faster! Is OBD more accurate due to its 5 time higher frequency of update?

It is difficult to get the exact differences, extrapolation quirks with charts of 1hz accuracy.

Is the difference based on extrapolation mechanisms in gps and obd (both set on default)? The tires are stock, so I doubt it is the speedo error issue.


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Harry
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Re: Internal GPS vs BT OBD accuracy

Postby Harry » Sun Oct 15, 2017 9:05 am

Interpolation necessary does not influence measurements as much as low (speed) accuracy or data to time mapping. The problem when using GPS and OBD is that data is fused before it is stored (and used to determine the time). In case OBD speed is available, LapTimer will use that speed for speed performance test. This results in the following scenario trends:

1) Low GPS update rate (1Hz) and high OBD update rate (>6Hz) ‣ high accuracy for OBD based compared to medium GPS based results
2) Low GPS update rate (1Hz) and low OBD update rate (2Hz) ‣ good accuracy for OBD based compared to medium GPS based results
3) High GPS update rate (10Hz) and low OBD update rate (2Hz) ‣ low accuracy for OBD based compared to good GPS based results
4) High GPS update rate (10Hz) and high OBD update rate (>6 Hz) ‣ good accuracy for OBD based compared to good GPS based results

All of this with low < medium < good < high quality.

You can select your scenario depending on the accessories used and rates achieved.

For most situations, I'd trust OBD more than GPS for speed tests. Absolute precision is not dominant here, what counts is volatility. There is one exception however: in case the OBD link is weak (e.g. disturbed WiFi / BT communication, frequent drops in rates), or your smartphone is at the limit processing data (i.e. older devices), there may be considerable variance in OBD data timestamps. This means runtime effects render data to time relation invalid (or at least not at the precision one would expect). The reason for this is that OBD data does not come in with the timestamp of the measurement like GOPS does. This in term means LapTimer cannot compensate runtime effects for OBD. In general, a gap of 1s sounds like one of the channels has an issue - this should not happen.

- Harry


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