[Ovmsdev] SIM808/SIM908 GPS accuracy

Michael Balzer dexter at expeedo.de
Tue Feb 7 05:38:15 HKT 2017

Hi Edward,

I'll try to test your new code on a SIM908 module this weekend.


Am 06.02.2017 um 10:50 schrieb Edward Cheeseman:
> Hello again,
> I have tried out the integer math approach for converting Lat and Long GPS values on my SIM808 unit.
> It seems to be reasonable for the short drive I had today. Going by the iOS app (which I think uses Apple maps?) I was on the road, although maybe on the wrong side of it!
> So, maybe within 5 meters?
> I haven’t compared this to the floating point implementation since the negative fix, so can’t say whether it is an improvement in accuracy.
> I’m reasonably certain it takes less cycles. It may take more program space with the constants used.
> The SIM908 code hasn’t been tested on a live unit, but worked ok on mac gcc. I’ve transferred the changes I needed to make that ensure the C18 data types behave correctly.
> Integer Promotion and char unsigned by default were interesting quirks. So was the “don’t make a line too complicated” piece of advice. (1<<8) just disappeared, where 256 worked?!
> Anyway, this minor change is on Github:
> https://github.com/evbuilder/Open-Vehicle-Monitoring-System/tree/lat-long-precision-fix
> Let me know if you want this as a pull request.
> Edward
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Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
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