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The GPS seems to be "faster" by almost a second at high speed tests, while the OBD2 is faster on everything from standing start (understandable) but also 80-120km/h.

So to get my head around this I started looking at altitudes, exact speed correlation between OBD2 and GPS and more.

My question is (finally!), what are these "DOP" units used for satelite precision? Is a lower figure better, and what can be considered "good" for accurate testing?

Also what's "3d precision"? I can understand coordinate and altitude, is 3d some kind of combination of these?

Is the number of satelites important at all or are these DOP values all that matters for precision?

## Comments

Your OBD2 speed readings can be affected by factors such as incorrect tire size.

A well calibrated OBD2 speed (compared with constants speeds and GPS) should still be reliable though, and it's the GPS logs I should be critical of (in my case, not use due to inaccuracy), right?

That means GPS measurements are very apt for inaccuracies, and that you really need to check the conditions of your GPS signals for any performance test using GPS to be of any interest.

They go up when the number of satelites go down, which seems right, but I constantly have below 1 on coordinate precision.

They could well be off by a power like obstacle says, 7,8 and 8,9 for coordinates seem to be a fair accuracy, considering it's no high grade GPS (Globaltop G66 5Hz). The values are stable as long as the number of satelites is constant. Maybe I should try sticking the GPS *outside* the car on the roof with Velcro or something and see what the precision is like :-)

I wonder if 8-9 in DOP is good enough for performance tests, where you want 0,1s accuracy. It seems doubtful considering my results.

EDIT: Oh, I never got an answer regarding "3d" precision. Is that just coordinate and altitude tacked into a single value?

I've got "Coordinate precision", "Altitude precision" and "3D precision" too choose from when selecting graphs to view in RaceChrono. I thought these were RaceChrono labels, are they from my GPS?