Quote:
Originally Posted by jeffk
Next, how accurate is your GPS positioning. Since speed is calculated by position and the sampling periods are relatively short, if the positions are accurate the speed calculation will be as well. Accuracy is a attribute of the design of the unit, the installation on your boat (antenna position, etc) and the ability to receive accurate satellite signals from minute to minute. There can be a lot of variability.
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Accuracy is not really that important, as long as the offset is consistent. It doesn't matter if the GPS knows exactly where you are, if it is off by 3 meters the calculations would be correct as long as the 3 meter offset error remained constant.
Most GPS units are doing several samples per minute, so even if you were traveling a non-linear route the speed calculation would still be accurate enough for anything other than a boat doing continuous donuts in a small area.