I've been working on a hobby site for some time that analyses GPS data, and calculates top speeds etc for sailors. GPS data is a bit unreliable, so there is necessarily some filtering that occurs, and one filter is on acceleration. It is unusual for a vessel to accelerate at more than 3m/s^2, for instance. Recently I've realised that I'm basing all acceleration calculations assuming the vessel is travelling in a straight line. With bad data, this is not necessarily the case. I could have a data point that shows a vessel travelling at 16m/s at 270 degree, then 14m/s at 100 degrees a second later. At the moment, I would be calculating a slight deceleration from this, but in fact this is an invalid point. I can't seem to find any straightforward reference to how to determine acceleration around a corner. Can someone please point me to a place where I can find this information? Here's a real example: data is duration (s), speed (m/s), direction (degrees) 2, 28.9, 155 2, 14.7, 310 2, 18.0, 354 2, 1.02, 270 At the moment, my software doesn't think that the acceleration between points 2 and 3 is unusual, but because of the almost 45 degree turn involved, it should be noticing a big difference. Dylan.