Quote:

I understood even the GPS indicated speed is an extrapolated speed rather than genuine speed.



That is exactly right! To get an accurate speed you need to be travelling at a steady constant speed for a few minutes really. Like height, gps units are totally pants for height, you need to leave them for a good long time to settle before it gives you a decent height reading, and even then its not height above the ground, its height above the wgs84 ellipoid!! So like on portsdown hill where i work a gps unit will give you 178 metres height, but the actual height above mean sea level is 133 metres. A bit off topic but something to be awar of!

As for calculating speed, the equation is...

Speed (in mph) = 2.23*(distance_in_metres/time_in_seconds)

or

Speed (in km/h) = 3.6*(distance_in_metres/time_in_seconds)

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