Why GPS is rubbish
Well,of course ,it isn't really rubbish.It is a useful guide to measurement and I use it myself to check courses,but at best,it is only a rough guide and is nowhere near accurate enough for race measurement.
A race directors nightmare is the runner who come after the race with "My Garmin says............" This has happened to me and one runner said it was 0.2k too long and the other said it was 0.2k short.
Many things affect the GPS signal. Tree cover and high buildings will lead to marked inaccuracies .It is quite common when examining recorded tracks to see "spikes" where the signal will veer 20-30 mtrs or more away from the actual position ,then return.Also GPS is quite good measuring in straight lines,but not so good round corners ,as it tends to measure in a series of straight lines.
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