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.
Go to the Roundhay park run page,and look at the Mapmyrun page.Zoom right in on the bend at the top left corner of the course,or the bend at the very bottom(or for that matter,any bend in between). I rode the same line on each lap,but you will see that the tracks vary wildly .
The acceptable error in measurement is a maximum of One Metre per Kilometre and even when you are stood still,a good GPS signal will only give you your position to within 5 metres.