"I have a unit that knows where the speed cameras are and I use the distance coundown to judge the accuracy of the GPS position."
FE Hoppy
Your position indicated by the GPS unit is accurate to within a few tens of metres.
It is the location of the speed camera that is in doubt, because the lists of locations are compiled from a variety of sources including amateur users and are frequently inaccurate.
If you repeat the same journey you will find that the indicated error at each camera is consistent - always the same every day.