Andy Bennet formulated on Thursday :
Only trust the satnav speed when on a level road. The speed is calculated by
rate of change of position on a flat surface. If you are going up or down a
hill it will under read.

Yes, it does make a difference, but for all practical purposes can be
ignored. The major difference to be taken into account, is acceleration
and deceleration - they provide accurate readings only at steady
speeds.


The derivation of altitude via GPS is not very accurate anyway and is ignored
in domestic satnav devices.

Correct, mine shows the altitude, but rarely moves from 114 feet ASL
irrespective of where it goes.