GPS signal strength is pretty weak, so things like tree cover, lots of high buildings, etc., will always have an impact on the receiving unit's ability to meaningfully process and output data.
GPS is also notoriously rubbish at calculating speed on the fly. It's only afterwards, when it has a series of timestamped coordinates, that it can produce an accurate picture of actual travelling speed.
GPS signal strength is pretty weak, so things like tree cover, lots of high buildings, etc., will always have an impact on the receiving unit's ability to meaningfully process and output data.
GPS is also notoriously rubbish at calculating speed on the fly. It's only afterwards, when it has a series of timestamped coordinates, that it can produce an accurate picture of actual travelling speed.