[baskinator]

A error correction technique I learned as a young land surveying assistant is to put a gps antenna on a known fixed point location. The delta between the fixed point and the point of measurement is cancelled out to get a more accurate read.

We did this to trial some new (at the time) surveying equipment when the primary equipment was optical. It would save time for really long measurements through the forest and mountainous terrain .

[crote]

You can even subscribe to services which do this for you! There are a few companies with large-scale networks of fixed receivers, and you can get the observed offset from a node near you via the internet, usually via "NTRIP".

Getting correction data from a node a few dozen kilometers away isn't quite as good as having your own fixed base station a stone's throw away, but it's way more convenient and for a lot of applications plenty accurate.