Hello, you are correct there are many things that contribute to accuracy of systems, for example our google image maps don't always line up to the base maps, we have our own base site which we have a application for as higher accuracy as we can get.
I have found from testing that there is lots of variation between units, GPS modules etc, I thought I would ask this question to aim for the best result first. There must be a GPS hardware company using the best GPS modules in the industry for the best results.
there is a term we use with technology......'rubbish in, rubbish out', if you feed a computer or information system junk, don't expect gold at the other end
we are not fortunate enough to have access to WAAS or EGNOS here in NZ, we have discovered with guidance computers we sell that GPS/GLONASS options give us a good result down here and the whole industry seems to have already moved there. we are getting better online times when machines go under trees, we are not getting any more accurate data, just more reliable data.
feel free to mention any more high end trackers, I am still interested in what units are out there