I’ve made some GPS traces on a small and almost unmapped Greek island.
Some of the traces cover a large pier used by ferries. Comparing the trace with the outline of the pier in OSM shows an error of around 20 meters. I checked this on several days and walked in different directions and the error is pretty consistent. Further checks where other roads meet the coastline within a mile or two also show the same error. BTW I noticed when arriving at the island that my Legend HCX with a downloaded OSM map of the area showed the ferry several meters inland of its actual position!
QUESTION: Do I trace my tracks as they are and then move the ways and points by the appropriate amount, or just trace them and leave them? Which is the best practice for OSM?
Thanks in advance for any advice more experienced mappers can give.
When I first had my GPS I checked it against Google Earth by using GPSBabel to convert it to a kml file. This gave me confidence that it was working correctly with no offsets (except under trees and if I had it in my pocket, when it was rubbish!).
You shouldn’t make any assumptions about whether your GPS is any worse or better than anyone else’s, but averaging over many traces is a good idea.
In general, I don’t think the GPS units are accurate enough to be able to do the kind of mapping you want to do and I read somewhere that OSM is “topographical” in that it is more important to know the relationship between what connects to what rather than the absolute positions.
Another smart way to check the accuracy of recorded gps tracks is to use rhe java app RouteConverter.
Have a look at http://www.routeconverter.de/index_en.html and simply load your gpx- or nmea- or whatever tracks to display them on different layers, among them are google maps, google sattelite and Openstreetmap data.
But be aware: sattelite pictures can have an offset to reality as well !
Opinions are like a…s, everyone has one. here is mine:
In good conditions eTrex hcx can be very accurate. In my opinion map should be accurate without offset. if the map has been drawn based on yahoo/landsat/younameit images and as a result have some offset, then the map should be adjusted based on several gps traces.