Hi All Thanks for the replies, I re-examined my tracks, existing roads and the Yahoo layer - if only we had an extra zoom level or two it would be more obvious ...
The island in question is Menorca. The road that I travelled every day, due to a one way traffic system, is along the edge of an east-facing limestone cliff with a sharp vertical drop to sea level. So high water and low water are the practically the same from a satellite view. The road-to-cliff edge distance is mostly about 1-2 times the GPS stated accuracy. The GPS tracks were consistently plotted further west than expected relative to the PGS data coastline, which at that point matches the Yahoo layer pretty well. Repeat journeys matched very well. Looking at other thin promontories, this was consistent along the coastline. Some inland roads also appear displaced. I then loaded the same GPS tracks into another well known satellite viewer which allows higher zoom levels. They all seemed to have a more accurate relation to the coast. I checked this at various locations around the island and a consistent shift is seen. So local coastal adjustments won't be sufficient. My naive conclusion is that both the whole PGS and Yahoo layers need to be shifted, together, in the south west direction to get everything lined up with the road data. Is this possible and acceptable? If so, any hints as to how to do this would be welcome. regards, Martyn _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk