Hi everyone, Forgive me if this has been covered elsewhere, but I am wondering what can be done about the need for aerial photo offsets. There's very little in the wiki[1]
In Korea (as elsewhere, I assume) the aerial photos are not always accurately aligned with reality. Across the country different offsets are needed (sometimes no offset). What I do is look for a set of GPS traces from a very visible landmark and then use them to align the aerials. If there are none I will often find a visible landmark, such as a park, or a running track in a stadium, and make several GPS traces of my own on different days, so that I can use them to line up the aerials. I know that one trace is insufficient. I know there are tools in JOSM and Potlatch to move the aerial photo layer around, and JOSM even stores them for future use, but surely we must all have the same problem around the world? Is there any project underway to record the offsets centrally, so that we don't all have to have our own (slightly different) offsets recorded? It's a big problem when mappers map something *only* from aerials, and don't compensate for the offset (this is different from the parallax error from tall buildings). When I map the same area later with GPS traces I often find that I have to move many objects. So, I have two suggestions: 1) Have all mapping software pop up a warning when a mapper chooses to display aerial photos. The warning could say "Warning: Aerial photos may be slightly offset. Please check photo alignment before tracing aerial features". 2) Have a central repository of offsets for the main aerial photo providers. In the application (JOSM, for example) we can choose to automatically use the offset for the current viewport, or our own local one, or submit a better one. Any comments? Any suggestions to improve my technique? Any pointers to something that already does the Right Thing? Thank you, Andrew [1] <http://wiki.openstreetmap.org/wiki/Yahoo!_Aerial_Imagery/Accuracy#Mapping_with_an_offset> _______________________________________________ talk mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk
