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Hi All, I have a question regarding the offset required in JOSM to ensure mapping accuracy in China. Fiddling about a bit, it seems that the offsets required in JOSM to put the imagery in the right position are 493.42; -237.87. These seem like massive numbers to me as I have seen somewhere that Bing is normally offset by 8.00; -6.00. Are my numbers correct are am I missing something? This link to Geofabrik's excellent map compare tool shows the discrepancy. Thanks in advance, Feilipu |
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The Bing imagery seems to be correctly aligned in this area: It fits the OSM roads perfectly, but more importantly it fits the available GPX data fairly well (tick "raw gps data" when downloading an area in JOSM; there isn't any traces rigth there, but there are a few further south). The big discrepancy you see is between GoogleMaps and OSM, but why would you assume that GM is more correct ? As it happens, the Chinese government is known to insert big random errors when they provide map data to the public and especially to foreign companies. Google is distributing this intentionaly-erroneous data as-is, whereas OSM has gathered it own data, without artificial errors. Aligning satellite imagery is hard. Search this help site or the wiki to get a few techniques. Ultimately, having many GPS traces available is still the best reference material. So if you can provide GPS traces of this area, please do :) 1
Thanks Vincent, nice answer :) I shall use 8.00; -6.00 in future. GPS traces here? No chance, for me anyway. Sensitive place you know, must be that scret sbmarine b*se that is visible on Google maps:) Best, Feilipu.
(25 Apr '13, 15:24)
Feilipu
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Whereabouts are you seeing GPS traces in order to align the map? I don't see any on that island at all...
@SomeoneElse There's a handfull of traces further south. I think the OP was using GM as a reference to think that the imagery needed aligning.