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I am new to this and started to make a local minor road in rural France near my home conform more closely to the satellite image. But then when I got to the main road I noticed it was a bit off too - and to correct it would have meant moving some buildings too! Is the satellite image always right? If I am going to do more work on my local area I need to know which to trust. |
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"Is the satellite image always right?" Defintely no. Note what we tend to call "satellite imagery" is actually taken from aeroplanes and needs to be georeferenced and corrected for distortions caused by viewing angle of the camera and varying elevation of the ground (hills, mountains etc.). Depending on well this processing is done, the imagery can have errors around dozens of meters down to a sub-meter range. This can change substantially even over short distances. For example where I live Bing imagery is quite good (sub 1m error), 10km away from here Bing has errors of over 30 meters. The only way to determine if the imagery has an offset in the immediate area you are working on, is to compare it to other sources. For example:
Once you have determined if or if not the imagery has an offset that is large enough to need correction at least Potlatch 2 and JOSM allow you to adjust the position of background imagery. No the Satellite image may be a bit out of alignment. The best thing to do is collect some gps traces upload them and also use what is already been uploaded to check alignment you should then have enough to get a good average. Then if you are sure alignment is wrong it is possible to drag the aerial image so that it aligns correctly. In potlach2 just hold space bar and drag. also see this Question alignment of track vs background imagery
(07 Jun '12, 22:00)
andy mackey
Many thanks to both. Can I just check then: Many of the buidings are from French cadastre information. It sounds like I can't go wrong with adjusting to conform with that?
(08 Jun '12, 12:17)
Oldnog
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I would check with the French community if there are any known issues in your area, but otherwise if it seems to be consistent with other information (GPS tracks mainly in this case): yes.
(08 Jun '12, 13:07)
SimonPoole ♦
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When I looked at this further, I found that when I matched the cadastre information on one view, all was OK and I could put the roads etc in the right place. But when I moved along a road to a new view, the two slipped out of alignment again. In other words there is no single "right" match. I dealt with this by re-aligning each time I moved the map, so the new road is always in the right place relative to local buildings. It works, but there is a risk of minor errors or disjunctions along the new way. My conclusion is that the cadastre information is accurate but that there is not an exact 1:1 relationship to the satellite image, in other words that the latter is slightly warped. Beam me up Scottie.
(09 Jun '12, 13:51)
Oldnog
... maybe http://wiki.openstreetmap.org/wiki/True_Offset_Process can also help.
(16 Jun '12, 21:54)
stephan75
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Similar question: http://help.openstreetmap.org/questions/1200/which-is-more-reliable-gps-traces-or-aerial-imagery