I read in P2 wiki that SPACE move the imagery.
How can be sure which is the right position for aerial imagery?
Take as example this link:
http://www.openstreetmap.org/?lat=45.548133&lon=9.208469&zoom=20
go to edit mode, in the current zoom the two refuel buildings seems drawn not exactly, with edges shifted.
Try to zoom in two times, then the edges are perfect.
Why different level imagery are shifted?
Different imagery, different georegistration, different accuracy. Look to the left of the buildings - in the original zoom (20) and plus 1 (21) you see some construction going on; also note the cars north of the buildings. If you zoom in again (zoom 22), you’ll see that in place of the construction there is a lorry parking lot now and the pavement is marked with arrows; plus that imagery is of noticeably better quality.
This is why one has to be careful while tracing Bing imagery. There is one spot near where I live where Bing imagery is off by several meters.
See this guide: http://wiki.openstreetmap.org/wiki/Using_Imagery
If possible, its a good idea to record some GPS tracks, when travelling along a road or path visible in the imagery. Then see how well these line up. Of course, GPS can be inaccurate, so best to get a number of traces, on different days and times, then take an average.
ok, understood.
Seems the one with higher zoom are recent, while the construction one (lower zoom) is older.
How can I know the shot date for every zoom level?