> ... it has always been > suggested to check and align them to gps traces from the area (while > keeping in mind that one or few traces might be all wrong, the centre > line of many traces being the best). >
This is certainly the best advice (unless you are in an area with high buildings or steep mountains, conditions which may produce systematic GPS errors, in which case there is no recipe) Apart from simple shift problems, satellite/areal photos may suffer, even considerably, from vertical parallax problems. In my area (Northern Italy) Bing maps have consistently suffered from Lat-Long shift in the order of even 5 meters plus a superimposed parallax error that deforms road shapes further, typically in the mountains. The parallax error becomes worse the steeper the hills/mountains are. To illustrate the parallax problem, have a look at this example in California: https://www.openstreetmap.org/edit?editor=id#map=18/35.99284/-121.48429 Compare the Bing map layer with the Mapbox map layer and also with the GPX tracks.
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