For mapping in sweden I am using DigitalGlobe satellite images. The alignment between the different DigitalGlobe images is creating mapping problems. DigitalGlobe standard and DigitalGlobe Premium have an offset of about 15-20m. Tracing the roads on the one makes them wrong on the other images. How should I tackle this problem?
Generally features should be mapped based on the existing offsets in the area, with a preference for using any on the ground surveyed GPS data. It may be necessary to distort shapes slightly if the dominant offset changes from one place to another. so as to avoid abrupt discontinuities, or overlaps.
In JOSM, you can move background imagery to align it with existing features.
DigitalGlobe is known to have offset issues: http://wiki.openstreetmap.org/wiki/DigitalGlobe#Tile_offset
Also remember that imagery can have parallax errors causing offsets to very with height.