Accuracy - Which positional accuracy can be achieved?

Unfortunately there is no simple answer to that question. At first hand this will depend on the reference data you are able to provide. When we have to rely on on-board GPS-recordings - may be supported by barometric altitude measurement - results will be less exact and may show an overall positional error (Root Mean Square Error, RMSE) of several meters. The performance of your on-board system is essential at this point - differential GPS or not, if differential, which reference signal .... WAAS/EGNOS, ground-based like SAPOS or BEACON??? One important factor is, that you operating the GPS on a moving platform, so you can't do average measurements over a period of time.
Positonal accuracy will improve greatly when you provide us with ground control points or survey data. Just to give an example:
Using GPS-measured ground control points we were able to achieve an RMSE of 0.25 meter for an image mosaic comprised of 310 CANON EOS 550D images, that were taken over a steeply sloped alpine landslide environment with height differences of 200 meter on a horizontal distance of 500 meter.