we adopted this approach, not only focusing on the the horizonal distance of the reference data but selecting the three-dimensionally closest reference point to the test data point. the nearest point of the reference data was only considered when it was within a distance of 0.5m.
if a normal distribution of the errors is assumed and there are no outliers present or have been removed, the following accuracy measures are usually used to describe the dem globally (hohle and hohle,2009)