A DEM with elevations varying from 181 to 1044 m derived from SPOT data has been used as a contaminated sample, while a manually derived DEM obtained from aerial photogrammetry has been regarded as the ground truth. That allows a direct performance comparison for the methods with real errors.

We assumed that once an outlier location is suggested, a "better" value can be measured or obtained through some methodology. The options are different depending upon the user (DEM producers might go to the original data and make another reading, while end users might only interpolate). In this experiment we considered both choices.

Preliminary results show that for the available dataset, the accuracy might be improved to some extent with very little effort. Effort is defined here as the percentage of points suggested by de methodology in relation with its total number: thus 100 per cent effort implies that all points have been checked.

The method proposed by López (1997) gave poor results, because it has been designed for errors with low spatial auto correlation (which is not the case here). A modified version has been designed and compared also against the method suggested by Felicísimo (1994).

The three procedures can be applied both for error detection during the DEM generation and by end users, and they might be of use for other quantitative raster data. The choice of the best methodology is different depending on the effort involved. The conclusions have been derived for a photogrammetrically obtained DEM; other production procedures might led to different results.