"On the improving of elevation accuracy of Digital Elevation Models: a comparison of some error detection procedures"
Centro de Calculo, Facultad de Ingeniería (11), Universidad de la República
Julio Herrera y Reissig 565, Montevideo, URUGUAY
The widespread availability of powerful desktop computers, easy-to-use software tools and geographic datasets have raised the quality problem of input data to be a crucial one. Even though accuracy has been a concern in every serious application, there are no general tools for its improvement. Some particular ones exist however, and we are reporting here results for a particular case of quantitative raster data: Digital Elevation Models (DEM). We tested two procedures designed to detect anomalous values (also named gross errors, outliers or blunders) in DEM, but valid also for other quantitative raster datasets.
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 (end users might only interpolate, while DEM producers might go to the original data and make another reading). In this experiment we simply put the ground truth value.
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 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.
KEYWORDS: DEM, quality control, blunder location, gross error location, accuracy
Presented at:ScanGIS'97 Stockholm, Sweden 1-3 June