1. Presented at SCANGIS 97 On the improving of elevation accuracy of Digital Elevation Models: a comparison of some error detection procedures Carlos López http://www.fing.edu.uy/~carlos Centro de Cálculo Facultad de Ingeniería Montevideo URUGUAY

2. Presented at SCANGIS 97 Our goals: n Improve elevation accuracy of grid-based DEM n Use general methods as far as possible n Satisfy both DEM producer & end user needs

3. Presented at SCANGIS 97 Some assumptions... n a better value for the elevation can be obtained, but it might be too expensive to acquire it directly (GPS?) n once detected, the errors are corrected (“perfect inspector” hypothesis) n the editing cost is proportional to the number of candidate points

4. Presented at SCANGIS 97 Organization of the presentation n Description of the three procedures n Results for the Mt. Sainte Victorie DEM n Discussion n Conclusions

5. Presented at SCANGIS 97 The method by López (1997) n A brief presentation of PCA n The method for an elongated DTM n The generalization to any DTM ( IJGIS 1997, 11, 7, 677-689)

6. Presented at SCANGIS 97 A brief presentation of PCA n attempts to explain the behavior of clouds of points in R w reducing the dimensionality of the data n it is usually applied to tabular (not raster!) datasets n the starting point is the cloud; all ordering among points (profiles) is lost

7. Presented at SCANGIS 97 The elongated DTM case n The process requires two phases: –Identify the ¨suspicious¨ profiles –Analyze each of those profiles trying to pick in each the best candidate(s) for being an error n Any other procedure for tabular dataset can be used instead

8. Presented at SCANGIS 97 Some remarks... n Even though we use PCA, our approach is not the standard one used in image processing n We do not use nor assume at all any model of covariance in respect with distance for the elevation n We locate errors based only upon the elevation (we will not consider slope neither curvature)

9. Presented at SCANGIS 97 The generalization to any DTM n Any DTM can be considered as build from elongated ones, without intersection n We might look within each of those to locate errors n The procedure can be applied row-wise as well as column-wise n The most likely errors are those which are candidates both for column and row-wise analysis

10. Presented at SCANGIS 97 The method of Felicísimo n Compares a local best fit with a low order polinomial; the gross errors are obtained after an analisys of the residual n It is based on very simple hypothesis –uncorrelated errors in space –gaussian distribution of the errors n It can be easily implemented ( JP&RS 1994, 49, 4, 29-33)

11. Presented at SCANGIS 97 Some problems of both methods n The assumptions of errors weakly correlated in space do not hold at least in the considered example. n Moreover, the performance of both method decreases if the spatial correlation increases

12. Presented at SCANGIS 97 The modified procedure n Since adjacent profiles are too correlated, we formed the strip choosing every k th. row from the DEM. n The implementation and the rationale are almost exactly the same as before. n It considers as a particular case the previous code. ( TOG 2000, 4, 1, 43-64)

13. Presented at SCANGIS 97 The experiment n We used a SPOT derived DEM as a test bed, and consider another DEM of higher accuracy as the ground truth. n Once a location is selected, we correct the noisy DEM using the values from the other. The same point cannot be corrected twice. n We corrected as much as 15 per cent of the DEM. Various measures of the accuracy were recorded.

14. Presented at SCANGIS 97 Discussion (1) n The new method outperforms the previous in the low effort region n The Felicísimo’s method in the long run gets mostly systematic errors n The RMSE might drop from 12.7 m to 11.0 m by checking only 1 per cent of the DEM; the max error drops from 193 m to 100 m n For efforts over 2.5 per cent the Felicisísimo’s method becomes better

15. Presented at SCANGIS 97 Discussion (2) n Given a DEM, there are some parameters to be defined for our method. The suggested rules gave reliable values. n Once a suspicious point is suggested, some action needs to be taken. Different users might have different goals. n Despite the complexity of the details, the procedure requires only modest computer resources.

16. Presented at SCANGIS 97 Conclusion (1) n Some advantages of the procedure –It is valid for any raster dataset –It might be of use both for data producers as well as for end users –It has some free parameters which can be tailored for specific needs, but we provide rules suitable for a first guess –It does not require any “model” for the dataset, neither at local nor global scale

17. Presented at SCANGIS 97 Conclusion (2) n Some drawbacks of the method presented –It has been tested only with one DEM –It left unexploited some (maybe) important information from the dataset like the spatial correlation n Future work –should compare the methods with other DEM’s representative of different terrain characteristics