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| ŠUMARSKI LIST 11-12/2018 str. 13 <-- 13 --> PDF |
output values of the analysis are always negative for concave DTM features and positive for convex DTM features (Figure 4) (Mitášová and Hofierka, 1993). When detecting error points, it does not matter if the points underestimate or overestimate the terrain, the absolute value |T| was used to create the resultant raster. Analogously to the slope analysis, if the areas with high |T| values (|T|>0) are in the nearest neighbourhood of a spot height or mass point, this may indicate a gross error at that point. By combining the slope and tangential curvature using the expression: R=|T|·S, the resultant raster (R) was calculated. From the resultant raster (R) the potential error point areas were selected (5% maximal values of R, according to Schultz et al., 1999) and extracted in a new binary raster RS. To simplify the raster geometry of selected areas, the two-step generalisation process (2 pixel expansion followed by -2.5 pixel shrinking) of the RS was performed (RS-2nd) (Ablameyko and Pridmore, 2012). In the final step, the generalized RS-2nd raster was vectorized and overlapped with the point vector data of the original DTMPHM. The error points were detected and removed from DTMPHM to produce the corrected point data DTM (DTMPHMc). The DTMPHMc and DTMPHM were generated in the Global Mapper software because the triangulation process is much faster than in Grass GIS. Accuracy assessment – Ocjena točnosti To evaluate the proposed method, a difference raster model between DTMPHM and DTMLiD, as well as between DTMPHMc |