Contour Line Interpolation by using Buffering Method
3. Evaluations of Buffering Method
Evaluations of buffering method were carried out by comparison with existing methods which are profile method and window method. And effects of contour line interval on DEM accuracy were concluded. Items for evaluation is not only elevation but also topographical feature such as slope gradient and slope aspect.
Verification data were generated from 5m grid DEM which were generated by profile method with the linear equation from 1:25000 topographical maps. The 5m grid DEM were resampled to 50m grid size by taking the average in order to make suitable DEM for verification. Moreover, slope gradient data and slope aspect data were generated from the verification DEM. lOOm, 200m, 300m and 400m interval contour line maps were created from the verification DEM. From those contour line maps, DEMs were generated by using each interpolation method.
3.1 Elevation Accuracy
An index of elevation accuracy is used percentages of correct pixels. In case of elevation evaluation, correct pixel means difference with verification DEM indicates inside of 20m. Figure 7 shows relationship between contour line interval and correct percentage in each method. In this figure, accuracy has tendency to drop with increasing contour interval. And buffering method is always located the highest accuracy in all contour intervals. Though window method shows almost same accuracy with buffering method in lOOm contour interval, the accuracy becomes 10% less in other contour intervals. Profile method shows the worst results, because radiated noises influence to accuracy
Figure 7 Relationship between contour line interval
3.2 Slope Gradient Accuracy
A slope gradient can be expressed from DEM, which is one of the most important items for topographical analysis. In this study, the slope mea n s maximum inclination angle at one target pixel. An index of slope gradient accuracy is also used percentages of correct pixels. In case of slope gradient accuracy,correct pixel means difference with verification slope gradient data indicates inside of 20 degree . figure 8.shows relationship between contour line interval and correct percentage in each method. Buffering method is always located the highest accuracy in all contour intervals. Though profile method shows almost same accuracy with buffering method in 100 m Contour line interval, the accuracy becomes 10% less in other contour intervals. The window method Shows the Worst results, because steep slope along contour line Influenced to accuracy .
Figure 8 Relationship between contour line interval and correct percentage of slope gradient
3.3 Slope Aspect Accuracy
A slope aspect can be expressed from DEM, which is one of the most important items for topographical analysis. In this study, the slope aspect Shows along the maximum inc inclination angle at one target pixel. An Index of slope aspect accuracy IS also used percentages of Correct pixels. In case of slope gradient accuracy, correct pixel means difference with verification slope gradient data indicates inside of 45 degree. Figure 9 Shows relationship between contour line interval and Correct percentage in each method. The Correct percentage Shows very lower than elevation or slope gradient, which is indicated less than 68%. However, buffering method is almost located the highest accuracy in all contour intervals.
Figure 9 Relationship between contour line interval and correct percentage of slope aspect
3. Conclusions
In this study, buffering method was developed for small scale maps. The developed method was compared with existing methods on elevation, slope gradient and slope aspect. In all items, the buffering method showed very good results.
Existing method had some problems in one items of topographical features. For example, profile method has much error in elevation value, because there are many errors along calculated profile. Window method has much error in slope gradient, because there are many errors along contour lines. Anyhow, existing methods didn't Show good results in case of wide contour intervals.
A contour line interval influenced accuracy of DEM. When contour line interval becomes over 300m, a Correct percentage becomes less than 50%. A percentage of pixels which are consisted contour line is about 10% in case of 300m contour interval. Moreover, in case of loom contour interval, the percentage becomes 30%. Therefore, 20% contour line information on whole map are required for reliable DEM generation.
In this paper, calculation speed was not evaluated. Actually, buffering method spends much time in comparison with existing two methods. However, if we need high accuracy DEMs, it will not be So serious problem. And Computer is progressing rapidly.
References
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Sukit Viseshsin and Shunji Murai (1990), "Automated Height Information Extraction from Existing Topographic Map", International Archives of Photogrammetry and Remote Sensing, Vol.28 Part 4, pp.338 -346
- Kiyonari Fukue, Yousuke Kuroda, Haruhisa Shimoda and Toshibumi Sakata (1990), " Simple DEM Generation Method from a Contour Image", International Archives of Photogrammetry and Remote Sensing, Vol.28 Part 4, pp.347 -355
- F. Ackermann (1994), " Digital Elevation Models -Techniques and Application, Quality Standards, Development", International Archives of Photogrammetry and Remote Sensing, V 01.30 Part 4, pp.421 -432
