A Development of DEM Generation System
4. Contour/Grid Conversion
Contour/Grid conversion is a so extensively researched topic and there has been reported
a lot of algorithms for this problem. However, the available algorithms often generates peculiar grid data which contains unnatural linear incontinuities as shown in Fig.4.
Figure 4 Quality Problem of the Previously Proposed Algorithms
In general contour/grid conversion process consists of the two sub-operations. This is, searching for the standard points for the interpolation and interpolation itself. Among the two sub-operation the later is straight-forward and not so much problem. Mainly the former is thought to be the cause of the peculiar results . for example, Fig.5 shows a well-known contour/grid conversion algorithm, in which a contour map is scanned, and the detected contours are interpolated by a spline curve. As shown in the figure, the standard points for interpolation abruptly changed at the scan point indexed by an arrow mark, which results in the incontinuity of the grid data as shown in (b) of the figure.
Figure 5 Cause of Incontinuity
In the developed system, a new contour/grid conversion algorithm which do not require the searching for the standard points for interpolation is adopted, In the proposed algorithm, instead of searching process, contour itself is propagated to the region by Euclidean distance transformation. As a result of the transformation, the distance to the contour to the contour from every to-be-interpolated points is automatically obtained2).
Fig.6 (a) is the experimental contour image, and Fig.6(b) is the result generated by adopted algorithm. The incontinuities scattered in Fig.4 is not shown in the result and the quality of the DEM is quite satisfactory.
Figure 6 Experimental Result
5.Evaluation
To evaluate the performance of the developed system, a throughput which is defined as a time to generate DEM from a topographic map is measured (a.1:25,000 topographic map published by Japanese geographical survey institute is used). also, throughput of the methods which are adopted by the organizations who generates DEM from topographic map is surveyed for the comparison.
Table 2 is the result. Because the flows of the surveyed methods vary, each operation steps are grouped into contour recognition(includes contour extraction and restoration), labeling and contour/grid conversion. As shown in the table, totally 40 to 207 hours for 1 page of topographic map. On the contrary, in the developed system, only 8.5 hours were necessary.
Table 2 Comparison on the Throughout
| |
Surveyed Methods |
Developed |
| |
A |
B |
C |
System |
| Contour Recognition |
32.0 |
92.0 |
200.0 |
3.5 |
| Labeling |
4.0 |
8.0 |
5.0 |
3.0 |
| Contour/Grid Conversion |
4.0 |
0.5 |
2.0 |
2.0 |
| Sum |
40.0 |
100.5 |
207.0 |
8.5 |
6.Conclusion
Even though there exist wide need for DEM, it has been very difficult to satisfy them because of lack of efficient DEM generation technology. In this paper, a new DEM generation system is reported .
In the developed system a new restoration algorithm is adopted, which resulted in 5 to 24 times improvement of throughput. Also, by using a contour/grid conversion algorithm which uses Euclidean distance transformation, DEM of satisfactory quality could be generated.
References
-
Lee, Fukue, Shimoda,Sakata,"Restoration of contour image by using adjoining property", journal of the Japan society of photogrammetry and Remote sensing, Vol.34.No. 2,pp45-53. 1995.
- Lee. Fukue, shimoda, sakata, "A contour/grid conversion method by using euclidean distance transformation," journal of the Japan society of photogrammetry and remote sensing, Vol.33.No.6,pp26-34, 1994.
