Ig points
criterion
ine Dapf
line and
ions can
cement,
Anselm Schmieder
4 AUTOMATED SYSTEM
The tasks of the various modules depicted in Figure 3 are:
Extraction: The number of height levels is calculated from map scale requirements and InSAR DEM elevation range.
To extract initial contour lines a line-following method is applied to the DEM.
Segmentation: Contour lines are either closed or open. Open contours touch the border, therefore previously closed
contours touching the border have to segmented into open contours.
Thresholding: Depending on the map scale, contours are drawn around objects of a given minimum extension. Thresh-
olding eliminates contours describing objects which can be neglected.
Approximation: This is the contour smoothing step discussed in the previous section.
Labeling: The inscriptions are placed constrained to some placing and direction rule.
Approximate continuity between independently processed mapsheets is ensured through processing of a larger view, i.e.
adding of a border, and cutting off this border after smoothing. Conversion of labeled contours to Drawing Exchange
Format (DXF) is a straightforward final processing step.
DEM
Contour line
extraction
Segmentation
ne and thresholding
Map ^N
rs
\ requirements /
3 ee a - .
Approximation
Labelling
Figure 3: Contour derivation, approximation and labeling from DEM
5 RESULTS
5.1 Performance Measurements
Table 1 gives estimates for the completion of contour line processing, including file output in DXF format.
| Image size | Number of polygons | Computation time |
512 x 512 20 17 sec.
1024 x 1024 50 48 sec.
2113 x 2077 140 5 min. 35 sec.
3000 x 1450 21837 12 hrs. 21 min.
Used machine: Pentium II, 233 MHz, 256 MB RAM
Used system: IDL 5.0 on Linux x86
Table 1: Performance measurements of the contour line extraction, smoothing and output process
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 275
