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Digital Image Processing 1
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Efficient preprocessing of generating DEM from digitized contour map
Topographic map for digitization
In this study, we use the 1:25, 000 scale contour map made by Geographical Survey Institute of Japan. Principal contour lines and intermediate contour lines are drawn on 1:25,000 scale contour map. The height interval of principal contour lines is 50 m and that the intermediate contour lines is 10 m. Therefore, there are four intermediate contour lines between neighboring principal contour lines. This contour maps has some map symbols such as cliff, rocks, depression in addition to the contour lines and numerals indicating the altitude. To make a multi level contour image, this contour amp was scanned by drum scanner with 50 um pitch. Binary this contour map was scanned by drum scanner with 50 um pitch. Binary contour image is made from multi level contour image by binarizing and thinning.
Attributes of contour
After removing the isolated point noise and unnecessary branches, all lines one the binary and multi level contour image is traced to collect the contour attributes as follows.
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Location
Coordinates of start point and end point of a contour line.
- Length
Total number of pixels which compose an traced line.
- Continutity of contour
Contour lines before connecting process can be classified into four types from their continuity as shown in
Fig. 2.
- Attribute of numerals
This is a flag indicating the possibility of certain traced line whether it is a numeral or not. In the line tracing process, lines on contour image are not identified whether they are contours or numerals. But it can be said that broken or closed short lines have a possibility to be numerals.
- Sum of digital count
Total sum of digital count in a line is obtained by tracing the multi level contour image. After connecting process, the average count of each line is calculated from its length and the sum for identification of the principal line.
Editing digital contour image
- Editing branch off points
First, branch off points are found by raster scan with 3 x 3 pixels window and analyzed their structure by line tracing. Most branch off points caused by binarizing and thinning process have simple structure. It is easy to edit them automatically.
Fig. 3. shows the branch off points which have simple structure and short branches. All branches are traced from branch off point, and the number of branches and their length and direction are examined. It is easy to find unnecessary branches and remove them automatically in the case of Fig. 3. Map symbols such as cliff, however, make complex branches. In this case, editing to remove unnecessary lines in themselves. Therefore, the interactive processing was proposed to edit appropriately.
- Connecting broken lines
Very simple algorithm was used to connect broken contour lines. That is "When searching inside of the circle with radius r, centered on a certain
break point , if another break point is found, connect these points by the straight line". Using this method with fixed radios r, however, many disconnected lines will remain. If r is small, it will be seldom to success the searching. In the opposite case, there will be high probability that more than two break points are found in the circle. Then iteration method with growing r is proposed in order to decrease the above effect.
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