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1 
AUTOMATIC COMPILATION OF CONTOUR LINES BY LOCAL DTM METHOD 
Bokuro Urabe 
Topographic department, Geographical Survey Institute, Japan 
Commission IV 
Abstract 
With the progress of computers applied to plotters, it has become available to obtain digital contour line data instead of conventional raw 
manuscripts. Therefore, it became possible to edit contour lines with the help of numerical calculation instead of manual redrawing. 
Usually computer aided editing is realized by interactive method, however, it also takes long time as manual editing. In order to reduce 
editing time, an automatic method to compile digital contour lines has been studied. 
Compilation of contour lines is a kind of smoothing of the surface plane expressed by contour lines. Therefore, simple curved surface 
(named local DTM, DTM=digital terrain model) is calculated to represent the local terrain within the neighborhood of the concerned point on a 
contour line. Then the point is to be moved horizontally so that the point is dropped on the computed surface. Local DTM of the neighboring 
points are so strongly correlated to each other that we can obtain smooth contour lines consist of the moved points. 
As an application of this method, generation of DEM(=digital elevation model) raster data is mentioned in this paper. The elevation on each 
intersection of the concerned grid is calculated out by generating its local DTM using neighbouring points. 
KEY WORDS: Local DTM, Editing, Contour lines, DEM 
1. THEORY OF EDITING 
Plotting work can be considered as observation on terrain 
surface by aerial photographs and a plotter. The observed data 
are presented on a mapping sheet by an analogue plotter, while 
they are recorded on a digital media as alist of coordinates by an 
analytical plotter. Observations always have errors. In the case 
of plotting contour lines, both of altimetric and planimetric val- 
ues are observed at the same time, and both observations have 
errors. These observing errors cause undesirable flutter or ille- 
gal crossings of contour lines. 
The editing process can be considered as correcting these 
observation errors for the real terrain. When editing work is 
done by hand, firstly the editor chooses a small area and he/she 
"guesses" the real terrain of the area from the drawn contour 
lines. After that he/she redraws contour lines and corrects them 
against the terrain image generated in his/her mind. 
The Local DTM editing method is developed to replace 
this job by a computer. Instead of guessing the real terrain, a 
local DTM is generated around every point which was recorded 
by encoders. Each point which has planimetric and altimetric 
errors are adjusted on the local DTM, and whole contour lines 
have been redrawn when all points are moved(Table 1). 
2. THE BASIC THEORY OF THE LOCAL DTM 
METHOD 
2.1 Concept of the Local DTM 
The coordinate stream (x.,y;) of a contour line with the 
elevation z, obtained by a plotter, can be considered as a group 
of planimetric and altimetric observations at each point. The 
contour lines consist of numbers of observations with various 
elevations. all over the area. À local DTM (z=f(x,y)) is gener- 
ated around each point collecting planimetric and elevation data 
Table 1 Contrast between manual and Local DTM editing 
  
Manual 
Local DTM 
  
Observation data Plotted manuscript 
Data correction 
  
  
An image guessed from the manuscript | Local DTM 
Redrawing lines by hand T 
| 
| 
| Obtained coordinates 
| (simple curved surface) 
| Calculating corrections 
of observations