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drawings, such as stereoscopic points, lines and circles, 
can be drawn in Video Random Access Memories 
(VRAM) using 3D graphic software which has been newly 
developed using two—dimensional graphic software 
provided in the PC operating system. The left image and 
drawing, and the right image and drawing, are overlaid 
respectively and displayed alternately. The observer can 
see a stereoscopic model by using liquid crystal shutter 
glasses, which are synchronized with the display. Photo 1 
is the overview of the system. The most significant 
feature of this system is that the total cost of the system 
is lower and the size is smaller because recent excellent 
digital equipment is employed in the system. 
  
  
  
  
  
Photo 1 The overview of the digital photogrammetric 
system 
2.2 Software for the digital photogrammetric system 
This system can extract DEMs of any spatial resolution 
from digital stereoscopic images and can draw contour 
lines of any interval by using the DEM. The stereo 
conjugate points in stereoscopic images are determined 
automatically by stereo matching, using an image 
correlation method. Because errors in stereo matching 
can be corrected easily by using the 3D display module, 
precise DEMs can be extracted from digital stereoscopic 
images. The details of this function will be explained in 
the next chapter. Photo 2 shows an orthoimage and 
overlaid contour lines, which are produced from 
Stereoscopic images by this system. 
2.3 Stereoscopic image processing software 
A stereoscopic image processing system must be very 
useful in fields which need topographic information, 
especially when it has a man- machine interface as 
mentioned above. Now there are scarcely any 
Stereoscopic image processing systems which have these 
267 
  
Photo 2 An orthoimage overlaid by contour lines. 
abilities. Therefore development of the stereoscopic 
image processing programs on this digital 
photogrammetric system was attempted. The functions of 
the programs are as follows: enlargement and reduction, 
brightness enhancement, affine transformation, calculation 
between images, cutting out and displaying a part of a big 
image, etc. A program has also been developed to 
produce stereoscopic images of any B/H ratio from a 
digital orthoimage using DEM. This system has the 
functions of both a digital photogrammetric system and a 
stereoscopic image processing system. 
3. PRECISE DEM EXTRACTION BY USING 
THE 3D DISPLAY MODULE 
3.1 Inputting positions of stereo conjugate points 
Generally speaking, image recognition is very difficult for 
a computer because there is too much information in 
images to be analyzed. It is also true in the case of stereo 
matching, which is one technique to determine stereo 
conjugate points automatically with digital photogrametric 
systems. Now the errors in stereo matching are a very 
serious problem for automatic DEM extraction. It seems 
that the best way is to get human help at appropriate 
stages. In order to get human help, a man- machine 
interface is necessary. The 3D display module of this 
system can provide the man- machine interface. In 
automatic stereo matching, the man- machine interface 
can be used to reduce matching errors at two stages, 
before and after the stereo matching. 
Before the stereo matching, some stereo conjugate points 
can be provided to a computer by using the 3D display 
module. Because this module can display stereoscopic 
images together with stereoscopic drawings of points, the 
height of the points can be easily adjusted to the surface 
elevation of the stereoscopic model by using a mouse 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996