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The stereo-orthophoto technique, developed at the National Research Council of Canada 
n, 4] , provides a means for reconstructing a geometrically correct stereoscopic model from the 
orthophoto, the latter being supplied with a "stereomate'' for this purpose. Since image displace- 
ment in the original photographs caused by differences in terrain relief are corrected during the 
orthophoto production process, the stereomate must have special geometrical characteristics. 
A. differential rectification process similar to that used in the production of orthophotos 
is also being applied for producing the stereomate ; however, the basic difference is that image 
shifts are introduced proportional to height differences. These shifts are parallel to the base of 
the original aerial photographs (x-direction) and replace the x-parallaxes in the original stereo- 
pair of central perspective photographs. 
The orthophoto and its stereomate could be produced from the same photograph. However, 
to provide stereoscopic interpretation of small terrain features (such as buildings and trees) and 
to assure that the continuity of the terrain surface is not affected by the scanning pattern, it is im- 
portant that the orthophoto and its stereomate are produced from different photographs of the ori- 
ginal stereo-pair. ; 
The artificial x-parallaxes in the stereomate should be introduced according to the approxi- 
mate base/height ratio of the original aerial photographs. From the theoretical point of view a non- 
linear approximation of the base/height ratio will give the best results (5] . For most applications, 
however, a linear approximation of this ratio will be sufficient. Whatever function is used for in- 
troducing the artificial x-parallaxes in the stereomate, it is essential that the same function be 
applied for reconverting these parallaxes into height differences, if the stereo-orthophotos are to 
be used for terrain height measurements. : 
To safeguard a satisfactory height accuracy of stereo-orthophotos the orthophoto and the 
stereomate should be produced from the same terrain profile information [3, 4]. This, of course, 
is also essential for an economical and fast way of producing the stereomate, either simultaneously 
with the orthophoto or, in the case of an off-line mode of production, on the basis of the same pro- 
file information that was used for producing the orthophoto. 
An instrument under devefopment at the NRC for simultaneous production of the orthophoto 
and its stereomate is the Orthocartograph [2]. It consists basically of an analog photogrammetric 
plotter and two printing units for recording the orthophoto and stereomate images. The photogra- 
phic images are transferred from the plotter to the printing units by an optical image transfer sys- 
tem, containing the necessary components for image correction. Eventually the instrument could 
be used in both on-line and off-line modes of operation. Orthophotos and stereomates are produced 
simultaneously also at the University of Guelph (Canada), using the SFOM 693 white light system. 
The original projecting table has been modified to contain both orthophoto and stereomate images 
using a 2.2x enlargement. 
In the off-line mode of operation stereo-orthophotos are produced on the orthophoto-printer 
developed for the Analytical Plotter at the Rome Air Development Centre (U. S. A.), using previously 
stored profile data [6, 7]. The off-line approach taken at Matra (France) produces stereo-ortho- 
photo pairs using the Matra-SFOM 910 third camera orthophoto system. 
An important feature of the stereo-orthophoto technique is that the y-parallaxes in the ori- 
ginal photographs are eliminated during the production process. The entire area covered by a ste- 
reo-orthophoto pair can be stereoscopically observed while the orthophoto and the stereomate are 
positioned on a plane support such as a.light table. No need exists for changing the relative posi- 
tions of the orthophoto and the stereomate once they have been properly aligned. This offers the 
possibility of an interpretation, measuring and plotting system, including the recording of height 
information, using stereo-orthophotos and simple inexpensive equipment. 
An instrument, the Stereocompiler, has been developed at NRC for this purpose. It con- 
sists basically of two horizontal plate carriers for supporting the orthophoto and the stereomate [1]. 
These carriers, which are mounted on one single carriage can be moved in a parallel fashion with 
respect to a stationary observation system which contains the reference marks for the measuring 
and plotting operations. In addition to this common movement the stereomate carrier can be moved 
with respect to the orthophoto carrier in the x-direction allowing the measurement of elevations. 
Plotting of planimetric and height data is done on a plotting sheet or scribe coat placed 
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