The image quality of modern orthophoto systems combined with stereo 
orthophoto interpretation, and the measuring and plotting facilities 
introduced by the National Research Council of Canada will without 
doubt result in an increased use of orthophotos. The possibility of 
a precise determination of X, Y and Z coordinates offered by the stereo- 
orthophoto technique, will extend the use of orthophotos to the measure- 
ment of terrain profiles, spot elevations, and heights of terrain 
features such as buildings and trees. Contour lines, representing the 
same amount of detail as obtained from conventional-type photogrammetric 
methods, can be plotted directly from the stereo-orthophotos. The 
stereo-orthophoto technique will therefore take an important place in 
transferring orthophotographs into orthophoto maps and line maps. 
‚The accuracy of vertical terrain information obtained from stereo- 
orthophotos has been evaluated theoretically and it was proven that 
the elevation errors are considerably smaller than the usual plani- 
metric error of orthophotographs (Ref.: Blachut, van Wijk 1970). 
Lately a number of large-scale mapping experiments, based on the 
stereo-orthophoto technique, have been undertaken. Some of the 
results obtained from this work are described here. 
The basic principles of the stereo-orthophoto technique 
  
o 
À stereo-orthophoto pair consists of an orthophoto and a stereomate, 
produced by differential rectificatios of two adjacent photographs 
which form the stereoscopic model. The stereomáte is produced in 
such a way that it contains image shifts proportional to the terrain 
heights and parallel to the flight direction. When the orthophoto 
and the stereo-mate are viewed stereoscopically, the terrain surface 
appears three-dimensional because of the x-parallaxes introduced by 
the image shifts in the stereomate. Terrain features which are not 
 Sübject to the differential rectification process, such as buildings 
amd trees, will be projected with their original parallaxes with 
respect to the tegrain, and they also appear three-dimensional. 
In order to create a smooth, three-dimensional model from stereoscopic 
orthophotos it is important that the artificial differential parallaxes 
be compatible with the basic parallax equation. In this case the 
vertical appearance of the three-dimensional model will not be affected 
by approximations in the rectification process, such as caused by 
dimensions of the scanning slit. 
Orthophotos and their stereomates should be produced simultaneously or 
' based on the same profile information to make the technique economically 
attractive. Moreover, simultaneous production is required in order to 
assure optimum accuracy (Ref.: Collins 1968). 
An instrument in which this principle has been realized is presently in 
an advanced state of completion at the N.R.C. This instrument, called 
the Orthocartograph, is based on simultaneous optical transfer of 
corresponding aerial photographs on two separate photographic emulsions 
(Ref.: Blachut 1971). 
-7- 
The geometry c 
ment in order 
its stereomate 
operations. À 
developed for 
with the mappi 
In the Stereoc 
on two horizon 
Movement of th 
X-movement of 
photo allows o 
observation sy 
done on transp 
upon the ortho 
plotting of co 
placed on the 
plotting opera 
is important i 
A later versio 
linear encoder 
Vertical accur 
If the surface 
of the film at 
mate, any rele 
and model heig 
would be used 
height differe 
certain featur 
plane of the f 
either below c 
of the scannir 
will create o[ 
to avoid discc 
x-parallaxes ( 
parallax relat 
where p = art 
x 
B = bas 
H = ave 
h = ele 
pre 
The following 
to profiling : 
fication proc: