e
27
b) Data-processing in an electronic computer, leading to the computation of
the heights of the profiles which in the next phase (c) will have to be
followed for orthophoto production.
c) Automated orthophoto production in an ortho-printer.
Several instrument manufactuers (Kelsh, Matra, OMI, Wild) have
developed the necessary orthophoto-printer for phase c.
33 Stereo-Orthophotographs
Orthophoto maps provide the photographic image of the terrain. As
indicated above, any map user interested in a certain aspect of the terrain can
in principle extract his own data from the photographic imagery. A conven-
tional photo map, however, does not allow for stereoscopic photo interpre-
tation. This is usually done using the original aerial photography. Any anno-
tations made on these photographs, while observing them stereoscopically,
can then be easily transferred to the photomap (much more easily than to a
line map).
For stereoscopic interpretation from orthophotographs, Collins developed
in 1968 the so-called Stereo-Orthophoto concept. For details, refer to the
Proceedings of the Orthophoto Symposium, Cracow, September 1974 (van
Wijk, Stereo-Orthophoto as a New Concept in Mapping).
From each stereo model, one normal orthophoto and one pseudo-ortho-
photo, the so-called "stereomate", are produced. When observing these to-
gether under a stereoscope a metrically correct, and levelled, terrain model is
obtained.
The planimetry of the normal orthophoto is correct and can thus be
copied (traced). With a parallax bar, correct terrain heights can be measured,
and contour lines plotted. Neither an expensive stereo plotter, nor relative
and/or absolute orientation is required. This system is ideal for photo-inter-
preters who also want to measure heights, slopes, etc.
Note: for the extraction of line maps from large scale orthophotos of cities
with high buildings, this method is not suitable because in the orthophotos
it is the terrain only that is orthogonally projected. All points above the
terrain—such as rooftops—are projected incorrectly, i.e. they are radially
displaced.
