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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Use of (stereo-) orthophotography prepared from aerial and
terrestrial photographs for engineering geological maps and plans
Niek Rengers
International Institute for Aerospace Survey and Earth Sciences (ITC), Enschede, Netherlands
ABSTRACT: Orthophotography, prepared by optical mechanical correction of aerial or terrestrial photography,
is imagery with a uniform scale and without relief displcement. Together with so called "stereomates", pre
pared from overlapping imagery, stereovision can be obtained. Examples of stereo-orthophotography are shown
with their engineering geological interpretation. A comparison is made of the interpretability of stereo
orthophotography at various scales as compared with normal stereo photography for various terrain details
which are used for delineation of engineering soil and rock units as well as for slope instability pheno
Stereo aerial photography and stereo terrestrial
photography are important tools for the engineering
geologist. Interpretation of the stereo imagery
under the mirror stereoscope is very helpful during
the early stages of preparation of an engineering
geological map or plan for a civil engineering pro
ject (IAEG, 1981).
One of the main problems encountered when using
photography is that due to the central projection
the image shows important variations in scale and
radial relief displacements when the terrain is not
completely flat. Thus the transfer of information
from a photo overlay to a topographical map (a ver
tical parallel projection of the terrain on the
horizontal datum plane) causes a large amount of
extra work and an important source of errors.
Orthophotography offers an elegant way out of
these problems. An orthophotograph is made from an
ordinary aerial photograph by optical-mechanical
correction of the radial relief displacement and
scale differences with help of an orthophoto pro
jector. With modern equipment orthophotography ima
gery of excellent quality can be produced. The
photographic quality is not notably affected by the
projection process. The topographic quality (pos
itional accuracy) depends to a large degree on the
accuracy of the available digital terrain model but
in normal circumstances errors in position should
not exceed the order of magnitude of 5-10 m on
orthophotography at a scale of 1:10,000 (< 1 mm in
the orthophoto). At these scales such an accuracy
for an engineering geological map is quite accept
able. Orthophotography can be printed on the (uni
form) scale of the map on which the engineering
geological information must be plotted. The transfer
of data from a photo to its orthophoto is still
extra work but the sources of error have decreased
strongly as features on the photo and orthophoto can
be correlated much easier with each other than fea
tures from photo and map.
Orthophotography can be made from aerial photo
graphy, but also from terrestrial photography if a
metric camera was used to make the terrestrial
photographs (Grabmaier, 1983).
Figure 1 shows part of an aerial orthophotograph
at a scale of 1:5,000 onto which the contour lines
of a topographic map with contour line interval of
10 m are printed. This orthophotograph has been
prepared from original negatives at a scale of
With help of the orthophoto projector a "stereo
mate" can be produced of photography overlapping
the photography from which the orthophotograph was
prepared. Into the stereomates an artificial relief
displacement in x direction is introduced which
causes x parallaxes between orthophotograph and
stereomate which equal the x parallaxes of the
original stereo model.
When orthophotographs and stereomate are viewed
stereoscopically with a mirror stereoscope a three
dimensional image of the terrain is visible which
is not distinguishable from the original stereo
When stereo orthophotography are prepared exactly
at the scale of the map to be prepared the overlay
on which the photo interpretation results are plot
ted can be used directly as a map after introduc
tion of the contourlines from a topographic base
Aerial photography.
The interpretation of aerial stereo orthophotogra
phy follows exactly the same lines as the inter
pretation of normal aerial photography. Engineering
soil and rock units, slope instability phenomena
and erosion features can be recognised and outlined
on the photo-overlay (Soeters and Rengers, 1981). A
comparison of the quality of the engineering geo
logical interpretation between stereomodels of
aerial photographs and their orthophotographs and
stereomates has shown that orthophotography gives
comparable and in some cases even better results
(Sissakian, Soeters and Rengers, 1983).
Terrestrial photography.
In cases where vertical outcrops have to be mapped
in detail for engineering purposes, the transfer of
information obtained from stereoscopic interpret
ation of overlapping terrestrial photography to a
base map is particularly difficult. The reference
plane should be a vertical plane on which the in
terpretation results must be plotted with parallel
projection in a direction normal to the reference
In this case orthophotography (fig. 3) can be
particularly useful and the digital terrain model
obtained photogrammetrically from the metric photo
graphy can be used to plot as well an equidistance