Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
665 
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 
mena. 
ORTHOPHOTOGRAPHY 
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 
1:22,000. 
STEREO ORTHOPHOTOGRAPHY 
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 
model. 
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 
map. 
INTERPRETATION OF (STEREO) ORTHOPHOTOGRAPHS 
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 
plant. 
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