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technology.
The final step is to take the orthophoto file generated
by the digital photogrammetric software and produce
a hardcopy output, which for this project was a laser
jet printer.
6. ORTHOPHOTO PRODUCTION
An orthophoto is a photograph which has been
corrected for the effects of aeroplane tilt and terrain
variation.
A stereoscopic model is formed from the
stereoscopic images by the photogrammetric
software. This process establishes the tilts of the
aeroplane at the times of exposure. Once the
stereoscopic model is established a digital elevation
model (DEM) is generated by the photogrammetric
software. The software uses automatic correlation
techniques to match corresponding points on the
imagery and to calculate individual point heights.
The established tilts of the aeroplane together with
the digital elevation model of the terrain are
combined by the software to produce an accurate
orthophoto by correcting for the effects of tilt and
relief displacement.
7. RESTRICTIONS WITH VIDEO IMAGERY
There are a number of limitations, restrictions or
special considerations which apply when using video
imagery for mapping purposes.
Firstly, the elements of inner orientation of the
camera, such as focal length, geometric lens
distortion and position of the principal point are either
unknown or unstable. These elements can be
determined by calibration but in this technique any
discrepancies are largely compensated for by the
adopted procedure.
Probably the most severe restriction is the limited
resolution and coverage which can be obtained using
a video camera. For example, the small format and
small focal length of a typical video camera means
that some 500 video images are required to cover
the same area at the same scale as a standard
aerial survey photograph. Accordingly, at this point
in time, the technique is most suited to project work,
and is not at all suited to large mapping projects.
One of the difficulties of using remotely controlled
aeroplanes is in determining their exact location and
161
altitude at any time. Both influence the coverage
which is obtained. This is somewhat offset by the
video imagery itself which can be acquired
continuously over a considerable length of time.
Video images can realistically only be processed
using digital photogrammetric systems. |n the past
this was a severe restriction because such systems
were either expensive or not readily available.
However, this is changing and systems are becoming
more affordable and more available.
The technological evolution can only continue to work
in favour of video imagery.
8. PROJECT DESCRIPTION
The technique was tested at a site in Tingalpa in
Brisbane.
As previously described, an onboard video tape
acquired continuous imagery over the test site.
Overlapping images were selected and written to file.
These image files were saved in standard TIF format
and were used in the processing stage.
Processing with the digital photogrammetric software
followed a standard procedure. First the individual
images were registered so that the image measuring
system was referenced to the centre of the image.
A number of control and check points were
coordinated in the field. Some of the points were
used to control the photogrammetric model while
others were used for checking. The control points
were identified in the images and labelled with their
allocated numbers.
The stereo model was formed and oriented to
control. Although one is able to view the model
stereoscopically, this was not required for the
procedure and accordingly was not done. The
purpose of the stereo model formation is to organise
and orient the geometry for DEM generation.
The DEM was generated by the software using an
auto-correlation technique. That is, the software
matches corresponding points in each of the two
images by identifying like patches in the images. A
match leads to the generation of a height point. The
process continues systematically until the model is
covered by a grid of height points, which of course is
the DEM.
There was a minor problem in that the correlation
algorithm in the digital photogrammetric system used
was not very sophisticated. Occasionally it would
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
