target in the photograph. Moving in this direction it was
decided that the shape of the target be square, with 40 x 40 cm?
dimensions, which is appropriate for the photo-scale used.
Figure 3.2
The specially designed artificial photo-target in the tidal terrain of Saint John
Harbour
b) Lack of features and stable patterns in tidal terrain.
Being a part of the seabed, the tidal areas are often featureless
without showing any pattern. But even if patterns exist, and
some features appear in the tidal area, there is a high probability
that the dynamic nature ofthe tide will change them rapidly.
Some additional problems are introduced in these cases:
* Difficulties in achieving stereo vision over featureless tidal
areas.
* Lack of features that remain stable during the time between
camera exposures (especially when the exposures occur on
different days). Therefore, it is very difficult to find features
that can be used as pass and tie points for connecting the
photographs into a block. A solution to this problem is the
mechanical marking of the images using a point transfer
instrument, in order to create artificial pass and tie points. But
this is not as accurate and often impossible in cases of extended
featureless areas, because of the very weak stereo perception in
those areas.
Features that create a pattern on the seabed and can be used as
pass or tie points are stones, rocks, and sea plants. The use of
stones can be misleading, because they may have moved during
the time between exposures. But usually big stones remain in
place during a low tide period. If pictures are taken on different
days, even big stones are unreliable. Sea plants can be used as
tie or pass points, but their use is limited due to their usually
large and irregular shape.
Color photography is preferred for tidal terrain mapping since
color photographs show more radiometric detail than the black
and white ones and help in both the stereoscopic perception and
identification of pass, tie and control points. For the extended
areas of difficult stereo vision a photogrammetric solution is
impossible. In these cases it is suggested to spread just before:
the flight a number of highly reflective items (like the white
arborite that it was used for the construction of the control
points) over the tidal terrain, establishing in this way patterns
and some of these items can also be used as tie and pass points.
20
4. USE OF NON-METRIC, SMALL-FORMAT CAMERA
TO MAP A TIDAL AREA AT LOW TIDE
When a non-metric camera is used for tidal terrain mapping, a
number of additional problems are introduced:
-The unknown and unstable internal geometry, and the large
systematic errors of the non metric cameras require the use of a
photo-variant self-calibrating bundle adjustment. The
application of such an adjustment introduces the following
problems when it is applied to the tidal terrain:
* The tidal terrain is nearly flat and the use of the bundle
adjustment is difficult when it deals with a combination of
a flat terrain and vertical photography because of the high
correlations between the unknowns.
* Self calibrating bundle adjustment requires dense and well
distributed control points. Its accuracy is also affected by
the density and distribution of object points in the images.
In the tidal terrain it is difficult to establish control points
and the number of well defined features that can be
observed in the images are very limited.
-The small format of the non-metric cameras greatly increases
the number of photographs covering an area and thus more tie,
pass and control points are needed. Furthermore more stereo-
models have to be connected to form the block which can result
in erroneous blocks, taking into account the weak connections
caused by the lack of appropriate pass and tie points and the
flatness of the terrain.
To solve the last of these problems the acquisition of the aerial
photography in a smaller than the required scale was proposed
and then enlarged to the desired scale in a high quality enlarger.
A non metric 500 EL/M Hasseblad camera was used. Aerial
photography was taken in 1/15000 scale and then enlarged by 4
times to the desired 1/3750 photo-scale which is a good scale
for achieving the required accuracy.
Even though this approach seemed to be promising it proved
inadequate, partly because it:
- results in an inability to closely approximate the principal
point of the images because of the cropping of the original
images during the enlargement process.
- degrades the image quality thus making the
photogrammetric observations of the tidal object points
even more difficult.
- reduces the side-lap and over-lap and eliminates precious
control and other object points on the images as result of
the cropping effect . It was found that the mean image loss
of the enlarged images was more than 10% of the original
image area.
- introduces very large image distortions which, if are not
effectively compensated for, can deteriorate the results
dramatically.
- makes the use of data snooping for gross error detection
impossible because of the low reliability and the
inadequate observation quality of the enlargements.
All the above combined with the correlation and over-
parameterization problem that accompanies the photo-variant
self calibrating bundle adjustment make the use of the small-
format non-metric cameras inappropriate for tidal terrain
mapping.
The analytical plotter DSR11 was used for the photogrammetric
measurements and the photo-variant self-calibration bundle
adjustment programs UNBASC2 (Moniwa, 1977) and
GERATV (El Hakim and Faig, 1981) were used for the
aerotriangulation process.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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