its can be repeated
bility of theodolite
ing.
s and a minimum
as calculated that
lld be required to
onfiguration and a
ity of the wall and
bject are shown in
thout obstructions
10 doubt suffer a
harper intersection
can be reduced by
ively improve the
of course be at the
to the increase in
on degradation can
1lations. Naturally
etwork would be
uments around the
iangulation system
ination of control.
is required to shift
the system. Based
) estimate the time
> using theodolite
1.5 hr
ach) 3 hr
nin. each) 1.5hr
6 hr
| targeting which is
method selected.
es which must be
2.5 metres and for
ing jig, giving an
pints on the top of
ase or manufacture
against the edge of
rements for offsets
Itiple targets which
1e point of interest,
296
based on the known geometry of the rod. The additional
measurements will obviously influence the overall measurement
time. In addition to these concerns, the use of such an offset
rod would make it difficult to exactly revisit the same points in
subsequent surveys. This would reduce the ability of the
surveys for self-checking and to detect changes within the
network between the two measurement epochs.
With regard to target intersection there are other issues which
bear consideration. Many of the targets along the base will be
difficult to intersect purely because of their orientation relative
to that of the instrument. Figure 7 demonstrates the effects of
viewing angle on target visibility. It is clearly evident that
certain viewing angles make accurate pointing to targets quite
difficult.
==>
Figure 7. Effect of viewing angle on circular targets
The personnel required to operate the system was also a
concern, as a three instrument system requires at least four
people for efficient operation. An operator is needed for each
instrument, plus a fourth person to operate the PC and act as a
task coordinator. A fifth person would be required in this case
to manipulate the offset rod.
Finally, the ability of the system to produce “real time” or
online coordinates is somewhat negated by the fact that all
coordinates are needed before the best fit planes and circles can
be computed. Hence coordinate information can not be used
until all targets for the particular surface have been triangulated.
DIGITAL PHOTOGRAMMETRIC TRIANGULATION
The alternative optical triangulation technique assessed was
digital photogrammetry. The development of “still video” CCD
cameras with onboard storage capabilities has simplified the
digital photogrammetric process. Image capture, transmission,
analog to digital conversion and the digital image storage is
carried out internally in a convenient and portable package.
The CCD camera can be operated via a direct link to a notebook
PC if required, or alternatively images can be stored on the
internal disk for subsequent image processing and mensuration.
The ability of these cameras to collect multiple, high resolution
digital images without a cumbersome computer interface makes
them a very useful digital photogrammetric tool (Fraser and
Shortis, 1995).
185
The Kodak DCS 420 is such a still video cameras. The CCD
chip present in the camera has a resolution of 1524 by 1012
pixels and is capable of producing maximum accuracies of at
least 1:80,000 (Beyer, 1995; Fraser and Shortis, 1995). The
DCS 420 uses a removable, credit-card sized PCMCIA
(Personal Computer Memory Card International Association)
type hard disk for convenience, portability and
interchangeability with other computers. Transfer of the image
data can be completed through a SCSI interface included with
the camera back, or by physically transferring the PCMCIA
hard disk card to a computer with an appropriate interface
(Ganci and Shortis, 1995b).
By far the most appealing features of photogrammetry for this
project were the speed of data acquisition and the ability to
optimise accuracy through the use of multi-station, convergent
imaging geometries. The time of observation needed to be only
as long as the time it would take to capture the necessary images
as defined by the simulation. This means that deformation
measurements can be made with minimal disturbance to the
production process. Photogrammetric networks are also
advantageous it that they can be readily designed to optimise
the accuracy and economy of the project at hand. They can
afford considerable flexibility in camera positioning and the
number of images involved. This flexibility is particularly
advantageous for adapting the network design to the prevailing
environment and conditions at the work site. With a single,
roving still video camera, the addition of new camera stations or
extra images at pre-designed camera stations is straightforward,
whereas the inclusion of extra theodolite stations is a major
exercise.
In addition, photogrammetry is advantageous as it permits non-
contact measurement even in the absence of a stable camera
platform. The gantry crane walkway used in this project is a
prime example of how a relatively unstable platform can still be
utilised for data collection. Theodolite triangulation from a
such a platform would of course be highly inaccurate if not
impossible. Finally, only one person is required to operate the
still video camera, whereas four or perhaps five people are
needed to operate and manage the theodolite system. This
represents a considerable saving in the cost of manpower.
METHODOLOGY EVALUATION
In almost all the criteria areas examined, the digital
photogrammetric measurement option was better suited to the
measurement task than the theodolite, or indeed any other,
approach. In this instance the online triangulation system was
deemed unsuitable as it could not satisfy many of the
requirements of the project. The lack of flexibility coupled with
the significant personnel requirements made it highly unlikely
that the project could be completed without the resolution of
significant concerns. This is especially evident in the face of
the alternative method. The flexibility of the digital approach is
clearly evident when comparing the relative size and complexity
of the equipment required for data collection. The theodolite
system requires three total stations, stands, cabling and an on
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
