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Photographs were taken at each station before and after
deformation with both cameras from the same stations and
with the same orientation. Due to the difference in the format
of the imaging area for the two cameras (i.e. the area of the
emulsion for the non-metric camera and the CCD frame in
the digital camera), and the difference in the focal lengths,
two options were available, namely to keep the camera-to-
object distance unchanged and get a different photo scale for
each camera, or else to keep the photo scale constant and
change the camera-to-object distance.
From a practical point of view, the first option was chosen.
The second option was difficult to achieve due to the
expected short depth of field, which would prevent a clear
and sharp image of the whole model. The camera-to-object
distance was chosen to be 2.5 m which resulted in a photo
scale of 1:90 for the non-metric camera, and 1:315 for the
digital camera. The deformation was facilitated by changing
the tension of the standing wires, which in turn affected the
hanging wires as well.
5. INSTRUMENTATION
The non-metric camera used is an Olympus OM10 equipped
with 28 mm nominal focal length, 36 mm x 24 mm image
format, f-stop ranges from 2.8 to 22, an automatic exposure
time adaptor and four artificial fiducial marks which had been
placed in the photo plane for another project. The recording
medium was a Kodak Elite 400 diapositive colour film.
While capturing the images, f/22 was selected as aperture,
which represents the smallest diaphragm opening of ~1.3
mm, to maximize the depth of field. The regular illumination of
the laboratory dictated an exposure time of 1 sec.
A Fujix DS-100 digital still memory card camera was also
utilized. It is an off-the-self, low cost digital camera, not
specifically designed for scientific or photogrammetric
purposes. It can be considered as a non-metric digital
camera because it has similar unstable geometric charac-
teristics as conventional cameras, except for the recording
medium.
Its focal length varies between 8 to 24 mm for the Fujinon 3X
power zoom lens with f-stop ranges from 2 to 11 and
exposure time settings of 1/4 to 1/750 sec. The picture
element is a CCD ( Charge-Coupled Devise ) solid state
image sensor. The camera has a frame of 720 x 488 pixels
with a pixel size of 8.5 um x 9.7 um. The storing medium is
a IM-85 Image Memory Card with 8 Mbyte capacity. The
images stored in the card can be displayed and resampled on
the computer using the DP-100 Memory Card Processor and
SD-PC (Digital Stil Camera System Image Handling)
software. The resampled image has dimensions of 640 x 488
pixels with a square pixel size of 9.7 uum x 9.7 um.
For image capturing, the minimum focal length 8 mm was
chosen to obtain the maximum angular field of view. Again,
the maximum f-stop=11 along with the maximum exposure
time ( t=1/4 sec) were chosen.
In addition to two sets of 6 images each ( hard-copy film and
digital ), a third set was obtained by scanning ( digitizing ) the
hard copy images. A Nikon scanner for slides was used for
this task. The scanning resolution was 59 DPMM ( dots per
millimetre) and the final resolution for the output image was
101.7 P/mm ( pixel per millimetre). This final resolution led to
9.8 um x 9.8 um pixel size.
The output image was chosen to have a format of 1.325 cm
x 1.000 cm which produced a frame of 13475 x 10170 pixels.
Due to the differences in image formats between the hard-
copy and scanned images, the photo scale for the scanned
images was calculated to be 1:157.5 with a hypothetical focal
length of 16 mm.
6. PHOTO MEASUREMENTS
Photocoordinates are the fundamental input for the analytical
photogrammetric adjustment. The accuracy of the final
results is directly affected by the accuracy of the
measurement of the photo coordinates. Due to the fact that
we have two different types of information, analogue images
and digital images, two different photocoordinate measuring
instruments were needed.
For the hard-copy images, the analytical plotter Wild BC2
was used in monocomparator mode to measure the photo
coordinates of the 6 photos. Each photo was observed three
times. The average photo coordinates and their standard
deviations were calculated. An average standard deviation for
each photo was also calculated to provide an indicator of the
observation precision. Table(1) illustrates the precision of the
observations for both epochs.
ODE
Table(1) The precision of the observations for hard copy
images ( in um at photo scale )
The photo coordinates of the digital images were measured
with the Leica DVP ( Digital Video Plotter ) system. Some the
advantages of DVP are its a very powerful photogrammetric
software, its low cost compared to any other digital
photogrammetric workstation, its suitability to be installed in
any ordinary PC with no need for a high capacity hard drive,
and its subpixel accuracy in measuring the photo
coordinates, which compares well with the accuracy of any
analytical plotter.
Again, each image, either digitally collected or scanned, was
measured three times and the average values and standard
deviations for all photo coordinates were calculated.
Tables(2) and (3) illustrate the precision obtained in
measuring the photo coordinates of the digital images in both
pixels and um.
149
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
