away from the object. One camera has the task of orien-
tation, the second has to measure the desired targets.
Many configuration variants can be realized. Of course
the orientation camera can additionaly be used for object
measurements and one camera can control the positions
of a set of several cameras. It may also be possible to
optimize the image scale for the control points and use
cheap hardware for the orientation task.
A variation of this method can be used to determine the
angles of rotating cameras. In this case the field of control
points can be very small and near the camera.
4.3 Beam splitter techniques
Instead of using additional cameras it is possible to use
beam splitters in order to get two different viewing direc-
tions with the same camera, one on the control points and
one on the object. This is a much cheaper solution com-
pared to additional cameras. The disadvantages are
reduced flexibility and lower image quality together with
illumination difficulties because of overlayed images.
4.4 Video theodolites
Photo theodolites are used in conventional terrestrial
photogrammetry for the purpose of obtaining photographs
with known orientations. The optical axis of the camera is
combined with the angle measurement facilities of a
theodolite. With known position coordinates all orientation
parameters are given for every viewing direction. Modern
theodolites have features that allow remote or automatic
operations. They have interfaces for data transfer and
device control and they can perform a remote controlled
positioning with built-in servo motors. Combined with a
CCD-camera they can operate as a very flexible online
system. The main disadvantage are the very expensive
theodolite components, which consist of precise me-
chanical and electronical parts. For many applications the
full flexibility is not really neccessary.
4.5 Reproducible camera positions
For moving cameras it is possible to build fittings with
several predefined positions. The simplest way is to have
two viewing directions represented by two limiters, where
the camera moves against. Intermediate stops can be
realised with mechanical centering devices. All camera
positions have to be calibrated. The main problem is that
the rotation angles have to be known with high accuracy,
which requires very precise mechanical parts. They have
to be protected against bad environment conditions like
dirt and shock and they may loose stability by wear.
5. DATA TRANSMISSION
An online measurement system has more or less de-
mands on transferring data between the different compo-
nents, depending on distances and configuration com-
plexity. Limits are given by cabling possibilities, cable
length and transmission speed. They reduce the layout
flexibility of a system and can lead to serious problems.
484
5.1 Data types
Three principal data types can be distinguished:
e video data
e controlling data
e resulting measurement data
Analog video data has to be transferred between camera
and frame grabber, which is the device for analog-to-
digital converting. In digital cameras the frame grabber is
built-in and therefore the image information has to be sent
digitally to the controlling computer. This data type re-
quires the highest transferring rates.
Controlling data is needed to remotely handle camera
functions and movement. The information amount is low
and there are usually no transmitting difficulties, excepte
the cabling itself in some cases.
Resulting measurement data mainly consists of coordi-
nate sets and accuracy information. The transport is usu-
ally done via printout or floppy disk. A connection to an
existing computer network is possible and has several
organizational advantages.
5.2 Influence on system configuration
The configuration of an online video measurement sys-
tem can be affected by data transfer problems. The worst
case is that for environmental reasons it is not possible to
find cabling paths anywhere. One solution might be to
place the controlling computer nearby each camera, an-
other could be found in wireless systems described be-
low. Limited cable length can have the same effect. In
most cases this will have the effect on the configuration
that certain good camera positions cannot be used, which
reduces accurracy and may complicate the orientation
process.
5.3 Standard video
Standard video means the analog signal transmission as
defined in the common TV-norms. For this type of con-
nections the cable length should be under 10-30 m, which
depends on electronical disturbance potentials in the
environment. The data rate is normally 25 images per
second (30 for USA), which at usual sensor resolutions
leads to a bandwith of about 4-5 Mhz. The speed of
measurement and evaluation is given by the controlling
computer.
5.4 Digital video
Digital video means in common that the camera has a
built-in frame grabber as mentioned above. Data rates
and permitted cable lenghts between camera and com-
puter depend on the interface type used, which may be
for example the serial RS422 interface or an SCSI-Bus.
The transmission speed of analog video is not reached in
usual systems. Data quality is high because of the digital
character.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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