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2.5 Conventional orientation methods
A simple way of determining an orientation is to provide
enough control points in the image and calculate a spatial
resection. In a block arrangement the images can be
oriented relatively, whereby it is necessary to have identi-
cal points which do not need to have known coordinates.
This operation can be performed by a bundle block ad-
justment. In both cases additional targets have to be
placed in object space, in the case of control points they
have to be determined in a separate calibration step. The
positions of such targets of course must also be covered
by the images, which may sometimes lead to additional
cameras. If point signalization on the surface or in front of
the object is not possible and an offline system calibration
does not come into consideration, a different orientation
approach has to be done.
2.7 Special demands in industrial environments
The limit for configuration layouts given by the industrial
production process is mainly that the optimal camera
positions are not accessible. The consequences can be
small object regions covered by images for short dis-
tances between camera and object and in most cases
unfavourable ray intersections which lead to reduced
accuracy. Important for the stability of camera positions
and control points are production activities or traffic, pos-
sibly causing that orientations have to be controlled per-
manently.
3. OPERATING RANGE ENLARGEMENT
Operating range enlargement in this context means to
optimize extensive configurations with the goal of a mini-
mum of cameras and a maximum of object size. It is
mainly a problem of camera orientation.
3.1 Object coverage
As mentioned before it is not always necessary to cover
the whole object with images. An example is the determi-
nation of profiles of a railway waggon of 18 m length,
which are only required in the middle and at both ends. A
complete coverage with a usual close range block con-
figuration would only be possible with a lot of cameras,
when an accuracy of 1 mm has to be achieved. As al-
ready pointed out many of these cameras only have the
task to build the block structure and do not show any
really interresting object point. When such cameras are
eliminated, the block becomes instable or unusable. The
remaining cameras have to be oriented independently.
This can be done in the same coordinate system for all
parts which is represented for example by a field of con-
trol points determined in a special calibration. The possi-
bilities and advantages of block adjustment are reduced
for this type of configuration because of lower redun-
dance and slighter connections between the block parts.
Also the elimination of tie cameras can result in remaining
images that have no orientation information. These im-
ages have to be oriented by a calibration, or in unstable
conditions with alternative methods described below.
483
3.2 Moving cameras
Camera movement in an otherwise static measurement
system can be used to obtain more images without addi-
tional video hardware. This is the same idea as the macro
scanning used in some digital photogrammetric cameras.
There are two principal movement types which have dif-
ferent effects: the rotation and the shifting.
Rotating cameras simulate a greater field angle. They can
cover a larger object region. Depending on layout and
precision of the fitting the images taken in different direc-
tions have none or up to five identical orientation pa-
rameters. The rotation parameters are given by the the
movement itself, whereby the cameras normally do not
roll around their optical axis. Position shifts are caused by
different locations of the rotation and the projection cen-
ter. This is a mechanical problem, but the effects can be
calibrated. Two different shots taken from an only rotating
camera of course cannot be used alone for a three-
dimensional evaluation because of the missing image
base.
Shifted cameras deliver fully independent images. Orien-
tation parameters may still be partly identical, suitable
mechanics presupposed.
A combined movement system with rotations and shifts
can be very flexible, but loses the advantage of identical
orientation parameters.
The disadvantages of moving cameras are the additional
time consumption, the sensitivity for disturbance and the
mechanical wear.
4. ALTERNATIVE ORIENTATION METHODS
Standard methods use their own image information for
the orientation process. The disadvantage is that addi-
tional targets have to be placed in object space, which
can be difficult, instable or simply impossible. In these
cases the orientation of cameras can be performed by
alternative methods which will be described in detail be-
low.
4.1 Laser
To obtain all orientation parameters with a laser system is
possible but very complicated and inefficient. The ad-
vantages lie in rotation control and determination. A laser
attached to the camera body can point to a far target
outside of the object space, where displacements of the
beam are equivalent to certain rotations of the camera.
The check can be performed manually, for example with a
scale, or an additional camera can be used for the auto-
mation of this step. Another possibility to control the beam
movement is given by PSD's (PSD - Position Sensitive
Device). These elements have a size of 10-30 mm? and
can register the position of a light point in one ore two
dimensions with high accuracy.
4.2 Orientation cameras
The fact that two cameras fixed together have sets of
orientation | parameters, which are directly dependent
from each other, can be used to place control points far
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
