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achieve. Therefore it is very important to know how accurate
measurements can be made and how the measurement accuracy
can be improved. The selection of the measuring points is also
important. Selected points should represent the object and its
interesting properties. The accuracy of unknown parameters
can be calculated by simulation.
In the design of photogrammetric measurement or network you
have to consider several things that effect to the measurements.
For example, Fraser (1989) has listed following questions to be
answered:
« How many camera stations are needed?
e What imaging geometry is selected?
e What scale is optimum?
e How many plots per station are needed?
e What is adequate depth of field?
e What targets are in which photographs?
e Camera self-calibration?
There are many more questions available. Many answers
depend on each other - so the designing of the measurement is
not a simple task. Not only an object to be measured does
effect to the design. In design you have to take into account the
measurement environment, too. The use of CAD-system is a
great help if you have some kind of CAD-models about the
object and the environment available.
In the design of the measurement, different requirements have
to be full-filled despite the restrictions. The goal of the
measurement design is to achieve accurate and reliable results
with an economical way. Because of the opposite requirements,
the simulation of measurement is the best way to inspect that
all design requirements are achieved. It is also a good way to
see how the changes of different parameters (for example,
sensor locations, sensor parameters, etc.) effect on the
measurement results.
When planning a photogrammetric measurement, the cameras
should be placed so that each point is seen at least from two
cameras. The reliability of the measurement increases when
the number of image rays increases. In special applications the
convergent imagery is usually used instead of normal situation
(stereo). Mason (1994) has given the following basic
constraints, which should be satisfied in camera placement:
* image scale constraint
* resolution constraint
* workspace constraint
* depth-of-field constraint
* incidence angle constraint
* number and distribution of image points constraint
* illumination constraint
There exists also a number of objectives that should be
considered in camera station placement (Mason, 1994).
* contribution to intersection angles
* field of view
* visibility
The influence of these constraints and objectives to camera
station placement can be visualized geometrically. An example
of this in 2D is given by Mason (1994). Because measurements
are done in 3D the influence of these constraints and objectives
have to be considered in 3D. This is one reason, why it is so
Important to design the measurements three-dimensionally.
433
2.2. The use of 3D-models in measurement design
The purpose of the measurements can be either to produce a
model of an object or to compare how well the real object
meets the design. The reverse-engineering tasks produce
models. In the quality control and deformation measurements
the real object is compared to an available model. In later cases
the model can either be a design or it can be a model based on
the previous measurements. In most cases, three-dimensional
CAD-models are used.
The planning of measurements have traditionally been done
based on two-dimensional maps or drawings. In addition, the
planning has been done by experts, who in most cases have a
long experience of similar measurements. The measurement
planning of complicated 3D-objects can be difficult using 2D-
documents, like maps and drawings. The sensor placement
demands several, often conflicting, constraints and objectives
to be taken into account, which is done easier in three-
dimensional planning. The visibility of measuring points and
the finding of common points for separate camera groups can
be done in CAD-environment using three-dimensional models.
The accuracy of the models available for the measurement
planning can vary. The more exact model of the object and the
workspace is available, the more exact the design of
measurement model and its simulation are going to be. The
most exact 3D-models are available in quality control and
deformation measurements. For example, the design of
deformation measurements using 3D-model can assure that the
most relevant information to detect the deformation is going to
be measured. The camera placement can be greatly assisted by
the use of 3D-models. Already a rough 3D-model can be very
useful, when the camera placement around an object (needing
several camera groups) has to be designed. If no other model is
available, the model of measurement environment can be done
from building drawing by extruding. When the constraints in
working space can be taken into account from the beginning of
camera placement design, there is no need to change camera
placement later due to difference in reality and optimal
planning (done in 2D without knowledge about these
difficulties).
In AutoCAD, there are different ways to model three-
dimensional objects. It can be done by using wireframe,
surface, or solid modeling. The MMD tool uses solid models of
the object. The measurement environment can be modeled
using wireframe or surface modeling.
3. THE MEASUREMENT MODEL
DESIGN TOOL, (MMD)
The measurement model design tool (MMD) uses three-
dimensional CAD-models of the object and measuring
environment. The user of the tool can define the camera
placements and orientations in AutoCAD. The user also selects
the camera parameters. If the real calibration data of the
camera is available, it can be used in simulation. The tool
visualizes the part of the object seen by each camera. The
definition of the measuring points is interactive. The tool
checks the visibility of the points for each camera. The point
has to be seen from at least two cameras. Because the visibility
checks are made three-dimensionally, possible occlusion points
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
