Real-Time Photogrammetry with Automated Direct
Orientation
The system can be used as a pair of CCD cameras
to do real-time photogrammetric work. In this
case, both the relative and the absolute
orientation parameters of the cameras can easily
be determined directly through the theodolites
no matter how the scene and the configuration
are unfavourable. This orientation procedure can
also be automated. The accuracy of this mode is
limited by the resolution of the CCD cameras.
Measuring while Tracking
The system is capable of measuring the geometric
status of a dynamic object as it is tracking it
for kinematic parameters. This function results
from the combination of motorized theodolites
and CCD cameras and can scarcely be found in
other systems. The accuracy of this mode is
again restricted to the resolution of the CCD
cameras.
Another justification for combining theodolites
with CCD cameras is that much photogrammetric
practice involves some sort of theodolite
operation, either in calibration, control
establishment, or in accuracy tests. Integrating
theodolites and CCD cameras in a system will
change the situation that a high accuracy system
cannot work without an even higher accuracy
system providing control for it.
The concept of combining theodolites with CCD
cameras has, in fact, been mentioned before
(West-Ebbinghaus, 1988). The fundamental
principle of such systems is no more than some
variation of the existing photogrammetric
theory. More important, however, is the effort
we need to bring this innovative idea into
instrumentational research in one form or
another.
The current investigation covered in this paper
is about the fundamental problems of such
systems -- calibration and orientation.
2. SYSTEM CALIBRATION
System calibration includes the determination of
the camera interior parameters and the geometric
relationship between the camera and the
theodolite. The latter is stated by three
positional parameters and three rotational
parameters with respect to the telescope
coordinate system. These six parameters can be
determined together with the camera interior
parameters by using the camera-on-theodolite
calibration method where only two targets have
to be used (Huang & Harley, 1989,1990). The
camera-on-theodolite method relies on the
rotation of the telescope and multi-exposure to
form an array of target images on the CCD camera
from only one or two physical targets. The
corresponding 3-D coordinates of those target
images with respect to the
telescope coordinate system can be determined
from the theodolite readings and the target to
theodolite distances so as to allow the solution
of space resection for the camera parameters and
the camera to telescope parameters. This process
can be fully automated if auto-theodolites such
as the Wild TM 3000 are available.
System calibration should also include the
determination and check of the axial correctness
of the theodolite; many theodolites we use today
are good enough to save this step and if not the
established methods in surveying can be used to
solve it.
3. SYSTEM ORIENTATION
System orientation involves the determination of
the relative geometric relationship between the
two theodolites. It is apparent that once the
system calibration as well as the theodolite
orientation have been completed, the orientation
of the cameras at any theodolite readings can be
derived straightaway. There are a number of
methods for theodolite relative orientation:
five-point method for non-levelled theodolites;
three-point method for levelled theodolites;
reciprocal pointing plus one point for non-
levelled theodolites;
reciprocal levelled
theodolites;
pointing only for
horizontal reciprocal pointing plus one point
for levelled theodolites;
theodolite orientation via the attached CCD
cameras.
The principles of those methods can easily be
understood by photogrammetrists and surveyors.
The detailed mathematical description of some of
them can be found in Kyle's thesis (Kyle,1988)
The theodolite orientation process can be fully
automated if auto theodolites such as the Kern
E2-SE or Wild TM3000 are used. For scaling a
known distance is required in using the methods
listed.
The absolute orientation which relates the
system to any specified real-world reference
system can be realized much more easily and
flexibly with the help of the theodolites which
is itself part of the system.
4. DIGITAL IMAGE PROCESSING
The targets which have been used in experiments
are black discs laser-printed on white paper.
The diameters are about seven pixels on images,
while larger targets are used for calibration.
A local thresholding method is applied to detect
target images, which is similar to the one used
by Zhou (1990). The method of least squares
parabolic edge interpolation followed by
elliptic fitting is used to locate target
centres on images to subpixel accuracy (Huang &
Harley, 1990). The correspondence of conjugate
target images is achieved by comparing the
closenesses of image points to epipolar lines
from left to right and from right to left.
5. EXPERIMENTS IN THEODOLITE SCANNING
PHOTOGRAMMETRY
Experiments have been carried out to investigate
the accuracy of theodolite scanning
photogrammetry and the effectiveness of the
calibration and orientation methods used. For
this purpose, a three dimensional target array
of dimensions of 2.3*1.7*0.9 m was coordinated
with the Kern E2-ECDS system to an accuracy of
0.01 mm. These coordinates were regarded as true
values in accuracy assessment. The accuracy of
the photogrammetrically determined coordinates
was then assessed by the RMS of the residuals
resulting from the coordinate transformation
from the determined coordinates to the true
coordinates with scale factor as unknown.
Two theodolite stations were set up at a stand-
off distance of about 3 metres with base line of
2.2 metres. A Philips frame transfer CCD camera
of a 4.5*6 mm sensor chip was used together with
a PCVISION Plus image grabber to capture digital
images of 480*512*8 bits. The focal length of
