3 IMPLEMENTATION IN PHODIS ST
The core tasks of an DPWS can generally be described as
shown in Figure 1 :
e Measurement of control and tie points for image orientati-
on
e Bundle block adjustment
e Stereoplotting
In an existing prototype, the tasks of control point measure-
ment as well as stereoplotting are carried out by PHODIS.
Bundle block adjustment for three-line imagery is carried out
by an external program (Ohlhof 1995). This program uses the
control and tie point measurement data supplied by PHODIS
and the approximate exterior orientation data. The result is the
adjusted exterior orientation of the individual lines. The used
bundle block adjustment program was developed by the
Technical University of Munich and can in principle be inte-
grated into the PHODIS environment.
Processing of line-scanner data essentially corresponds to the
classic method. The particularities and some implementation
details are described below. The following items are covered :
e Sensor data management
e Model setup
e Point measurement for orientation process
e Stereoplotting
The processing of three-line data in PHODIS starts with the
definition of a sensor platform carrying at least three sensors.
Figure 2 illustrates the MOMS-02 platform. The spatial locati-
on of each sensor on the platform can be defined relative to a
reference point. Up to 6 additional parameters are obtained
that have to be managed for every sensor, namely a three-
dimensional translation vector with the elements dx,dy and dz
and the directions of the CCD array defined by the angles do,
do and dk. The individual sensors of a platform and the as-
sociated parameters (see section 2.3) are defined in PHODIS as
regular cameras.
Model setup for three-line imagery allows the definition of 3
models for each strip. Normally the forward and aft look are
combined into a stereo scene for stereoscopic viewing. Howe-
ver, to exploit the high resolution of the nadir look for high-
precision stereoplotting, it makes sense to provide also for
forward-look / nadir-look models and nadir-look / aft-look
models. This involves the need to compensate the scale diffe-
rences in the images resulting from the use of different camera
focal lengths. PHODIS offers this option. The image scale
differences of the two scenes can be adjusted by means of
continous Zoom.
The image coordinates of the tie and control points can be
measured interactively or semi-automatically in PHODIS (see
Table 1) using an Intensity-based least-Squares Matching
(ISM) algorithm (IfP 1996).
Stereoscopic display of the model area and stereoplotting itself
are based on the finally adjusted orientation parameters for the
individual lines.
The stereoscenes can be processed with either one of two
product versions of the stereoplotter :
e PHODIS ST 10 with MIFC
e . PHODIS ST 30 with FIMC
For stereoscene setup, PHODIS ST 10 uses the original images
and allows moving the images behind a fixed floating mark
(MIFC), while in PHODIS ST 30 the floating mark is moved
over the preprocessed epipolar image pair (FIMC). Further
particularities of PHODIS ST 10 are the independent continous
zoom for the two images and subpixel-accuracy positioning
with real-time resampling in the greyscale or color imagery.
Scale adjustment during processing is especially relevant for
the plotting of three-line or terrestrial imagery.
During stereoplotting of line imagery, the parallel prespective
in the flight direction requires accounting for a special geome-
tric model. Since each image line has a separate exterior
orientation the required time for computing and moving to a
position is immense (see real-time loop).
In PHODIS ST, the 3D mouse (P-Mouse) supplies motion
pulses every 20 msec. This high frequency ensures jerk-free
and precise positioning. The motion pulses act on the so-called
model coordinate system (see section 2.3). In the real-time
loop, the corresponding positions in the object coordinate
system, the photo coordinate system, and the pixel coordinate
system are determined for each incoming motion pulse. The
position obtained in this way is used to create the stereoscene
in the MIFC case. During interactive measurement, two trans-
formation cases are distinguished :
e Point measurement
e Move to a position
These cases differ fundamentally. During point measurement,
the two pixel positions are known. The exterior orientation of
the two lines can be accessed directly and the associated posi-
tions in the model and object coordinate systems are deter-
mined in one step.
When moving to a point, the position in the image space has to
be determined iteratively by the real-time loop as described in
more detail in section 2.5. Iterative computation for moving to
a position has to be done in the real-time loop of the stereoplot-
ter and is therefore subject to the high time requirement of 50
complete iteration cycles per second. Measurements made with
SUN SPARC 20 computer showed for the used geometric
model that one single successful iteration cycle can be perfor-
med in about ] msec.
When points are to be measured, the coordinates are transfor-
med in the stereoplotter. The linked data acquisition system
only receive the object coordinates. However, if vectors are
superimposed in the stereoscene, the coordinates are transfor-
med by the linked program. Since PHODIS supports the linka-
ge of various data acquisition systems, special attention has
been paid to this fact. The functions required for coordinate
transformation have been included in a program library where
they are provided to both, the attached measuring program and
the PHODIS system. By implementing modular transformation
functions, it is furthermore possible to support different pro-
gram packages with identical transformation geometries, but
without modifying the programs specifically for three-line
geometry.
4 CONCLUSIONS
Three-line geometry has been integrated sucessfully in the real-
time loop of PHODIS ST. Convenient processing of this data
with a digital photogrammeric workstation is possible by use of
this standard product. Workflow optimization is relatively
easy, e.g. by integrating the bundle block adjustment for three-
76
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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