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From today's point of view a digital stereo plotter is for GIS
the ideal system to acquire 3D-data. Besides the floating mark
control device there is no special hardware required. GIS and
the digital stereo plotter can be implemented on one and the
same computer. These are very good starting positions to join
GIS and DP closely.
PHODIS ST is the digital stereoplotter within the PHODIS
product family. It contains a modular and open programming
interface which already is used by the PHOCUS, CADMAP,
and MicroStation™-based CADMAP/dgn mapping packages.
AIO and ARO are standard components of PHODIS ST. The
interactive measurement of ground control points is supported
by matching procedures which deliver accuracies of up to one
tenth of a pixel size. This fulfills photogrammetric accuracy
requirements. The user can apply in all important image
orientation steps easy-to-learn automatic program modules.
Every single module can be integrated into a GIS on request. In
addition to the orientation steps PHODIS ST also administers
an arbitrary number of stereo models per project. The main
purpose of PHODIS ST is stereoscopic display in order to be
able to acquire metric 3D-data and to superimpose 3D-data.
The digital stereo plotter described above represents a raw
stereo device which finally becomes a stereo plotter when
connected to a 3D-data acgisition system. A more detailed
description of PHODIS ST can be found in (Dérsel 1995).
The raw stereo device in PHODIS ST possesses special
functionalities which make its usage as a 3D-digitizer for GIS
attractive. These are the on-line Z-correlation, draping, and
superimposition in transparent mode. These properties will be
presented in the following chapters.
3.1 On-line Z-correlation
Initially the floating mark is set to ground in an oriented stereo
model. After activating the measuring mode of on-line Z-
correlation the Z-wheel can be switched off. Changing the
position of the floating mark in object space by moving the
floating mark control device, which in PHODIS ST is called
the P-mouse, contributes one part to the movement of the
floating mark in the image space. The remaining part of the
image space movement of the floating mark is caused by the
result of the Z-correlation. The implemented algorithm is
based on an adopted and optimized image matching process.
It is possible that an individual can measure in stereo without
the requirement to also see stereo. This apparent paradox is
called OneEyeStereo and is also the name of the on-line Z-
correlation proprietory to PHODIS ST. OneEyeStereo is
implemented as a robust algorithm. It can distinguish between
three states. These are good correlation, acceptable
correlation and no correlation. The correlation results can be
visualized on-line as colored floating marks. Such an approach
can be compared to the states of a traffic light. It cannot be
guaranteed that OneEyeStereo always will deliver good or
acceptable correlation results. For this reason it is necesary
that the user is be able to see stereo and capable of using the
P-mouse.
OneEyeStereo can be used in two different modes, as of now.
The first operating mode is called move and correlate. The
user drives to an arbitrary point and switches on-line Z-
correlation on. The second mode allows continous correlation
and registration of XYZ where the coordinate triple may be
555
accompnied by the attibute good, acceptable, or no
correlation. OneEyeStereo is integrated in the raw stereo
device of PHODIS ST. Hence it is available to any connected
3D-data acquisition system. For GIS-users this represents an
important improvement towards easier 3D-measurements. The
previously required capability of the operator to see stereo is
greatly reduced.
3.2 Draping
Draping is the capability to visualize 2D-data in a
stereoscopically correct superimposition. In order to achieve
this photogrammetric paradox a DTM is assigned to the stereo
model. All 2D-data, i.e. eastings and northings, are used as
arguments for the interpolation of the corresponding
elevations. The 2D-data are thus draped over the DTM. Using
this functionality, any 2D-data sets can be stereoscopically
visualized. The accuracy of the stereoscopic superimposition,
however, is dependent on the quality of the DTM and, to a
much lesser extent, on the quality of the model orientation.
Usually orientation parameters are correct as otherwise there
would not be a stereo model. As some GIS consider the DTM
an external data set it is easy to connect it to the raw stereo
device. There is a generic DTM interface built into PHODIS
ST which defines the general DTM data access. Basically any
DTM data can be connected this way to the digital
stereoplotter. Draping allows simple and efficient DTM
checking. The planimetric coordinates of a point are used as
arguments for the Z-interpolation, Zpaw. At the same time the
floating mark can be set on the ground, either manually or by
means of OneEyeStereo and one obtains Zmeasurea- The
difference Zair = Zmeasured - ZboM. gives a measure of the
quality of the DTM. In GIS this can be applied to assess the
quality of a DTM by checking single points and evaluating all
Zeifr.
3.3 Superimposition in transparent mode
When visualizing defined areas in GIS often these areas are
covered in a transparent mode. This emphasises a particular
area, for example with a colored tint, but still lets the image
contents of this area be seen through the overlay. This
technique is known from thematic cartography. Such a
visualisation mode puts a tremendous amount of graphics
performance on a digital stereoplotter.
4 Conclusions and outlook
The achieved technical status quo in DP forms a solid position
for introducing DP as a technology to be applied by GIS for
3D-data acquisition. The reasons for this are more than that DP
is implemented on the same computer hardware as is GIS. In
particular, the automatic procedures, like AIO, ARO, semi-
automatic GCP measurements, OneEyeStereo, and Draping,
are the functionalities which give the photogrammetrically
unskilled GIS user access to 3D-data acquisition and let him
still use and stereoscopically superimpose his 2D-data sets.
Photogrammetric tasks like interior, relative, and absolute
orientations are either fully- or partly automated. Three-
dimensional measuring is separated into an interactive
identification step and an automatic determination of the
coordinates. The combination of 2D-data with available DTM-
data enables the stereoscopic superimposition of existing 2D-
GIS-data-sets. This can be used e.g. to check the topographic
part of a GIS data base for completeness by superimposing it
over the stereo model. This makes data base updating both
easier and faster. Increasing demands in the quality of data
make the elevation a necessary component in GIS. However, in
order to represent and administer true 3D-coordinates and
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
