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to the project area, respectively close to the flying aircraft.
Available experience suggests safe maximum distances of
about 30 km only. This represents a serious restriction, at
least in some conditions of application. It is to be
investigated, therefore, how far the stationary and the roving
receivers can be separated and still give reliable ambiguity
solutions. The second point of research is to develop
methods or conditions which make the OTF ambiguity
solutions safer and more robust.
The paper reports about a number of experimental tests on
fast ambiguity solutions which are undertaken by the GPS
working group of OEEPE. In summer 1995 three test flights
over test areas have been flown, each with several
stationary dual frequency receivers which were positioned
at different distances from the project area. The range
extends up to several hundred km. Receivers of different
manufacturers have been used, in order to cover different
cases. Several more test flights are scheduled for fall 1995.
The tests include smooth flying, avoiding loss of lock as
much as possible, as well as rough flights with sharp and
steep turns, producing loss of lock by intention.
The GPS data processing (and checking via aerial
triangulated camera positions) is going to use several
software programs for the ambiguity solutions. The results
will be analyzed with regard to the accuracy and the
sensitivity of fast ambiguity solutions, possibly related to
different external circumstances.
The theoretical development of fast ambiguity solutions of
higher performance is not the task of the experimental
investigations. The available data will allow, however, to
investigate whether the use of several ground stations can
robustify the method and extend its range of application.
The experiments concern a problem, the investigation and
solution of which is a highly urgent topic for the practice of
airborne GPS application. It is not expected that all test data
can be processed in time for the paper. But some tests will
have been worked out completely and valid results will
certainly be demonstrated and analyzed in the paper.
6.2 Updating complex databases; the next step (by M.
Brand)
Digital topographic databases are fundamental to
management of an ever increasing range of business
functions within a GIS environment. It follows that currency
of such databases is critical to effective use of the new
technology.
The more complex the topographic information involved the
greater are the implications to the updating procedures,
particularly if in addition to current data, archiving of a
historical perspective is a requirement. To date these
matters have remained largely unaddressed by software
houses generally.
The issue increases in importance daily and it is logical that
OEEPE should play a leading role in promoting this follow-
up activity. Such activity must focus on the use of newer
technology, typically digital photogrammetry and
105
mono-plotting, and identify means to effectively apply such
methodologies to complex topologically structured, multi-
featured coded databases.
The recently completed OEEPE Commission | project
"Updating of Complex Digital Topographic Databases" was
a major attempt to tease out the principles involved and the
difficulties to be overcome in seeking solutions. The paper
outlines these and develops on its findings.
6.3 First experiences from the OEEPE Scanner test (by
O. Kólbl)
In modern aerial photogrammetry, image acquisition is still
largely done with photographic cameras. Consequently, the
conversion of photographic images into a digital form by
Scanning represents a key operation for digital
photogrammetry. Currently, a considerable number of
scanners specially developed for the requirements of
photogrammetric processes are available.
In order to develop criteria for a systematic analysis of
scanners and to gain experience on different systems, the
OEEPE (European Organization for Experimental
Photogrammetric Research) has set-up a working group,
which besides other activities launched a test.
The test material which included 7 different test-patterns
and aerial photographs, was sent to different vendors. Up
till now, test results have been received from the following
firms, using the referred systems: Agfa Horizon Plus of
Agfa, Mortsel, Belgium; PS1 of Intergraph, Huntsville, USA;
RM1 of Wehrli & Assoc, Valhalla, USA; and DSW200 of
Helava, San Diego, USA. The results of the scanner test
confirm the high standard reached by photogrammetric
scanners. Nevertheless, open questions remain and
differences can also be distinguished between the different
Systems.
The scanning of the original negatives is still not very
satisfying due to the limited dynamic range of the scanner
and high image noise in the darker areas. Problems also
arise when scanning colour, especially false colour
photographs due to the lower sensitivity in the blue area.
The scanning process also affects the resolution of the
original images and a thorough control of the MTF of the
scanner is highly recommended.
Furthermore, it is apparent that the scanning process is still
a very time consuming operation, mainly because each
photograph has to be treated individually and hardly none
of the scanners is equipped for film roll. A problem linked to
the management of the scanning process is the
organization of the data. The repartition of the disc in units
of 2 Giga bytes, or the storing on tape considerably
complicates the management of the data, and it appears
necessary to conceive a special data base for this task.
7. CONCLUSION
High costs in data acquisition, and increased user demands
for sophisticated applications, and thus for transparent
access to information, regardless of the hosts on which it
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996
