3. CONDITIONS OF STEREO ORTHOPHOTO
POPULARIZATION
Stereo orthophoto enhances flat photographic form of the
world representation by introducing the third dimension. Both
photogrammetric products have many features in common,
however only orthophotos are generally recognized. Fact is,
that stereo orthophoto was originally introduced when technical
incapabilities limited ^ straightforward use of all product
advantages. Rapid technology development in recent years
dramatically improved the standards of photogrammetric
products, especially in terms of the radiometric quality.
However technology development has not affected significance
of stero orthophotography, but has considerably strengthened
the orthophoto position in GIS environment and in geoportal
sites.
Strong orthophoto position in the GIS is justified by two-
dimensional, layered GIS information structure, the third
dimension is loaded from DTM (2D+1D strategy, also known as
2.5D). Orthophoto is treated as one of the many information
layers, which satisfies most of the end users. The GIS
transformation into full 3D system is currently at the initial
stage and results achieved so far could be qualified more as 3D
cartography than GIS.
Could stereo orthophoto technology gain more popularity? The
authors believe that the answer to this question is positive but
only after meeting a number of conditions. One of them is
perceiving the stereo orthophoto as a support material, not as an
othophoto substitute. Inducing potential users to complete
resignation from orthophoto for the benefit of the product that
requires additional hardware for stereoscopic observation,
would be a mistake. Stereoscopy effect can be achieved by an
anaglyph technique, which requires very simple and cheap filter
glasses. This technique, however, cannot guarantee preservation
of full colour palette on the image. On the other hand
professional stereoscopy requires an appropriate graphic card
and special monitors with adequate glasses, which could be a
major investment. Therefore, stereo orthophotography should
be implamented in the GIS as an additional option, used only
when orthophoto does not provide sufficient information. The
possibility of 3D measurement is an additional advantage, but
not necessarily appreciated by all GIS users.
Another important aspect of stereo orthophoto propagation
strategy is utilization of the popularity of open source software.
Several valuable GIS tools have been created within the open
source market with a number of users around the world. The
first open source GIS software worth pointing out is fast
developing QuantumGIS, which despite of being used in typical
amateur applications, works well in a major, professional
projects. Key advantage of such tools is possibility of casy
extension of the program by adding specialist plugins, such as
an application for displaying prepared sets of stereo orthophotos
in anaglyph technique with the possibilities of 3D measurement
and vectorization.
Next aspect worth consideration is the availability of the data.
And once again orthophoto is in the better situation. In many
countries orthophoto data are gathered from governmental
projects in cycles of several years. Such data are usually
available for a small fee, while reviewing the othophothos with
WMS services is free. Stereo component generation procedure
from single orthophoto is quite easy, but stereo model obtained
in this way applies only for the terrain. In order to achieve
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
“full” stereoscopy effect both DTM and all othophotos (not
only mosaic of them) or all original images with orientations
are needed.
Method proposed by Li and Wang (Li et al 2002, Wang 2004,
Li et al 2009) of using the odd and even images separately from
photogrammetric block with 60% coverage is interesting from
stereo orthophoto populization point of view. In this solution
two mosaics are generated — one based on the othophotos from
odd images and other mosaic from stereo mates created for even
images. Production cost of such data should not be
considerably higher than production of traditional orthophoto
based mosaics. Commonly used all orthophotos mosaicing
method into sections of orthophotomaps is an anachronism,
because today's photogrammetry products guarantee proper
level of accuracy not only for regions around the centre of
photo but for all image area.
As the above arguments suggest conditions of stereo orthophoto
populatization are complex and difficult to meet. On the other
hand the strength of the open source movement is so immense
that is worth entrusting. Therefore, authors decided to develop
QuantumGIS plugin application, allowing stereo orthophoto
observation and 3D measurements.
4. APPLICATION BASIC ASSUMPTIONS FOR
STEREO ORTHOPHOTO
After several considerations for building computer application,
designed for displaying and measurement of stereo orthophoto,
following assumptions were made:
a) application will be working as a plugin in
QuantumGIS environment and will be written using
C++ language, with possible bindings to Python,
b) stereoscopy will be carried out with orthoimage
prepared from left photograph and stereo component
prepared from right photograph. Optionally DTM
used for orthorectification may be utilized,
c) plugin will provide possibility of stereo observation
with full stereoscopy (chapter 2) and XYZ coordinate
measurement,
d) anaglyph images will be used to achieve stereoscopic
effect.
QuantumGIS environment imposes requirement for using
dynamic loading libraries, "dll" files in case of Windows
system, or “so” libraries in case of Linux. During compilation
those libraries have to satisfy not only dependencies of
QuantumGIS itself, but also dependency of popular Qt library
have to be fulfilled. It is used as a toolkit for providing user
interface, and initially Qt is meant to serve as facility to enable
specification of parameters for anaglyphs prepared previously.
But wide variety of functions included in this library does not
exclude possibility of creating accelerated OpenGL context and
following potential of stereoscopic visualisation thorough
shutter glasses, or even equally popular polarisation technology.
At the same time QuantumGIS standard libraries provide
QgsRasterLayer class, member functions of which allow for
adding images in all image formats supported by GDAL library.
Furthermore, available public member function setCrs, which is
inherited from its parent class QgsMapLayer should allow for
manipulating layer coordinate system, providing way to impose
proper parallax between stereo pair. One could assume that
those features should be enough to build application In
question.
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