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in the duration of the technological cycle of a survey, in
particular, aerial survey, has allowed the company to make
is image more attractive for potential customers and
investors. These have been traditionally used to protracted
mapping processes in Russia. An extremely high level of
efficiency, along with the highest quality of data provided
create, among other factors, a very favourable climate in
customer relations. The customer is given the opportunity
to monitor quality and completeness of data and even to
make adjustments to the operation plan throughout the
entire technological process.
7. THEORETICAL ASPECTS
The progress in LIDAR technology and its numerous
topographical applications have been rather impressive
during the last few years. On the one hand the development
of sophisticated algorithms for geomorphological analysis
is still ongoing, enabling improved recognition and detailed
description of objects. On the other hand, combining with
other datasets results in augmented information. In this
respect, LIDAR and digital imagery data perfectly
supplement each other. LIDAR allows an effective DTM
generation while enabling automatic recognition of many
classes of objects having a clear geomorphological
structure. Digital imagery provides the most natural kind of
scene representation, including complete information
concerning surface texture. In some cases, this pure visual
information may be supplemented with infrared or multi-
spectral data. LIDAR data and appropriate processing
technology enable the following:
- Avoidance of the stage of on-ground geodetic support
for both corridor and area survey;
- Use of Ilaser-derived DTM for ortho-image
rectification;
- Creation of new procedures for aerial triangulation in
particular, and frames georeferencing in general, by
combined geomorphological analysis of laser data
and imagery.
Simultaneous recording of LIDAR data and digital imagery
requires joint operation of the two sensors on board of an
aircraft, which apparently is associated with the solving of
a number of technological problems.
Generally speaking, from a practical point of view the
concept of real-time mapping proposed by Geokosmos may
be regarded as an alternative to a classical
stereotopographic method in cartography. Though it would
be more accurate to say that this technology is derived from
a wide variety of classical geodetic and photogrammetry
methods. Numerous particular limitations of the
stereotopographic method are well known. These have been
studied in detail therefore there is no sense in their
thorough description here. Let us just mention the
following:
Such problems are quite diverse in their nature, but they all
are in general associated with the issue of points correlation
on stereopair. In certain cases this leads to complete
inapplicability of the method, for instance in snow-covered
or sandy landscapes with absolutely no visual texture. In
other cases the quality of results largely depends on a
number of factors, such as average forest elevation and
density when surveying forestry, or buildings shape when
mapping city landscapes.
The above-mentioned limitation of the
stereophotogrammetry method manifests itself mostly in
the most practically important applications connected with
surveying complex scenes with numerous objects. Largely
70
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
for this reason large-scale mapping of city landscapes with
a significant share of multilevel buildings cannot be done
by exclusively aerial survey methods, thus forcing massive
carrying out of on-ground topographic survey, which is
extremely expensive in city conditions. Besides, there are
seasonal limitations restricting aerial surveys in the
presence of heavy snow cover or foliage. For the most part
of the territory of the Russian Federation, for example, such
limitations leave only 1.5 — 2 months a year for aerial
survey.
Practically, such problems often lead to a serious
deformation of technology that causes doubts about the
accuracy of the results. Thus, generation of a DTM of a big
city area considered as compulsory within the
stereotopography method, is regarded as such a labor
consuming and expensive task affecting the overall cost of
the project that a 'compromise' has been offered, namely, to
use a relief model taken from an existing topographic map
of appropriate scale. Given the extremely low metrologic
quality of the existing topography basis in Russia, one may
only guess what consequences in future would be caused by
such decisions when doing, for example, a cadastre system
to regulate real estate relations.
It would be reasonable to note here that the main advantage
of the real-time mapping technology is that it is practically
free from all the limitations mentioned above. This explains
its great attractiveness for potential customers engaged in
various kinds of topographic activity.
The proposed digital technology of mapping in real time is
free from major disadvantages of the traditional
stereotopography method which, as has already been
mentioned, include a necessity of on-ground geodetic
support, inevitable manual labour at the stages of frames
mutual orientation, DTM production and correct
combination of orthorectified photos.
8. SOME PRACTICAL RESULTS
The proposed technology of real-time mapping assumes the
carrying out of aerial survey missions for an area of interest
using Optech ALTM and other aerial survey equipment.
The aerial data is to be processed using Geokosmos'
specialised software according to the following scheme:
Logically, the first step is true DTM separation from a full
cloud of laser points. As it was mentioned above, the
implementation of such a procedure is a complicated task.
The solution is the application of special topologic analysis
algorithms classifying laser points by criteria "belong/not
belong" to true ground. Such algorithms are based upon the
two obvious postulates:
- A true ground point has a minimal value of
geodetic elevation in comparison with other
ones in its vicinity.
- The spectrum of spatial frequencies of a true ground
surface has no high frequencies.
Practical realisation of such algorithms is normally carried
out by modelling mathematical surface, which delineates
the laser point cloud from beneath. Spectral selection for
the given surface is expressed by limiting the values of the
first and second differentials of the surface regarding as a
two-dimensional function of planar coordinates. In practice
such an approach provides quite satisfactory results.
The next step is the aerial photos automatic mutual
orientation:
- The special DTM processing is implemented with a
view to detecting the DTM fragments for which the
correlation algorithms of point matching can be
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