International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
more important that data so presented can be
effectively analyzed by software products and used to
generate vector models, which, for the above-
mentioned reasons, is extremely significant in
implementng present-day design approaches. For
instance, laser data processing software systems make
it possible to automatically identify types of towers of
power transmission lines and their spatial coordinates
with an accuracy of 10-15 cm. Besides, such
important parameters can be automatically
determined as the height of supports, length of
insulator strings, structural damages, etc.
- Unlike traditional methods, laser scanning is to a
great extent free from seasonal constraints caused by
foliage. In most cases laser scanning can be used to
survey objects under trees’ canopies.
- There are no limitations on the use of laser scanning
to survey scenes with no or indistinct surface textures
like sandy beaches, snow-covered areas and bodies of
water. lt is known that stereophotogrammetric
measurements of such scenes are impossible as no
associated points can be identified in stereopairs.
5. LASER SCANNING IN TOPOGRAPHY
Speaking about the applied aspect of laser scanning
techniques, we can conventionally distinguish between the
two major groups of applications. The first one includes
topographic applications where laser data are used to
reproduce landforms and plot important contours to be
shown on topographic maps and plans. Another major
group of applications includes a wide range of tasks which
are not directly associated with topography. To handle such
tasks, laser data are used to generate vector models and
determine a set of morphological properties of various
natural or artificial formations. In most cases, the collection
of such data is an essential part of engineering surveys.
Anyway, in analyzing the significance of laser scanning, it
would be logical to consider it, above all, as an alternative
to the stereotopographic method of producing maps and
plans or its analogues based on ground photogrammetry.
Technological and economic advantages of laser scanning
should be therefore considered using the stereotopographic
method for comparison. The following arguments can be
given in favor of stereotopographic method as a basis to
evaluate the efficiency of laser scanning:
- . Until now, stereotopographic method has been the
main tool used to produce and update topographic
data in the most general sense. The application of the
method is obligatory which is officially prescribed by
applicable regulations. At the same time, in terms of
nature, detail and accuracy of laser data obtained,
laser scanning techniques help perform largely the
same tasks as the traditional stereotopographic
method which involves aerial photography, geodetic
control and complex procedures of photogrammetric
data processing. In this sense, it is quite appropriate to
compare laser scanning techniques and
stereotopographic method. Another argument in favor
of this choice is the tendency of applying laser
scanning techniques to the production of topographic
materials. There is a clear tendency that
stereophotogrammetric methods are being ousted by
laser scanning in production of topographic plans,
cadastral projects as well as in engineering surveys in
such areas as power engineering, oil and gas industry,
construction. Generally, it would be more correct to
speak about the evolution of the stereotopographic
178
method rather than about its being substituted, as the
terrain data and major contours are obtained directly
through laser scanning.
- The comparison of laser scanning techniques with
other currently known methods of airborne remote
sensing providing 3D data directly, in particular, with
interferometric side-looking radar systems, cannot be
regarded as absolutely correct. Here we must keep in
mind the following. Speaking very roughly, a radar
image shows the distribution of dielectric and
magnetic permeability over the scene so that the
intensity of a reflected signal is determined directly as
the product of dielectric and magnetic permeability of
the medium. The distribution of the radar image
intensity is also largely affected by such factors as
morphological conditions of the surface (ripples on
the water surface), presence of contamination, etc.
The main practical consequence here is that, in spite
of their capability to ensure that the terrain geometry
is measured directly, interferometric radar systems
occupy an ecological niche other than laser scanning
techniques and therefore may not be regarded as an
analogue to be compared with in terms of technical
and economic performance. The main characteristics
of radar data (resolution at a flight height of 2000 m
at the level of first meters, accuracy of geodetic
elevations at the level of 3-7 m) allow them to be
used in a number of areas such as geology.
Based on the foregoing, it seems to be quite appropriate to
speak of a laser scanning method meaning a complex of
procedures associated with the application of laser
detection and ranging and related techniques in topography
and engineering surveys. The laser scanning method is
composed of the following thematic issues:
- applicability of laser scanning techniques to a
particular group of objects and scenes;
- management of aerial surveying and selection of
optimal modes of operation for equipment to meet
specified objective;
- assessment of accuracy and validity of obtained data;
- compatibility of laser data and their integration with
data obtained using other methods of remote sensing
and ground-based measurements as well as the
processing of data for further use in various thematic
applications.
The laser scanning method is described above in a most
general way. Of course, all considerations above need to be
detailed when applied to conditions of a certain survey.
6. THE CONCEPT OF REAL-TIME
MAPPING
LIDAR technology has today completely proven its
effectiveness. Applications like Digital Terrain Model
(DTM) generation and power-line corridor mapping are
already classical. The technology is still progressing, its
main advantage being its combination with other airborne
remote sensing data, such as aerial photography. The
immanent ‘3D nature’ of laser data allows fully automatic
spatial orientation, orthorectification and geopositioning of
imagery. It is obvious from practical perspective that the
simultaneous recording and combined processing of
LIDAR data, aerial imagery and some other kinds of
remote sensing data accelerate the processing cycle and
increase data accuracy and reliability. Such an approach
encapsulates the concept of real-time mapping.
Focus on the development and implementation of systems
and software for real-time geodetic mapping remains a high
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