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7/31/2019 01. Airborne Laser Scanning - Present Status and Future Expectations, 1999
1/4
.ISPRS Journal of Photogrammetry & Remote Sensing 54 1999 6467
Airborne laser scanningpresent status and future expectations
Friedrich Ackermann )
Pfeilstrasse 22, D-70569 Stuttgart, Germany
Keywords: airborne laser scanning; digital terrain model; continuous wave laser
1. Present performance and application
Airborne laser scanning represents a new and
independent technology for the highly automated .generation of digital terrain models DTM and sur-
face models. Its development goes back to the 1970s
and 1980s, with an early NASA system and other
attempts in USA and Canada.
Then, the GPS solution of the critical positioningproblem made high accuracy performance feasible.
Thorough investigations at Stuttgart University from
19881993 with a laser profiler proved the high
geometric accuracy potential, especially for DTM
generation, and clarified the essential system parame-
ters. The scene was set for the development of
genuine scanning systems, which then followed in
quick sequence. The method has successfully estab-
lished itself within a few years, and quickly spread
into various practical applications. In this issue of the
ISPRS journal, a number of technical and application
papers display the status and performance of air-borne laser scanning. Here, in this paper, some addi-
tional considerations are submitted on the potential
further development and application of the method.
Naturally, these will be personal views.
)
Fax: q49-711-2288111
The development of airborne laser scanning has
been technology driven. It became initially possible
by pulse lasers operating in the near infrared, which
gave clearly recordable return signals after diffusion
and reflection on the ground. The travel times are
recorded to nearly 10y1 0 s and converted to distance. .Recently, also continuous wave CW lasers are
used, which obtain range by phase measurements.
Precise kinematic positioning of the platform bydifferential GPS and inertial attitude determination
by IMU now provide the accurate reference to an
external co-ordinate system.
Laser scanning systems furnish geometric results
in terms of distance, position, attitude, and co-
ordinates. For each shot, the spatial vector from the
laser platform to the point of reflection is estab-
lished, thus providing the XYZ co-ordinates of the
footprint. The overall vertical system accuracy is
usually in the dm order. Most systems presently
operate at flying heights of up to about 1000 mabove ground. The scan angle is generally -"308,
in most cases -"208. Some laser scanning systems
provide, in addition to range, information on theintensity of the recorded signal or information range,
.and in some cases, also amplitude for multipleechoes within one pulse see paper by A. Wehr and
.U. Lohr in this issue .
0924-2716r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .P I I : S 0 9 2 4 - 2 7 1 6 9 9 0 0 0 0 9 - X
7/31/2019 01. Airborne Laser Scanning - Present Status and Future Expectations, 1999
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( )F. AckermannrISPRS Journal of Photogrammetry & Remote Sensing 54 1999 64 67 65
The high measuring rate of laser scanning is of
particular importance. Present measuring rates lie
between 2 kHz and 25 kHz, one system reaches 80
kHz. Accordingly, from 1000 m flying height, the
sampling densities on the ground range from about 1
point per 20 m2 up to 20 points per m2. The actual
sampling density depends on the system and on the
balance between flying speed, pulse rate, scan angle,
and flying height. The geometric sampling pattern on
the ground is pre-determined by the system design.
But it is not rigidly fixed, as it also depends on the
irregular flying path, and on the 3-D structure of the
terrain.
Laser scanning is not capable of any direct point-
ing to particular objects or object features. The re-
sulting co-ordinates refer to the footprints of the
laser scan as they happen. In that sense, it is a blind
system, but particularly distinguished by high accu-
racy, high sampling densities, and a high degree ofautomation.
The laser footprints directly measure the visible
ground surface or objects on it. However, objects
without a well-defined surface, like trees or corn-
fields, may produce several separately recordable
reflections of one incident pulse. Hence, a laser pulse
can penetrate partly into and possibly through the
vegetation cover of the terrain. This potential of
passing through forest canopies was the original
motivation to study laser systems for the purpose of
generating DTMs in forest areas at our laser workinggroup at the University of Stuttgart. It was found that
with near vertical incident angles of the laser system,
penetration rates to the ground, in European type
coniferous and deciduous forests, of 2040% can be
expected, and up to nearly 70% in deciduous forests
in winter time.
The multiple signal returns from forests or other
vegetation covers do not represent any particular
surface. The required ground surface must be derived
by mathematical modelling, on the basis of data
analysis and data redundancy. Still, some types of
vegetation present difficulties. In particular, the pen-
etration capability of laser signals through dense
tropical rain forest remains questionable, although
successful attempts have been reported.
Laser pulses may also be reflected from objects,
which are below the resolution as suggested by the
footprint diameter. Examples are electric power lines
or steel structures, which can be captured, indeed, by
airborne laser scanning.
The described technical features of airborne laser
scanning outline the present fields of application.
The primary application concerns the generation of
high quality topographic DTMs, described by mostly
regular grid patterns. It is the unique advantage of
airborne laser scanning that it is equally applicable to
open terrain as well as to areas which are partly or
completely covered by forest or other vegetation.
Naturally, the interactive editing efforts in the latter
case are higher. Another important application of
laser scanning also concerns the generation of DTMs
in coastal areas or wetlands which are difficult to be
obtained by other methods.
It is a general feature of new technologies that
their technical potential soon opens up new applica-
tions. Airborne laser scanning is presently in that
process, spreading into other fields beyond the DTMgeneration. Multiple returns from vegetation covers
imply, for instance, that information about the vege-
tation itself can be obtained. Also, the survey of
electric power lines together with the under-growing
vegetation has become a highly interesting special
application, the demand for which is growing fast.
A particularly interesting new application of air-
borne laser scanning concerns the automatic capture
of buildings in built-up areas for city modelling
purposes. Buildings and constructions, masking the
ground surface, were originally considered as ob-structions to be removed in the DTM generation. In
the meantime, the recognition and capture of build-
ings has become an important independent task. In
built-up areas, many laser points lie on the super-
structure of buildings, in particular, on flat or gabled
roofs. With high sampling densities, of e.g., several
points per square meter, the vertical geometric distri-
bution of the raw laser data allows the delineation of
buildings in very close approximation, i.e., the auto-
matic detection and geometric capture of buildings.
There are other cases where detailed terrain fea-tures and structures are discernible and can be de-
rived from the geometric information alone which is
provided by laser scanning of high sampling density.
For instance, breaklines of the terrain can indirectly
be extracted to some extent. Other examples are
dunes, hedges, walls, ditches, dams etc., which can
be delineated from laser points, especially in flat
7/31/2019 01. Airborne Laser Scanning - Present Status and Future Expectations, 1999
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( )F. AckermannrISPRS Journal of Photogrammetry & Remote Sensing 54 1999 64 6766
terrain. Such applications were originally not antici-
pated but simply follow from the technical perfor-
mance, which airborne laser scanning has reached.
2. Some comparison aspects to photogrammetry
The above review has shown that airborne laser
scanning is a new, versatile, and to a high degree,
automatic method for obtaining terrain information.
Data processing and object modelling provide infor-
mation which is more than just geometric surface
description. This method is still expanding and has
become a serious competitor to aerial photogramme-
try, with regard to some applications.
Airborne laser scanning is comparable in some
ways to the photogrammetric method of automatic
generation of DTMs by digital processing of image
data. Both methods are highly automated, althoughphotogrammetry still is to a lesser degree. Their
results are geometric and can reach, in the applica-
tion to high precision DTMs, similar accuracies.
With either method, more or less extended areas canbe covered although flying time per unit area is
.much shorter for photogrammetry .
On the other hand, there are highly essential
differences between both methods. Laser scanning is
an active system, applicable even at night. It pro-
vides ground points in a certain pattern which is
primarily determined by the system design and onlyinfluenced to some extent by the geometry of the
terrain surface and its cover. The photogrammetric
points, measured automatically or interactively, may
be arranged in a pre-fixed rigid pattern. But quite
often, they are quite arbitrarily selected, depending
on image texture and image features. In case of
vegetation, they would lie on the canopy, while laser
points can be on or within and below the vegetation
cover. Whether the photogrammetric restriction to
the visible canopy of vegetation is a disadvantage or
not depends on the purpose of the intended terrain
and surface model, respectively. Orthophoto rectifi-
cation, for instance, usually asks for the vegetation
surface.
As far as buildings are concerned, both methods
have, in a way, supplementary properties. The laser
system provides high density of points, but it does
not directly capture features like breaklines, roof
ridges or the like. On the other side, photogrammetry
has image information about the objects which, in
principle, allows capturing of breaklines or linear
and spatial objects. Especially, all necessary informa-
tion exists for the identification and extraction of
buildings and other man-made constructions. How-
ever, there are still great problems with regard to the
automatic measurement of buildings from image data.
3. Expected further developments
Airborne laser scanning has had a fast and most
successful development. Today, it is an established
method with high technical and economic perfor-
mance. Where does it stand now? Has it reached its
culmination, or can further developments be ex-
pected? If we evaluate and extrapolate its present
status, the trend becomes clearly evident that theapplication of airborne laser scanning will continue
to expand, in combination with a further deployment
of the technological potential.
It can be safely anticipated that the present techni-
cal performance of laser scanning systems will be
extended and used in more diversified applications.
Pulse rates and resolution, in terms of size and
spacing of footprints, may become more adaptive.
Platforms on low-flying helicopters can provide re-
fined ground information for special applications up
to monitoring of local scenes. On the other hand, theabsolute system accuracy may still be increased, and
higher flying heights will provide larger area cover-
age. Such pending developments will mainly be
application-motivated. We may also see refined elec-
tronic analysis of the return signals from which
additional information about surface characteristics
of the footprints on the ground can be derived.
Another item will be the comparison between pulsed
and CW lasers. It may also be mentioned that there
is a potential competition with DTMs derived by
interferometric SAR, although the latter still operatesin a different accuracy and scale range.
It can be anticipated that a certain consolidation
and extension of the laser scanning method will take
place in the near future, concerning data processing,
in view of extended and also more specialised appli-
cations. This progress will concern intelligent filter-
ing and thinning out of data. It will also imply more
7/31/2019 01. Airborne Laser Scanning - Present Status and Future Expectations, 1999
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( )F. AckermannrISPRS Journal of Photogrammetry & Remote Sensing 54 1999 64 67 67
complete information extraction by more sophisti-
cated object modelling, in particular, with regard to
objects and features, which are not directly captured.
Examples may be geomorphologic structures, land-
scape modelling, city models, change detection or
integration and comparison with existing databases.
With increasing and extended applications, new
business opportunities will certainly emerge with the
result that airborne laser scanning, in combination
with other techniques, will constitute an indispens-
able, powerful, and highly economic method in the
world of geographic information acquisition. Here,
however, a critical remark may be appropriate. The
user community hopes very much that, contrary to
the present trend, in future, the system parameters
and algorithms will be sufficiently disclosed to allow
judgment of performance. In addition, standards
should be established for ensuring operational relia-
bility and quality.In view of more fundamental further develop-
ments of airborne laser scanning, we take a look at
the present restrictions. There is the basic limitation
set by the geometric nature and the sampling system
of the method, with its blindness about the capture
and identification of objects and object features.
What is missing is additional image information.
This limitation has not been too restrictive, so far, as
it has partly been overcome by modelling assump-
tions in the data processing. But complex cases still
have to be edited by interactive interpretation basedon pre-knowledge or on any available object visuali-
sation.
Present laser scanning systems can provide image
information taken by video cameras during the flight
mission. But the video images are usually no inte-grated part of the laser data system i.e., no
.spatialrtemporal co-registration with laser data or
give only supplementary support for interactive edit-
ing and object modelling. Results and performance
could certainly be enhanced, if image information
would become an integral part of automated data
processing. Hence, most likely, the laser scanning
systems will become supplemented with digital cam-
eras. It would mean the direct and possibly automatic
merging of geometric scan data with digital image
data for the purpose of object recognition and object
capture. City modelling is a promising primary ex-
ample.
The systematic combination of digital laser and
image data will constitute an effective fusion with
photogrammetry, from a methodical and technologi-
cal point of view. It would resolve the present state
of competition on a higher level of integration and
mutual completion, resulting in highly versatile sys-
tems and extended application potential. A total fu-
sion would certainly agree with the general trend
towards universal multi-sensor and multi-data sys-
tems.
A similar fusion can be expected by the combina-
tion of geometric laser scanning with multi-spectral
imaging systems. In that way, the integration with
photogrammetry would be extended to include re-
mote sensing applications in a wide range. Equallyconceivable is the combination with hyperspectral
lidar systems.
The concept of data fusion can be pushed very
much further. A particularly interesting possibility is
the recording of the intensity of the return signal.
Unfortunately, such digital images are presently lim-
ited to monochromatic image data and imply some
under- or over-sampling. Therefore, they are not yet
fully comparable to digital photogrammetric images.
Nevertheless, it is a fascinating aim to obtain in
principle digital image data together with polar posi-tion data for each image element. It would be a
complete revolution in photogrammetry if image data
could directly be combined with spatial position
data. The consequences could be as dramatic as what
happened in surveying, when polar geometry re-
placed the century-old intersection methods of point
determination.
Summarising this brief outlook, it can be stated
that airborne laser scanning will certainly continue to
proceed technically and to expand its applications.
The potential integration with imaging sensors isexpected to put airborne data acquisition on a revolu-
tionary new level of system performance with far
reaching prospects.