01. Airborne Laser Scanning - Present Status and Future Expectations, 1999

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.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


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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


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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


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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.

Documents

01. Airborne Laser Scanning - Present Status and Future Expectations, 1999