History of Airborne laser scanning -a new and independent technology in GIS-Photogrammetry, Part 2

The high measuring rate of airborne 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 for airborne laser scanning 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. Airborne laser scanning is not capable of any direct pointing to particular objects or object features. The resulting 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 accuracy, high sampling densities, and a high degree of automation.

The laser footprints directly measure the visible ground surface or objects on it. However, objects without a well-defined surface, like trees or cornfields, 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 working group 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 20–40% 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 modeling, on the basis of data analysis and data redundancy. Still, some types of vegetation present difficulties. In particular, the penetration 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 applications. Airborne laser scanning is presently in that process, spreading into other fields beyond the DTM generation. Multiple returns from vegetation covers imply, for instance, that information about the vegetation 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 airborne laser scanning concerns the automatic capture of buildings in built-up areas for city modeling purposes. Buildings and constructions, masking the ground surface, were originally considered as obstructions to be removed in the DTM generation. In the meantime, the recognition and capture of buildings has become an important independent task. In built-up areas, many laser points lie on the superstructure of buildings, in particular, on flat or gabled roofs. With high sampling densities, of e.g., several points per square meter, the vertical geometric distribution of the raw laser data allows the delineation of buildings in very close approximation, i.e., the automatic detection and geometric capture of buildings.

There are other cases where detailed terrain features and structures are discernible and can be derived 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 terrain. Such applications were originally not anticipated but simply follow from the technical performance, which airborne laser scanning has reached.