Comparison between airborne laser scanning and aerial photogrammetry

The discussion in last posts 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 information which is more than just geometric surface description. The airborne laser scanning is still expanding and has become a serious competitor to aerial photogrammetry, 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, although photogrammetry still is to a lesser degree. Their results are geometric and can reach, in the application to high precision DTMs, similar accuracies. With either method, more or less extended areas can be covered although flying time per unit area is much shorter for photogrammetry..

On the other hand, there are highly essential differences between airborne laser scanning and aerial photogrammetry. Laser scanning is an active system, applicable even at night. It provides ground points in a certain pattern which is primarily determined by the system design and only influenced 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 rectification, for instance, usually asks for the vegetation surface. As far as buildings are concerned, both methods have, in a way, supplementary properties.

 The airborne laser scanning 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 information exists for the identification and extraction of buildings and other man-made constructions. However, there are still great problems with regard to the automatic measurement of buildings from image data.