International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
UTILIZING AIRBORNE LASER ALTIMETRY FOR THE IMPROVEMENT OF AUTOMATICALLY
GENERATED DEMS OVER URBAN AREAS
K. MceIntosh!, A. Krupnik!, T. Schenk?
! Department of Civil Engineering, Technion - Israel Institute of Technology, Haifa, Israel
{kerry, krupnik} @tx.technion.ac.il
“Department of Civil and Environmental Engineering and Geodetic Science, The Ohio State University, Columbus, Ohio, USA
schenk.2@osu.edu
Commission III, Working Group 2
KEY WORDS: Data Fusion, Laser, Photogrammetry, Surface Reconstruction.
ABSTRACT
Airborne laser altimetry is a highly efficient and accurate method of obtaining data for the determination of visible surface topography.
With minimal processing, the laser data can provide coordinates of points on the visible surface with high spatial frequency and
precision. Although this technology has benefits compared to photogrammetric techniques, there are limiting factors due to the laser
data having no thematic information.
photogrammetry in the surface determination process.
These limitations may be overcome by utilizing aspects of both laser altimetry and
The research described in this paper has been undertaken to accurately determine the visible surface in urban areas using airborne laser
scanner data and digital aerial images. In this research, edges detected in the aerial images are used to refine the digital surface model
(DSM) produced from airborne laser scanner data. The three dimensional edge information allows improvement of the laser DSM by
providing accurate horizontal locations of the surface discontinuities. Therefore the laser data and the edge information are merged to
obtain the benefits of each data set, facilitating the generation of an accurate surface model.
The paper presents preliminary results of testing undertaken using an algorithm developed to combine laser data and photogrammetric
data. The data set used in the testing is an urban site covering Ocean City, Maryland, USA.
1 INTRODUCTION
Highly accurate models of the visible surface in urban areas are
becoming widely used in many applications, such as digital
orthophoto production, three dimensional (3D) city modelling
and 3D building reconstruction. Methods for generating surface
models of urban areas include using laser scanner data and using
digital photogrammetric methods. Both methods of DSM
generation have advantages and limitations.
The research presented in this paper utilizes information from
laser scanner data and photogrammetric data to produce an
accurate model of the visible surface. There are two phases to the
project. In the first phase, surfaces are created from each data
source and are accurately registered to the same coordinate
system. The second phase consists of the extraction of edge
information from the photogrammetric data, which is used to
delineate surface discontinuities in the urban scene. By merging
the edge information and the laser data for surface generation, a
DSM which more closely represents the actual scene can be
produced.
Digital photogrammetric methods of automatic surface
reconstruction have become widely used due to the efficiency and
cost effectiveness of the production process, especially in open or
flat areas, and when using small and medium scale imagery
(Krzystek and Ackermann, 1995). However, most software
packages perform poorly in areas with abrupt height differences,
such as those occurring frequently in urban areas (Haala, 1999).
The degradation in performance can be caused by failures of the
image matching process (Axelsson, 1998). Such failures may be
due to factors like lack of texture in the images (Haala, 1994),
poor image quality, shadows in the images, occlusions, surface
discontinuities (Haala et al., 1997) and foreshortening (Schenk
and Toth, 1992). The problems occur when using digital
photogrammetry in urban areas result in inaccuracies in the DSM,
which can be seen in the smoothing effect on surface
discontinuities (Baltsavias, 1999; Haala, 1999; Toth and Grejner-
Brzezinska, 1999). Research continues into overcoming the
problems of deriving DSMs using large-scale imagery and digital
photogrammetric techniques (Cord et al., 1999; Gooch et al.,
1999; Veidman and Krupnik, 1999).
One of the benefits of photogrammetry is that the imagery
contains more information than just the position of pixels in the
images. Grey-value changes in the images allow the identification
and classification of objects, such as buildings or vegetation, and
can be used to detect edges in the images, which often indicate
the location of surface discontinuities (Baltsavias, 1999; Fradkin
and Ethrog, 1997; Haala and Anders, 1997).
Laser scanning is recognized as an accurate data source for DSM
generation in urban areas (Haala et al., 1997). The spatial
resolution of the data is dependent on several factors, such as
