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International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
REGISTRATION OF AIRBORNE LASER DATA TO SURFACES GENERATED BY PHOTOGRAMMETRIC MEANS 
Y. Postolov, A. Krupnik, K. McIntosh 
Department of Civil Engineering, Technion — Israel Institute of Technology, Haifa, Israel 
{yurip, krupnik, kerry} @tx.technion.ac.il 
Commission III, Working Group 2 
KEY WORDS: Surface Registration, Laser, Photogrammetry. 
ABSTRACT 
Laser altimetry has provided a source of elevation information, which is both accurate and spatially dense. This information is beneficial 
for the production of visible surface models, especially in areas where traditional photogrammetric methods are unable to provide 
accurate heights. Although laser altimetry has many benefits, it also has limitations due to its lack of thematic information and due to 
calibration errors that may occur during data acquisition. Therefore, it would be beneficial to use both laser data and photogrammetric 
data to achieve the best results. To work with both data sets simultaneously, it must be ensured that the data sets are accurately 
registered. The research presented in this paper describes an algorithm developed specifically for registering surfaces acquired using 
different methods, and in particular, laser altimetry and photogrammetry. 
The surface registration algorithm uses the difference in elevation between the surfaces and the gradients of the surfaces to produce 
observation equations. These are solved using an iterative least-squares adjustment. The transformation parameters that are determined 
by the algorithm include scale, translations and rotations. Testing was undertaken to assess the capabilities of the algorithm. Initial tests 
were carried out using synthetic data sets with known transformations. Further testing was undertaken using airborne laser data and 
aerial imagery covering an urban site located at Ocean City, Maryland. The results of the testing with this data set showed a systematic 
error in the location of the laser data as compared to the photogrammetric data. This paper details the approach taken, including the 
presentation of the equations used to determine the relevant transformation parameters, and the results of the initial experimentation. 
1 INTRODUCTION 
Airborne laser altimetry provides accurate surface points for 
obtaining a digital surface model (DSM). The trend towards 
using laser altimetry is motivated by the high spatial frequency of 
the data, the efficiency of the data capture, and the minimal data 
processing required. Laser altimetry has benefits as it can 
provide measurements in areas where traditional 
photogrammetric techniques encounter problems. Such areas 
include urban areas, wooded areas and areas that produce little or 
no texture or contrast in the digital imagery being used. 
Following this determination, laser data can be considered as 
complementary to photogrammetric techniques and would 
provide benefits when combined with data obtained from these 
methods. This approach has been suggested recently by many 
researchers (Ackermann, 1999; Axelsson, 1999; Baltsavias, 1999; 
Brenner, 1999; Csathó et al., 1999; Fritsch, 1999; Haala, 1999; 
Haala and Anders, 1997; Toth and Grejner-Brzezinska, 1999; and 
Vosselman 1999). 
Utilizing data from both laser altimetry and photogrammetry 
requires that the two data sets relate to the same coordinate 
system. To ensure this is the case, the surfaces generated from 
the data sets must be registered as accurately as possible. The 
algorithm presented in this paper is specifically designed for 
registering surfaces derived from laser data and photogrammetric 
data. 
The transformation parameters between two surfaces, which both 
contain irregularly distributed points, are determined without 
requiring the surfaces to be interpolated to a regular grid. These 
parameters represent a three-dimensional conformal 
transformation, and include scale, translations and rotations. 
Observation equations are based on the difference in elevation 
between the surfaces and the local gradients. The parameters are 
estimated using an iterative least-squares solution. 
Testing was undertaken to assess the capability of the algorithm 
to accurately register two surfaces. Initial tests were carried out 
using synthetic data sets with known transformations. These tests 
were useful to show the validity of the algorithm, and also to 
eliminate any implementation flaws. The sensitivity of the 
algorithm to random errors was investigated by introducing such 
errors to the data sets. Further testing has been undertaken using 
airborne laser data and aerial imagery covering an urban site over 
Ocean City, Maryland. 
In section 2, the suggested approach is described in detail, 
together with the mathematical model. Section 3 presents the 
results obtained so far. Results from both synthetic and real data 
experiments are shown. 
2 MATHEMATICAL BACKGROUND OF THE 
PROPOSED SURFACE MATCHING PROCEDURE 
The aim of the surface matching procedure is to register the 
airborne laser data to the surface generated by photogrammetric 
means, thus allowing the surfaces to be transformed to a common 
coordinate system. The most common methods for determining 
the orientation parameters between two data sets are based on 
conjugate points. These methods are not applicable when using 
airborne laser data as the laser measurement is referring to a 
footprint, and not to a specific point which can be identified on 
the ground (Baltsavias, 1999). The similarity between the height