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Helén Burman 
  
ADJUSTMENT OF LASER SCANNER DATA FOR CORRECTION OF ORIENTATION ERRORS 
Helén Burman 
Department of Geodesy and Photogrammetry 
The Royal Institute of Technology 
Stockholm, Sweden 
helen @geomatics.kth.se 
Working Group III/1 
KEY WORDS: Laser scanning, GPS, INS, Adjustment, Matching. 
ABSTRACT 
GPS and INS measurements are used for georeferencing of laser scanner data. Errors in the GPS and INS measurements 
are propagated to the co-ordinates of the ground point reflecting the laser beam. In this paper, discrepancies between 
overlapping laser strips are modelled as orientation errors. The discrepancies are measured, both in elevation and in 
intensity data, through matching in height and plane. Special interest is put on the alignment between the INS and the 
laser scanner co-ordinate system. It is shown that all three alignment angles; roll, pith and heading, can be found 
without ground control if the same area is covered by at least three laser strips flown in different directions providing 
there are height or intensity gradients in the area. To get redundancy, the recommended configuration is to cover the 
area with four strips in four different directions. The same method used for alignment can be used for adjustment of 
blocks of overlapping laser strips. 
1 INTRODUCTION 
The LRF (Laser Range Finder) measures distances from the sensor to the reflecting target and the reflected energy 
(intensity). The co-ordinates of the reflecting target can be calculated if we know the laser beam orientation, which can 
be measured by GPS and INS. A difference between two or more laser strips covering the same area can be caused by 
orientation errors, which in their turn can be caused by positioning errors, misalignment between the LRF and the 
inertial system or by drift or bad initialisation of the inertial system. Many of these errors can be corrected for by a set 
of shift and/or drift parameters, see e.g. (Kilian ef al. 1996) or (Lemmens 1997). This method is used in GPS-supported 
block triangulation where each strip has its own set of parameters. The same method can be used for adjusting laser 
strips to make them coincide in overlapping areas. Both elevation and intensity data in overlapping lasers strips are 
matched. The two different types of observations complement each other as they often create large gradients in different 
areas. A more extensive description of the work presented in this paper can be found in (Burman 2000). 
2 THE OBSERVATION EQUATION 
A laser scanner observation (a laser shot) is denoted (X, Y, Z),, and is obtained from the processed laser data. The co- 
ordinates of a laser shot is a function of the exterior orientation of the sensor (the Laser Range Finder (LRF)) and the 
laser shot vector. We introduce a shift in the GPS co-ordinates, ( X ,, Y,,Z,)' , to model errors in the GPS co-ordinates, 
e.g. caused by errors in the datum transformation or tropospheric delay. Assuming the GPS antenna vector is known the 
following equation is formulated. 
X Xp X, I, 
LRF 
Y|=[Yo: HIT, + Rus (Rs ) # (D 
£ I Zo Zi [ LRF 
(X,Y,Z y laser shot co-ordinates in the local frame 
(X 19:70 y position of the laser scanner (GPS measured position corrected for antenna eccentricity) 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 125