ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision“, Graz, 2002 
  
finding the “model keypoints”. If there is a large elevation 
difference in elevation, which is the case with discrepancy 
between two strips, more points are needed. Figure 10 shows 
the effect on model keypoints before and after laser strip 
adjustment. 
  
  
Figure 10. “Model keypoints” in the Gävle project. In the left 
image are model key points before and in the right after the 
strip adjustment. 
3.6 Summary of results 
A summary of the results is presented in table 7. As one can 
see, the standard error of unit weight presents 
  
  
  
  
  
  
  
  
  
  
  
so [m] | Observation | Time [s] No of 
density obs. 
[n:th point] 
Calibration 1 | 0.0465 5 1033.6 5 000 
Calibration 2 | 0.0350 1 56.4 19 000 
Svinesund 0.0992 100 2335.5 5 600 
Toensberg | 0.1093 100 563.4 20 000 
Gávle 0.0960 200 409.0 10 000 
  
  
  
Table 7. List of result in practical test sights 
The strip adjustment method used in this case depends on 
measured elevation differences between strips and difference 
towards known points. There are often many observations and 
a few unknowns. In most cases this is a favourable situation 
but in this case it can cause slow convergence. Assume there is 
an area of flat terrain with a couple of ditches. Planimetric 
discrepancies between laser strips will only cause discrepancy 
in the position of the ditch. Assume that you make observations 
in all laser points, most of them will show no elevation 
difference while just a few, e.g. 1-5 % will show elevation 
discrepancies. This means that you only wish to make 
observations where you have discrepancies, i.e. in the areas 
with undulated terrain. Elevation and roll errors are on the 
other hand easier to solve for if the observations are spread 
evenly over the terrain. 
4. SUMMARY AND CONCLUSIONS 
Orientation errors cause systematic errors that in some cases 
can be modelled and corrected for in strip adjustment. There is 
however often a strong correlation between unknowns, which 
limits the possibility to compensate for all errors in all cases. 
In the tests presented in this paper, elevation differences and 
roll offsets were easiest to solve for, as they only need 
elevation difference measurements. Planimetry (X and Y), 
pitch and heading are dependent on gradients in different 
directions, i.e. undulated terrain. In addition to this, unknowns 
are strongly correlated and needs certain fly pattern and/or 
control information to be solved. 
Issues of future improvement in the laser strip adjustment 
procedure can be derived from the practical tests: 
: The procedure of selecting areas of interest for 
matching should be improved — this will speed up the 
convergence 
Further investigation of how to solve for different 
orientation unknowns should be made — this will 
increase the reliability of the method 
The error model should be extended to include 
modelling of strip deformation (might be roll mirror 
scale factor) — this can improve the result in many 
cases 
= More effort should be put to matching laser 
reflectance intensity — this will add information in 
flat areas and make it possible to use e.g. painted 
crossroads as ground control 
In all practical tests presented here there was an improvement 
of the result by doing a strip adjustment. There are still 
investigations to be made for further development of laser strip 
adjustment. The method is necessary for laser data calibration 
and accuracy verification. 
5. ACKNOWLEDGEMENTS 
Many thanks to TopEye AB, Fotonor AS and Terrasolid OY for 
their cooperation, without their help there would not be any 
paper. Thanks also to the National Land Survey for letting us 
take part of their test. 
6. REFERENCES 
Axelsson, P., 2000. DEM Generation from Laser Scanner Data 
using Adaptive TIN Models, IAPRS Amsterdam, 2000 
Burman, H., 2000a. Adjustment of Laser Scanner Data for 
Correction of Orientation Errors. IAPRS Vol. XXXIII 
Amsterdam 2000. 
Burman, H., 2000b. Calibration and Orientation of Airborne 
Image and Laser Scanner Data Using GPS and INS. PhD- 
Thesis, Photogrammetry Reports No 69, 2000. 
Crombaghs, M.J.E., R. Brugelman, E.J. de Min, 2000. On the 
Adjustment of Overlapping Strips of Laseraltimeter Height 
Data. IAPRS Vol XXXIII. Part B3, Amsterdam 2000, pp 230- 
237. 
Maas, H-G, 2000. Least-Squares Matching with Airborne 
Laser Scanner Data in a TIN Structure. IAPRS, Vol XXXIII, 
Part B3, Amsterdam 2000, pp 548-555. 
Maas, H-G, 2001. On the Use of Pulse Reflectance Data for 
Laserscanner Strip Adjustment. IAPRS, Vol XXXIV 
Annapolis, 2001.