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point on the ground to be determined. LiDAR works on the direct measurement of range using a laser, from a known 
point in a known direction determined by GPS and IMU techniques. 
Before, the DSMs from either technique are used, it is important to assess the quality of the measurements. To do this it 
is important to understand the principles of each technique. A fundamental question to be answered is “what surface is 
being measured?” In addition to the determination of heights it is important to compare what other information is 
available for example, a photographic image is available for interpretation if photogrammetry is used. 
This paper will present a brief background to the technique of automated DSM creation by digital photogrammetry with 
particular reference to the influences on the quality. This will be followed by a description of LiDAR as this is perhaps 
less well known. A discussion is given on the possible quality of a LIDAR DSM with reference to some examples of 
applications. Some conclusions will be drawn and future plans for this project will be proposed. The poster presentation 
will show results obtained so far from the trials undertaken at Nottingham where a test site has been established. 
2 DIGITAL PHOTOGRAMMETRY 
2.1 Digital images and DSM generation 
Although there are scanners that can provide digital images they have often been developed as a multi-spectral sensors 
for remote sensing applications, not for the more traditional mapping purposes. The push broom type scanners create 
images with perspective geometry across track and the accumulation of scan lines along track. These sensors have had 
their mapping potential improved by the integration of GPS and Inertial Navigation System (INS) (Shukla, 2000). 
However, most digital images for photogrammetric applications use scanned standard aerial photography. The DSM 
that is created from this imagery is defined by what is captured on the traditional film. This in turn is dependent on the 
type of film used. This is normally film capturing the visible spectrum as opposed to for example, infra red film. The 
type of film is the first of numerous influences on the quality of the resulting DSM. The choice of camera, if there is 
one, will be partly dependent on the lens principal distances available. The automatic generation of a DSM by digital 
photogrammetry is then going to be dependant on the choice of a number of parameters some are basic and are like the 
choice of film and camera, they are perhaps going to affect all image matching algorithms. Other parameters are 
dependent on the type of matching algorithms used. The following is a list of some of the basic parameters which 
influence the quality of DSM:- 
Camera Scale of photography 
Film Image band used (red, blue or green if using colour photography) 
Scanning resolution DSM grid spacing 
Type of terrain Image quality 
The parameters that directly control the image matching algorithms for example in ERDAS IMAGINE OrthoMAX 
(1995) include the following :- 
Minimum precision 
Rejection factor 
Noise threshold 
Minimum and maximum template size 
Maximum parallax 
Minimum threshold 
The parameters affect the DSM in different ways for example, a number of these parameters have an effect of smoothing 
the DSM created (Smith et al., 1997). Even with a very fine grid spacing some of the sharpness of features will not be 
retained. This could be particularly important in urban studies where building outlines are important. However, the 
DSM is not the only information that is available as there is also the image itself which can be processed to extract 
further information. These various forms of information can be analysed by the use of image processing tools or GIS to 
assist in understanding the landscape. 
3 LIDAR TECHNOLOGY 
LiDAR stands for “Light Detection And Ranging”. The use of this technology from aircraft for ground height 
measurements goes back to the 1970's. However, it hasn't been involved in commercial applications until the last few 
years. It has both sounding and ranging remote sensing applications. In the sounding application, the back scattering 
properties (reflection) of laser can be applied in atmospheric studies (Rees, 1990). However, the ranging method is 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 45