DIGITAL SURFACE MODELLING FROM SPOT 5 HRS IMAGERY USING THE AFFINE 
PROJECTIVE MODEL 
C. S. Fraser ?, P.M. Dare ?, T. Yamakawa * 
a : S ^A. . : . ” ; 3 s 
Department of Geomatics, University of Melbourne, Victoria 3010, AUSTRALIA 
c.fraser@unimelb.edu.au; yamakawa@sunrise.sli.unimelb.edu.au 
^ Airborne Research Australia, Flinders University, PO Box 335, Salisbury South, SA 5016, AUSTRALIA 
Paul.Dare@AirborneResearch.com.au 
Commission I, WG 1/2 
KEYWORDS: SPOT, DEM/DTM, high resolution, matching, accuracy 
ABSTRACT: 
This paper presents the results of work carried out under the auspices of the joint ISPRS CNES Scientific Assessment Program of 
DEM generation from SPOT 5 high resolution stereoscopic (HRS) image data. À stereo pair of HRS images of Melbourne, Australia, 
with a ground sample distance of 5m in the along track direction, and 10m in the across track direction, was used to create digital 
surface models of varying grid spacings of the urban area surrounding the city of Melbourne. Conjugate points from the SPOT 
stereopair were located using intensity based image matching, and triangulated using the affine projective model. The resultant 
digital surface models were compared to precisely located ground control points and a reference digital terrain model. The results 
showed that high quality digital surface models can be produced from HRS image data using the algorithms described. The 
comparison between the surface model and ground control points revealed an RMS height error of 3-5m or 0.6-1 pixel. 
1. INTRODUCTION 
The purpose of the SPOT 5 Scientific Assessment Program 
(SAP) is to evaluate the utility of SPOT 5 high resolution 
stereoscopic (HRS) imagery for generating digital terrain 
models (DTMs) (Baudoin et al., 2004). Under this program, 
tests have been carried out by many different research teams on 
SPOT 5 HRS data for many different regions of the world. The 
study described in this paper has made use of data acquired 
over the city of Melbourne, Australia. Since the research teams 
involved in the SAP are independent, a range of different 
processing techniques have been brought to bear on the issue of 
extracting digital surface models (DSMs) from SPOT 5 HRS 
imagerv. 
Previous research carried out at the University of Melbourne on 
the subject of geometric modelling of high-resolution 
spaceborne sensors has focused on the use of ‘alternative’ 
mathematical models, such as the rational polynomial 
coefficient (RPC) model (Fraser et al., 2002; Hanley et al., 
2002) and the affine projective model (Yamakawa et al., 2002, 
Fraser ‘and Yamakawa, 2004). The success of these studies has 
led to these models being incorporated into the digital surface 
modelling algorithms created specifically for high-resolution 
satellite imaging sensors, such as IKONOS and QuickBird. 
Results of studies utilising stereopairs of IKONOS images have 
shown that both the RPC model and affine projective model can 
be used to create high quality DSMs (Dare, 2004; Dare and 
Fraser, 2004). 
Since SPOT 5 HRS data is not supplied with RPCs, the 
research presented in this paper concentrates on the use of the 
affine projective model. Note that traditional photogrammetric 
techniques can also be used to generate surface models from 
SPOT 5 data, but such investigations have been left to other 
members of the SAP. 
2. SPOT 5 DATA 
The data used in this study covered an area of roughly 140km 
by 50knf around the city of Melbourne, Australia (Figure 1). 
The image was acquired at 10:20am local time on February 
19^. 2003. The land cover types in the scene include water, 
urban areas, agricultural land and forests. The elevation varies 
from sea level to around 600m above mean sea level. A portion 
of the western side of the region was partially obscured by 
cloud cover during image acquisition, but this made little 
difference to the generation of the surface models in this study, 
since these were centred on other areas within the scene. 
The pixel size of HRS imagery is 5m in the along track 
direction, and 10m across track. Since the stereo images were 
acquired in the same orbit in a fore and aft configuration, the 
parallax differences occur in the along track direction. 
Therefore, the along-track pixel size (5m) is relevant when 
discussing the accuracy of surface models with respect to the 
pixel size of the original images. 
3. AFFINE PROJECTIVE MODEL 
Since the launch of the first high-resolution satellites, and even 
before, much attention has been focused on the use of 
alternative geometric models, especially RPCs, for restitution, 
orthorectification and terrain modelling (Dowman and Dolloff. 
2000; Dial, 2000; Grodecki, 2001; Grodecki and Dial, 2001). 
Significant research has also been carried out on other models, 
such as the direct linear transform (DLT) model and the affine 
projective (AP) model. An inter-comparison of these three 
models (RPC, DLT and AP) with IKONOS data showed that 
although the degree of complexity of the models differs widely, 
differences in the results can be expected to be small (Fraser et 
al., 2002, Hanley et al., 2002).