COMPARISON OF TWO DIGITAL PHOTOGRAMMETRIC SYSTEMS WITH EMPHASIS ON DTM GENERATION: 
CASE STUDY GLACIER MEASUREMENT 
E. P. Baltsavias, H. Li, A. Stefanidis, M. Sinning S. Mason 
Institute of Geodesy and Photogrammetry Department of Surveying and Geodetic Engineering 
Swiss Federal Institute of Technology (ETH) University of Cape Town 
CH-8093 Zurich, Switzerland 7700 Rondebosch, South Africa 
Commission IV, Working Group 4 
KEY WORDS: Softcopy Photogrammetric Systems, Comparison Softcopy Systems, Glacier DEM/DTM Generation, Matching, 
Accuracy, 3-D, Test 
ABSTRACT 
The aim of this project was twofold. Firstly, we wanted to compare two digital photogrammetric systems and especially check their 
performance regarding DTM generation. The two systems are the Leica/Helava DPW 770 and VirtuoZo, that has been developed by 
the Wuhan Technical University of Surveying and Mapping, China and Geonautics Pty Ltd, Australia and is being marketed by 
VirtuoZo Systems Pty Ltd, Australia. The second aim was to test whether DTM generation could be applied in mountainous regions 
that include glaciers. Due to the irregularity of the terrain and partly the form and image texture of glaciers it was not clear whether 
such approaches could yield usable results. The project was of particular interest to the glaciologists at ETH that regularly monitor the 
volume and displacement of glaciers in Switzerland and other parts of the world. After an overview of the two systems, the matching 
algorithms and the accuracy analysis of the automatically generated DTMs by using ca. 8,700 manually measured points are 
presented. The accuracy is equal to that of manual measurements, if big blunders are excluded. The paper also addresses and 
comments on the DTM strategies and editing tools of the systems, makes a short comparison between analytical and digital systems, 
and reports on different system aspects like ease of use, flexibility, ability to import and use external a priori information, ability to 
access intermediate results, time aspects etc. 
1. INTRODUCTION was used in both digital systems, so that the DTM comparison 
would be more objective. A reference DTM including breaklines 
The Leica/Helava DPW 770 is an advanced system with very covering the glacier and the surrounding mountain slopes was 
extensive functions and a price of over 300,000 SFr. The software measured on the analytical plotter in order to assess the DTM 
runs on a 50 MHz Sun Sparc 10 (in our case) and uses a second accuracy of the digital systems (ca. 8700 points). Initial tests for 
monitor for stereoscopic display of images with passive polarised automatic DTM extraction showed that results were poor in 
glasses. The VirtuoZo is purely software based and runs on regions that had little texture or shadows. This was partly due to 
Silicon Graphics (in our case an IRIS Indigo Elan Graphics) with the dynamic range of the linear CCD of the scanner which was 
stereo capabilities using CrystalEyes active shutter glasses. Its not high enough to accommodate the high contrast of the scene 
functionality is not as extensive as the one of the DPW 770 but its and the way the photographs were taken (exposure was optimised 
price is correspondingly lower (ca. 50,000 SFr.). More details on for glacier, hence the surrounding terrain appeared very dark). 
the systems can be found in Miller et al., 1992, and Zhang et al., Thus, a strong contrast enhancement was performed with a 
1994. Wallis filter (see Figure 1) which lead to a significant 
improvement in feature recognition and image matching. 
The data source for the study consisted of three photographs 
along a strip with 8576 overlap at 1:10,000 scale. The images 
were taken in September and included the glacier tongue (the 2. DTM GENERATION 
most important glacier part for the glaciologists). The height 
range in the scene was ca. 700 m. The negatives were scanned on Using the digital photogrammetric systems, DTMs, orthoimages 
a Zeiss/Intergraph PS | with 15 microns and were later and 3D perspective views (see Figure 2) were generated from one 
subsampled to 30 microns pixel size. The 15 micron images were model. In both systems the algorithms for automatic DTM 
too large to be processed by VirtuoZo, so the automatic DTM generation use epipolar images, and image pyramids for 
generation and the accuracy comparison was based on the 30 derivation of approximations for the position of conjugate points. 
micron images. The control points were measured geodetically Both systems use crosscorrelation for image matching, whereby 
with an accuracy of under 1 dm (3-5 points per stereopair). Due VirtuoZo also uses another global relaxation matching technique 
to the terrain difficulty and the movement of the glacier only a at the end phase in order to check whether the matching results of 
few permanent control points could be established for glacier each point are consistent with the results in its neighbourhood 
monitoring. The image quality of the control points was poor due and thus detect blunders. DPW 770 matches at a regular grid in 
to their small size, making their identification difficult. A Wild object space, VirtuoZo uses a regular grid in image space (thus, a 
ACI analytical plotter was used for the measurement of the subsequent DTM grid interpolation is necessary). Both systems 
control and tie points, and the sensor orientation was computed permit the selection of one out of many matching strategies. In 
by a bundle adjustment using all three images. This orientation both cases a strategy for "steep terrain" was selected (for DPW 
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