International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August - 01 September 2012, Melbourne, Australia 
271 
EXTRACTING PRECISE AND AFFORDABLE DEMS DESPITE OF THE CLOUDS. 
AJAX: THE JOINING OF RADAR AND OPTICAL STRENGTHS 
L.Cunin 3 , P.Nonin b , J. Janoth c , M.Bernard d * 
a IGN Espace , 6 avenue de l’Europe , 31520 Ramonville Saint Agne, France. Laurent.Cunin@ign.fr 
b ASTRIUM ASV GEO, 2600 Route des Crêtes , 06905 Sophia Antipolis Cedex, France. Philippe.Nonin@spotimage.fr 
c ASTRIUM ASV GEO Infoterra, D-88039 Friedrichshafen, Germany. Juergen.Janoth@astrium.eads.net 
d ASTRIUM ASV GEO, 5 rue des Satellites, BP 14359, 31030 Toulouse, France. Marc.Bemard@spotimage.fr 
Commission IV, WG IV/6 
KEY WORDS: Radar, SPOT, DEM/DTM, Fusion, Precision, Three-dimensional, Quality, Cost 
ABSTRACT: 
On the one hand, onboard SPOT 5, the HRS instrument systematically collects stereopairs around the Globe since 2002. Each 
stercopair can encompass an area up to 600 km x 120 km within a single pass (i.e. 72 000 km 2 stereoscopic strips). Covering now 
more than 120 millions sq.km of the Earth landmasses, SPOT 5 stereoscopic imagery has become one of main satellite data sources 
for accurate DEM extraction, at least where the cloud coverage leaves a chance to do so ! 
On the other hand, the TerraSAR-X satellite, launched in June 2007, is able to collect radar data through the clouds in several modes. 
An approach to extract height information by radargrammetry was developed, and the commercial distribution of Digital Elevation 
Models based on TerraSAR-X StripMap and SpotLight Modes (resp. 3m and lm resolution) has started in 2010. To improve the 
overall height accuracy of the DEM, acquisitions from both orbit directions are utilised, each point on the ground being thus imaged 
at least 4 times by TerraSAR-X. 
Since 2002, Spot Image and French National Cartographic Institute (IGN) are building a worldwide database called 
Elevation30/Reference3D™, which includes a Digital Elevation Model at 1-arc-second resolution (DTED level 2) extracted from 
HRS stereopairs. 
To answer the wide demand of precise DEMs over Tropical and Northern areas, frequently covered by clouds, a study was 
performed to integrate StripMap radargrammetric TerraSAR-X data into the Reference3D process, and two prototype products were 
issued, over Colombia and Congo areas. During this experiment, efforts have been made to stick to technical steps that could be 
integrated within a standardized production process, in order to keep offering affordable prices while maintaining a high standard of 
horizontal and vertical accuracy. 
The DEMs extracted from TerraSAR-X and HRS proved extremely consistent with each other, showing a mean difference of 0.80m. 
This allows to propose a unified Elevation30 product to the users, with a guaranteed accuracy materialized into the product through a 
dedicated vertical Accuracy Commitment Mask. 
1. INTRODUCTION 
On the one hand, onboard SPOT 5, the HRS instrument 
systematically collects stereopairs around the Globe since 2002. 
Each stereopair encompasses an area up to 600 km x 120 km 
within a single pass (i.e. 72 000 km 2 stereoscopic strips). 
Covering now more than 120 million sq.km of the Earth 
landmasses, SPOT 5 stereoscopic imagery has become one of 
main satellite data sources for accurate DEM extraction, at least 
where the cloud coverage leaves a chance to do so ! 
On the other hand, the TerraSAR-X satellite, launched in June 
2007, is able to collect radar data through the clouds in several 
modes. An approach to extract height information by 
radargrammetry was developed, and the commercial 
distribution of Digital Elevation Models based on TerraSAR-X 
started in 2010. 
Since 2002, Spot Image and French National Cartographic 
Institute (IGN) are building a worldwide database called 
Elevation30/Reference3D™, which includes a Digital Elevation 
Model at 1-arc-second resolution (DTED level 2) extracted 
from HRS stereopairs. 
To answer the wide demand of precise DEMs over Tropical and 
Northern areas, frequently covered by clouds, a study was 
performed to integrate StripMap radargrammetric TerraSAR-X 
data into the Refercnce3D process, and two prototype products 
were issued, over Colombia and Congo areas. 
2. DEM EXTRACTION FROM RADAR AND OPTICAL 
“-GRAMMETRIC” DATA 
Photogrammetry and radargrammetry principles are well 
known: both rely upon image matching. Main difficulties to be 
overcome are respectively: water bodies, shadows (steep 
mountainsides), poor texture (deserts) and low sun-illumination 
(high latitudes) for optical imagery, and water bodies, steep 
slopes and sharp edges (mountain tops), consistency over