agricultural areas, for radar. Scientific literature offers plenty of 
brilliant papers about principles and methods to extract DEMs 
from radar and optical pairs 
2.1 DEM extraction from Optical data 
DEM extraction commonly uses 2 optical satellite images, ie a 
stereopair, from more or less symmetric incidence angles, and 
can strongly benefit of the addition of a 3" image, preferably 
under a near-vertical incidence, to better render the steep areas 
and deep valleys (“tristerco mode"). 
As one knows, the clouds do hinder image collection by optical 
sensor like SPOTS/HRS. This task requires a careful monitoring 
and patient re-tasking over reluctant (ie cloudy) areas. And 
sometimes patience itself is not enough: after 11 years of 
continuous attempts (2002-2012), some areas of the Equatorial 
belt remain not feasible, from a DEM-extraction point of view, 
due to persistent cloud cover. The example of French Guyana 
(84,000 kn») is self-explaining: since the launch of SPOT 5 in 
May 2002, more than 2,360 HRS stereopairs have been 
collected, to achieve only 42% of a cloud-free coverage, though 
every place in F. Guyana has been imaged (obviously mainly 
under clouds) more than 202 times since 2002 ! 
2.2 DEM extraction from Radar data 
Meanwhile, radargrammetry needs 4 images over most places, 
ie one ascending pair plus one descending pair. Only in some 
cases (over gently hilly areas), one radar pair could be sufficient 
to achieve a good DEM; however as it remains difficult to 
predict exactly where this will occur, the collection of 
radargrammetric TerraSAR-X images systematically plans 2 
pairs. 
As one knows, the clouds do not hinder the collection of 
TerraSAR-X images. This opens the way to the collaborative 
extraction of DEM between optical and radar data: the AJAX 
project. 
3. AJAX CONCEPT AND REQUIREMENTS 
3.1 AJAX concept and goals 
AJAX did not aim at re-exploring DEM extraction 
methodologies from optical and radar —grammetric data, fairly 
well-known, but rather to experiment the joint use of both optic 
and radar pairs to provide a single consistent, accurate and 
affordable DTED2 DEM to complement the Elevation30 
product line over Equatorial areas. 
Two test 1°x1° geocells were chosen to demonstrate the 
potentialities of this blending: one over Colombia (NO7W074) 
and one over Congo (N02E018). Over these two geocells, the 
cloud-free HRS (ie optical) coverage was below 50%, which of 
course made impossible the extraction of any reliable DEM 
(within our production flow, a 98% ratio is considered as a 
minimum). 
This paper will focus on the Colombian prototype, as the results 
over Congo are very similar. 
3.2 AJAX accuracy requirements 
Being bound to be integrated into the Elevation30 Product range, 
the AJAX prototype should meet the following accuracy 
requirements: 
10m LE90 for slopes lower than 20 9o 
18m LE90 for slopes between 20 and 40 % 
30m LE90 for slopes greater than 40 % 
Since 2002, numerous accuracy assessments of the Elevation30 
products from HRS (also known as Reference3D) have been 
performed at international level by independent users: NGA, 
European Commission/JRC, ImageONE (Japan)... and many 
others. All concluded that the product fully met its 
specifications. [Kay, Winkler et al., 2004] [Yoshino et al., 2008] 
[Le Hir et al., 2010] 
4. PRODUCTION STEPS 
4.1 Colombian prototype area - Input data 
The Western part of the 110km x 110 km geocell consists in a 
rather flat plain divided by a large river gently flowing 
Northwards (see Figure 2). A mountain range with very steep 
slopes occupies the Eastern part of the geocell. Elevations span 
from 50m to 3,150m above sea level. 
The following Figures 1 and 2 show the input data that were 
used to produce a combined DEM. 
  
  
Figure 1 - SPOT 5 HRS coverage over N07W074 
Blue and white colours show cloudy areas 
  
  
Figure 2 - TerraSAR-X coverage over N07W074 
For the purpose of this test, the full geocell was collected by 
TerraSAR-X, notwithstanding the existing HRS coverage.