International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B3, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
3D CAPABILITIES OF PLEIADES SATELLITE
M. Bernard, D. Decluseau, L.Gabet, P. Nonin,
ASTRIUM GEO-Information Services — (Marc.Bernard, David.Decluseau, Laurent.Gabet,
Philippe.Nonin)@spotimage. fr
KEY WORDS: Photogrammetry, Automation, Matching, DEM/DTM, Satellite, Accuracy, Stereoscopic
ABSTRACT:
End of 2011 a new optical satellite, called Pléiades, was launched by the French space agency (CNES). It provides 20kmx20km
images at 0.5 meters. This agile acquisition system is able to relocate very rapidly and scan the earth in any direction. The agility of
the system offers the ability to acquire multi viewing angle images of the same area during the same orbit. This ability to capture,
from a single stereoscopic pair, to a sequence of 25 images, allows enhancing the quality and the completeness of automatically
extracted 3D maps.
The aim of the study is to validate and quantify the capacity of the Pléiades system to perform 3D mapping. The analysis explores
the advantages in terms of quality and automatism to use more than 2 stereoscopic images.
In the last 10 years, automatic 3D processing of digital images became more and more popular and efficient. Thanks to aerial images
with very large overlap and very high resolution satellite images, new methodologies and algorithms have been implemented to
improve the quality and accuracy of automatic 3D processing. We propose to experiment the same type of approaches using Pléiades
images to produce digital elevation models (DEM). A focus is made on analysing the 3D processing using video like (multi viewing)
acquisitions. Different reference sites with very accurate 3D control points are used to quantify the quality of the Pléiades DEM.
Different acquisition modes are explored from a single stereo pair to a sequence of 17 images.
1. INTRODUCTION
The agility of the new generation of earth observation satellites
aims at increasing the acquisition capacity, allowing nearby
areas to be acquired during the same pass. A side effect is the
ability to acquire multiple views from the same area, with
different incidence angles. Using multiple views for 3D
automatic extraction is a common practice in the airborne
survey community. The main motivations are the improvement
of the robustness and accuracy, due to the redundancy of the
measurements, and the removal of any hidden part in urban
area, due to the multiplicity of the viewing angles.
Based on these considerations, the applicability of airborne
multiview techniques to agile satellites imagery is a worthwhile
question. This paper analyses the performances of this approach
with Pleiades imagery.
2. BACKGROUND
The automatic matching of images stereo pairs has been widely
used for more than thirty years in the mapping industry for the
production of digital elevation models (DEM). For airborne
frame cameras, the usual acquisition scheme is based on 60%
overlaps in the flight direction with a minimal overlap between
adjacent tracks. This scheme insures that every point on the
ground is seen from, at least, two different points of view,
which seems to be sufficient for an open landscape, with
moderate slopes and a manual digitizing process. In an urban
environment, more points of view are required to ensure the
completeness of the elevation model, all around the highest
buildings. Moreover, the redundancy of the stereo
measurements from multiple points of view improves the
robustness of an automatic images matching process. In any
cases, the accuracy benefits from the averaging of the multiple
measurements. Nowadays, acquiring up to 80% overlap images
sequences (in both directions) is a common practice among
airborne survey companies. In the case of airborne push-broom
cameras like the Leica ADS40/80, a side overlap above 50%
provides six well distributed points of views for each point on
the ground.
In the computer vision community, several projects aim at
producing dense 3D city models from thousands of images
found on the Web, which means tens or hundreds of views for a
single point.
Up to now, satellite imagery has been a scarce resource and
most of space based elevation data have been extracted through
single stereoscopic measurements. Nevertheless, the ASTER
GDEM and SPOT Ref3D programs have achieved extensive 3D
coverages of the earth surface at a nominal 30meter resolution
with forward/backward stereoscopic acquisition. Locally, in
mountainous areas, the rendering of the Ref3D DEM has been
improved, thanks to the triple viewing mode of SPOTS with its
two instruments HRS and HRG.
The new generation of sub-metric earth observation satellite is
able to provide more than two images of the same area in a
single pass. From our experience with airborne imagery, it
should improve greatly the quality and the completeness of a
digital elevation model, especially in urban area. On the other
hand, satellite imagery is still expensive, and the benefit of
multi-view rendering has to be balanced with the extra cost of
the acquisition.
One should also notice that, during a one pass acquisition, the
satellite is bound to its orbit, which is a single one dimensional
path. This is not quite similar to a multi-tracks airborne
acquisition with viewing angles evenly distributed around each
point on the ground. However, this situation can be partially
recreated by using several single-pass stereo pairs acquired from
different orbits. Here again, cost matters.
3. THE STEREO DILEMMA
The best choice of viewing angles for stereoscopic measurement
is always a matter of compromise: A wide stereo angle provides
a good geometric accuracy, but the matching of two images
points may be difficult, or even impossible if the two points of
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