Accurate geometric references from low B/H stereoscopic airbone acquisition

Abstract

Since early 1999, the CNES has worked on defining and equipping supersites for its needs in image quality assessment. Various criteria guided the definition of this supersite involving the characteristics of high resolution earth observation satellites and the image quality assessment methods. The main goal of this work is to have high resolution geometric references in order to calibrate any high resolution spatial sensor. The accuracy of sensor calibration that can be obtained using image matching techniques depends both on planimetric and altimetric accuracy of the reference. In order to produce this kind of references, very high resolution aerial images are used (10 cm ground sampling distance). Two very high resolution cameras fly over several sites in order to produce stereoscopic images with narrow angular fields of view. This original acquisition system allows us to produce a high resolution 3D reference with similar images containing no shadows effects. Moreover, thanks to this system, the overlap between a couple of images is nearly 100% if the stereoscopic angle is low. One of the key factors of the quality of the 3D reconstruction is the value of the stereoscopic angle used. This paper shows that low stereoscopic angles are sufficient to build the elevation model including superstructures. This characteristic is the determining factor to make spatial native stereoscopy. The essential condition to compute an high accurate elevation model with the aerial data is to control the geometrical model of all the images. A lack of knowledge of the geometrical model could be interpreted as a default of altitude of the scene (or a part of the scene). Therefore to build the digital surface model (DSM), the planimetry is first controlled through particularly accurate aerotriangulation based on lot of tie points and also, if available, based on ground control points measured with high accuracy through a GPS measurement campaign. The accurate geometry of all the images is computed with a complete aerotriangulation. Once the geometrical models are accurately known, a high resolution DSM can be created using a dense correlation process between images couples with a low stereoscopic angle. The aerotriangulation and the DSM allow us to have a high resolution geometric reference that ensures both planimetric and altimetric accuracy which are key factors for the geometric quality of a spatial sensor calibration. This kind of reference will be used to calibrate a spatial sensor like Pleiades.