|Most geoscientific applications using georeferenced cartographic data need a good knowledge and visualization of the topography of the Earth's surface. For example, mapping of geomorphological features
is hardly feasible from a single image; three-dimensional (3D) information has to be generated or to be added for a better interpretation of the two-dimensional (2D) data. |
Since the early emergence of earth observation satellites, researchers
have investigated different methods of extracting 3D information using satellite data. Apart from a few early stereo-images acquired with hand-held cameras during the Gemini and Apollo missions, the first experiments to extract 3D data using stereo
viewing from space began with the Earth Terrain Camera (ETC) flown onboard SkyLab in 1973-74.
Since these early experiments, various analog or digital sensors in the visible or in the microwave spectrum have been flown to provide researchers and
geoscientists with spatial data for extracting and interpreting 3D information of the earth's surface. Although the shape-from-shading technique can be applied to optical sensor (OPS) images, stereo-viewing using space camera or digital scanner
images was, and still is the most common method used by the mapping, photogrammetry and remote sensing communities.
However, side-looking synthetic aperture radar (SAR) data gives also the opportunity to extract 3D information using
image-processing techniques appropriate to the nature of the data. With SAR data, three main methods have been developed: radargrammetry, clinometry and interferometry. Radargrammetry (similar to the stereoviewing of optical data) uses two images
acquired from different viewpoints to generate a stereopair and stereoviewing. Clinometry takes advantage of the SAR shading and shadowing in the image, and interferometry uses mainly the SAR signal data instead of the image.
The paper will
review the different methods and sensors used to extract absolute or relative elevation and assess their performance using the results from various research and commercial organizations. It will also discuss the respective advantages, difficulties
and constraints of the sensors, the methods, and the technologies used to take into account the strength of each. It will also assess how they perform as complementary sources and systems for extracting elevation data in an operational context.