Elevation Modeling from Satellite Data

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Abstract
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 (3‐D) information has to be generated or to be added for a better interpretation of the two‐dimensional (2‐D) data.Since the early emergence of earth observation satellites, researchers have investigated different methods of extracting 3‐D information using satellite data. Apart from a few early stereoimages acquired with hand‐held cameras during the Gemini and Apollo missions, the first experiments to extract 3‐D data using stereoviewing from space began with the Earth Terrain Camera (ETC) flown on board 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 3‐D information on the earth's surface. Although the shape‐from‐shading technique can be applied to optical sensor (OPS) images, stereoviewing 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 also give the opportunity to extract 3‐D 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. Other methods (polarimetry and altimetry) are also used for 3‐D information extraction.This article reviews the different methods and sensors used to extract absolute or relative elevation and assesses their performance using the results from various research and commercial organizations. It also discusses the respective advantages, difficulties and constraints of the sensors, the methods and the technologies used to take into account the strength of each. It further assesses how they perform as complementary sources and systems for extracting elevation data in an operational context.

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