Géoréférencement

Georeferencing or georegistration is a type of coordinate transformation that binds a digital raster image or vector database that represents a geographic space (usually a scanned map or aerial photograph) to a spatial reference system, thus locating the digital data in the real world. It is thus the geographic form of . The term can refer to the mathematical formulas used to perform the transformation, the metadata stored or within the image file to specify the transformation, or the process of manually or automatically aligning the image to the real world to create such metadata. The most common result is that the image can be visually and analytically integrated with other geographic data in geographic information systems and remote sensing software. A number of mathematical methods are available, but the process typically involves identifying several sample ground control points with known locations on the image and the ground, then using curve fitting techniques to generate a parametric (or piecewise parametric) formula to transform the rest of the image. Once the parameters of the formula are stored, the image may be transformed dynamically at drawing time, or resampled to generate a georeferenced or orthophoto. The term georeferencing has also been used to refer to other types of transformation from general expressions of geographic location (geocodes) to coordinate measurements, but most of these other methods are more commonly called geocoding. Because of this ambiguity, Georegistration is preferred by some to refer to the image transformation. Occasionally, this process has been called rubbersheeting, but that term is more commonly applied to a very similar process applied to vector GIS data. Georeferencing is crucial to make and , usually raster images, useful for mapping as it explains how other data, such as the above GPS points, relate to the imagery. Very essential information may be contained in data or images that were produced at a different point of time.