Georeferenced Photographs
Introduction
Georeferencing[a] is the technique of spatial positioning of an entity in a unique and well-defined geographic location in a specific coordinate and datum system. It is a common operation within geographic information systems (GIS) for both raster objects (pixel map images) and vector objects (points, lines, polylines, and polygons that represent physical objects).
Georeferencing is a fundamental aspect in the analysis of geospatial data, as it is the basis for the correct location of map information and, therefore, for the adequate fusion and comparison of data from different sensors in different spatial and temporal locations. For example, two georeferenced entities in different coordinate systems can be combinable after an appropriate affine transformation (either to the coordinate system of the first object or to that of the second).
Geodetic coordinate and datum systems
Coordinate systems can be broken down and represented into two large groups of great importance:
Transformation to map coordinates
One of the main objectives in the field of georeferencing is the generation of functions that allow converting the original coordinates of an image into map coordinates. These map coordinates are characterized because the separation between two pixels remains constant throughout the image, and a georeferencing of this type is perfectly defined through only four parameters:
This method is the one used internally by the GeoTIFF georeferenced image format. There is no univocal way to store in which system the coordinates that describe the geographical window are expressed: the most common methods are its EPSG code or its Well Known Text (WKT).
This definition of geographic windows is especially useful when overlaying raster data with other types of cartographic information (for example, vectors), or when fitting images in multi-temporal or mosaic studies.