To georeference something means to define its existence in physical space. Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another That is, establishing a relation between raster or vector images to map projections or coordinate systems. In Computer graphics, a raster graphics image or bitmap, is a Data structure representing a generally rectangular grid of Pixels Vector graphics is the use of geometrical primitives such as points lines, Curves and shapes or Polygon (s which are all based A map projection is any method of representing the Surface of a sphere or other shape on a plane. In Mathematics and its applications a coordinate system is a system for assigning an n - Tuple of Numbers or scalars to each point This procedure is thus imperative to data modeling in the field of geographic information systems (GIS) and other cartographic methods. When data from different sources need to be combined and then used in a GIS application, it becomes essential to have a common referencing system. This is brought about by using various georeferencing techniques.
Need
- Georeferencing is crucial to making aerial and satellite imagery, usually raster images, useful for mapping as it explains how other data, such as the above GPS points, relate to the imagery. Aerial photography is the taking of Photographs of the ground from an elevated position Satellite imagery consists of photographs of Earth or other planets made by means of Artificial satellites. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth
- Very essential information may be contained in data or images that were produced at a different point of time. It may be desired either to combine or compare this data with that currently available. The latter can be used to analyze the changes in the features under study over a period of time.
- Different maps may use different projection systems. Georeferencing tools contain methods to combine and overlay these maps with minimum distortion.
- Using georeferencing methods, data obtained from surveying tools like Total Stations may be given a point of reference from topographic maps already available. A total station is an optical instrument used in modern Surveying and archaeology as well as by police crime scene investigators private accident reconstructionists and
Methods
There are various GIS tools available that can transform image data to some geographic control framework, like ArcMap or ERDAS Imagine. ArcGIS is the name of a group of Geographic information system software product lines produced by ESRI. ERDAS IMAGINE is a Raster graphics editor and remote sensing application designed by ERDAS Inc One can georeference a set of points, lines, polygons, images, or 3D structures. For instance, a GPS device will record latitude and longitude coordinates for a given point of interest, effectively georeferencing this point. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement A georeference must be a unique identifier. In other words, there must be only one location for which a georeference acts as the reference.
To georeference an image, one first needs to establish control points, input the known geographic co-ordinates of these control points, choose the co-ordinate system and other projection parameters and then minimize residuals. Residuals are the difference between the actual co-ordinates of the control points and the co-ordinates predicted by the geographic model created using the control points. They provide a method of determining the level of accuracy of the georeferencing process.
Books
- Hill, Linda L. (2006). Georeferencing. The MIT Press. The MIT Press is a University press affiliated with the Massachusetts Institute of Technology (MIT in Cambridge Massachusetts ( USA) ISBN 0-262-08354-6.
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