Under the georeferencing, geocoding, geotagging or localization process refers to the allocation of spatial information, the geo-referenced to a record. The process plays an important role in the Computerkartografie, remote sensing and geographic information systems, but is also home applications such as the archiving of photos and videos.
There are essentially three reasons why one would want to perform a geo-referencing:
- You want to fit data in a geodetic reference system, ie provided with real-world coordinates ( geocode ).
- One would like to geometric distortions in data sets, especially in image data, eliminate ( rectify ).
- You want two differently oriented and scaled records against each other to adapt ( transform ).
Types of georeferencing
The following types of geo-referencing can be distinguished:
- The assignment of a postal address ( address coding )
- Assigning a coordinate ( Geocoding, Geocoding, Geotagging, Geo - Imaging)
- The assignment of a transformation (implicit geocoding, maps, calibration, see also internal and external orientation)
- The application of a transformation (explicit geocoding, see also rectification, rectification )
The terms georeferencing and geocoding are not uniform, even used partly contradictory. So is meant by geocoding the one hand, the allocation of individual coordinates, on the other hand, the application of a transformation to the complete data set. Georeferencing is used both for allocating individual coordinates, as well as an umbrella term.
In the address encoding the spatial data set is assigned to a postal address, thus creating an indirect spatial reference. Using geocoded addresses (which are points that carry both postal address as well as real-world coordinates) can be the direct spatial reference of the data produced (data ↔ ↔ Address coordinate). Address point data sets are created, among others, of the land management or navigation data record producers.
Geotagging ( geocoding )
Geotagging is a spatial data set ( such as an image, a website, an article ) is provided with a coordinate. The coordinates can be attached as a tag, attribute or meta information. It allows the spatial arrangement of the information. The data can be for example in a digital map (such as Google Earth) place at the right place. On the Internet, the terms of this geotagging and geocoding have spread. The attached attribute is named according to geotag or geocode. For the geocoding from Web pages are available in HTML, for example, the meta element geo.
See also: Geotagging ( Photography )
Implicit geocoding (card calibration, image orientation )
In implicit geocoding a two-or three-dimensional spatial data, a transformation rule is attached. The data will remain in the model coordinate system. The record is oriented with respect to the real world. Thus the conversion of the picked coordinates in real world coordinates. The georeferencing of maps is often referred to as a map calibration ( calibration card ).
Explicit geocoding ( rectification, image rectification )
The explicit geocoding the transformation is applied to the spatial data. This allows all data real-world coordinates. Besides the determination of the transformation parameters is determined by the geometric transformation, equalization (also rectification or orthorectification of Engl. Rectification ) was achieved.
Determining a transformation
To a spatial dataset implicitly or explicitly to geocode, a transformation equation must be found. Typically this control points are used. The control points must be easily identifiable in the record. The coordinates of the control points in the real world coordinate system are either known or can be taken from a reference data set. For vector data, the coordinates are tapped or interpolated. For image data, the image coordinates of the control points are measured. The transformation should be determined in consideration of the imaging geometry. For photos, thus the central projection is taken into account at the appropriate card network card design. The automatic finding common ground in digital images and the determination of the transformation is called in image processing, image registration. Registration of laser scanning point clouds can be done with the ICP algorithm.