The process of georeferencing , geocoding or geocoding , geotagging or localization is understood to be the assignment of spatial information, the georeferencing , to a data record . Georeferencing plays an important role in computer mapping , remote sensing , geographic information systems and geomarketing , but it also occurs in home applications, e.g. B. archiving photos and videos.
There are essentially three possible reasons you might want to georeference:
- One would like to fit data into a geodetic reference system, i. H. provided with real world coordinates (geocoding).
- One would like to eliminate ( rectify ) geometric distortions in data sets, in particular in image data .
- You want to adapt ( transform ) two differently oriented or scaled data sets to one another .
Types of georeferencing
A distinction must be made between the following types of georeferencing:
- the assignment of a postal address (address coding)
- the assignment of a coordinate (geocoding , geocoding , geotagging , geo-imaging )
- the assignment of a transformation (implicit geocoding , map calibration , see also inner and outer orientation )
- applying a transformation (explicit geocoding , see also rectification , rectification )
The terms georeferencing and geocoding are not used uniformly, sometimes even in contradiction. Geocoding means, on the one hand, the assignment of individual coordinates and, on the other hand, the application of a transformation to the entire data set. Georeferencing is used both for the assignment of individual coordinates and as an umbrella term.
Address geocoding / geocoding of postal addresses
The geocoding of postal addresses is a (complex) process in which postal addresses are checked, changed and given a spatial reference with the help of a reference database. After the process, the geocoded addresses should be as complete and correct as possible in structure and location and have geo-keys and / or x, y coordinates. Both the addresses themselves and their additional information (e.g. from the CRM) receive a direct (via x, y) and / or indirect spatial reference (geo key). So can
- the addresses are enriched with additional spatial data
- the additional address information such as B. purchase data are spatially evaluated
Special geocoding systems, also called geocoders for short, which are available as offline and online services, are used to geocode postal addresses. With online geocoding, an address is transmitted, which in turn is relevant to data protection.
Inverse geocoding (also called reverse geocoding ) describes the opposite: using geocoordinates, textual location information is sought, e.g. city names, street names and so on.
In geotagging, a spatial data record (e.g. an image, a website, an article) is provided with a coordinate. The coordinate is added as a tag , attribute or meta information . It enables the spatial classification of the information. The data can be placed in the right place on a digital map (such as Google Earth ), for example . The terms geotagging and geocoding have spread on the Internet for this purpose . The attached attribute is called a geo-tag or geocode . For geocoding of websites are available in HTML for. B. the meta-element geo .
Implicit geocoding (map calibration, image orientation)
In implicit geocoding, a transformation rule is attached to a two- or three-dimensional spatial data set. The data remains in the model coordinate system. The data set is oriented in relation to the real world. This makes it possible to convert the tapped coordinates into real world coordinates. Georeferencing of maps is often called map calibration (card calibration referred).
Explicit geocoding (rectification, image rectification)
Explicit geocoding applies the transformation to the spatial data set. This gives all data real world coordinates. In addition to determining the transformation parameters, the "geometric transformation" also achieves rectification (also rectification or orthorectification from English rectification ).
Determine a transformation
In order to be able to geocode a spatial data set implicitly or explicitly, a transformation equation has to be found. As a rule, control points are used for this. The control points must be clearly recognizable in the data record. The coordinates of the control points in the real world coordinate system are either known or are taken from a reference data set. In the case of vector data, the coordinates are picked up or interpolated. In the case of image data, the image coordinates of the control points are measured. The transformation should be determined taking into account the imaging geometry. In the case of photos, the central projection must be taken into account, in the case of maps the corresponding map network design . The automatic finding of similarities in digital images and the determination of the transformation is called image registration in image processing . The registration of laser scanning point clouds can be done with the ICP algorithm .
- Bollmann; Koch: Lexicon of cartography and geomatics. ISBN 3-8274-1672-8 .
- Markus Böhmer: Geocoding. Page 127f, from the Geomarketing Handbook. ISBN 978-3-87907-453-2 .
- Jens Gladis: Geocoding. Page 137f, from the Geomarketing Handbook, 2nd edition. ISBN 978-3-87907-653-6 .