Georeference
Encyclopedia
To georeference something means to define its existence in physical space. That is, establishing its location
in terms of map projection
s or coordinate systems. The term is used both when establishing the relation between raster
or vector
images and coordinates, and when determining the spatial location of other geographical features. Examples would include establishing the correct position of an aerial photograph within a map
or finding the geographical coordinates of a place name
or street address
. 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. Most georeferencing tasks are undertaken either because the user wants to produce a new map or because they want to link two or more different datasets together by virtue of the fact that they relate to the same geographic locations.
, PCI Geomatica
, or ERDAS Imagine
. 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. A georeference must be a unique identifier. In other words, there must be only one location for which a georeference acts as the reference.
Images may be encoded using special GIS file formats
or be accompanied by a world file
.
To georeference an image, one first needs to establish control points, input the known geographic coordinates of these control points, choose the coordinate system and other projection parameters and then minimize residuals. Residuals are the difference between the actual coordinates of the control points and the coordinates predicted by the geographic model created using the control points. They provide a method of determining the level of accuracy of the georeferencing process.
In situations where data have been collected and assigned to postal or area codes, it is usually necessary to convert these to geographic coordinates by use of a definitive directory or gazetteer
file. Such gazetteers are often produced by census agencies, national mapping organizations or postal service providers. At their simplest, these may simply comprise a list of area codes or place names and another list of corresponding codes, names or coordinate locations. The range and purpose of the codes available is country-specific. An example is the UK's National Statistics Postcode Directory which shows each postcode's membership of census, administrative, electoral and other geographical areas. In this case, the directory also provides dates of creation and deletion, address counts and an Ordnance Survey
grid reference
for each postcode, allowing it to be mapped directly. Such gazetteer files support many web-based mapping systems which will place a symbol on a map or undertaken analysis such as route-finding, on the basis of postal codes, addresses or place names input by the user.
Location (geography)
The terms location and place in geography are used to identify a point or an area on the Earth's surface or elsewhere. The term 'location' generally implies a higher degree of can certainty than "place" which often has an ambiguous boundary relying more on human/social attributes of place identity...
in terms of map projection
Map projection
A map projection is any method of representing the surface of a sphere or other three-dimensional body on a plane. Map projections are necessary for creating maps. All map projections distort the surface in some fashion...
s or coordinate systems. The term is used both when establishing the relation between raster
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
or vector
Vector graphics
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon, which are all based on mathematical expressions, to represent images in computer graphics...
images and coordinates, and when determining the spatial location of other geographical features. Examples would include establishing the correct position of an aerial photograph within a map
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
or finding the geographical coordinates of a place name
Toponymy
Toponymy is the scientific study of place names , their origins, meanings, use and typology. The word "toponymy" is derived from the Greek words tópos and ónoma . Toponymy is itself a branch of onomastics, the study of names of all kinds...
or street address
Address (geography)
An address is a collection of information, presented in a mostly fixed format, used for describing the location of a building, apartment, or other structure or a plot of land, generally using political boundaries and street names as references, along with other identifiers such as house or...
. This procedure is thus imperative to data modeling
Data modeling
Data modeling in software engineering is the process of creating a data model for an information system by applying formal data modeling techniques.- Overview :...
in the field of geographic information systems (GIS) and other cartographic
Cartography
Cartography is the study and practice of making maps. Combining science, aesthetics, and technique, cartography builds on the premise that reality can be modeled in ways that communicate spatial information effectively.The fundamental problems of traditional cartography are to:*Set the map's...
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. Most georeferencing tasks are undertaken either because the user wants to produce a new map or because they want to link two or more different datasets together by virtue of the fact that they relate to the same geographic locations.
Need
- Georeferencing is crucial to making aerial and satellite imagerySatellite imagerySatellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
, 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. 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 stationTotal stationA total station is an electronic/optical instrument used in modern surveying. The total station is an electronic theodolite integrated with an electronic distance meter to read slope distances from the instrument to a particular point....
s may be given a point of reference from topographic maps already available. - It may be required to establish the relationship between social surveyStatistical surveySurvey methodology is the field that studies surveys, that is, the sample of individuals from a population with a view towards making statistical inferences about the population using the sample. Polls about public opinion, such as political beliefs, are reported in the news media in democracies....
results which have been coded with postal codes or street addresses and other geographic areas such as censusCensusA census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common...
zones or other areas used in public administration or service planning.
Methods
There are various GIS tools available that can transform image data to some geographic control framework, like ArcMapArcGIS
ArcGIS is a suite consisting of a group of geographic information system software products produced by Esri.ArcGIS is a system for working with maps and geographic information...
, PCI Geomatica
PCI Geomatica
PCI Geomatica is a remote sensing desktop software package for processing earth observation data, designed by PCI Geomatics Inc. The latest version of the software is Geomatica 10.3, released in 2010. Geomatica is aimed primarily at raster data processing and allows users to load satellite and...
, or ERDAS Imagine
ERDAS IMAGINE
ERDAS IMAGINE is a remote sensing application with raster graphics editor capabilities designed by ERDAS for geospatial applications. The latest version is 2010, version 10.1. ERDAS IMAGINE is aimed primarily at geospatial raster data processing and allows the user to prepare, display and enhance...
. One can georeference a set of points, lines, polygons, images, or 3D structures. For instance, a GPS device will record latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
coordinates for a given point of interest, effectively georeferencing this point. A georeference must be a unique identifier. In other words, there must be only one location for which a georeference acts as the reference.
Images may be encoded using special GIS file formats
GIS file formats
A GIS file format is a standard of encoding geographical information into a file. They are created mainly by government mapping agencies or by GIS software developers.Metadata often includes:...
or be accompanied by a world file
World file
A world file is a plain text computer data file used by geographic information systems to georeference raster map images. The file specification was introduced by ESRI....
.
To georeference an image, one first needs to establish control points, input the known geographic coordinates of these control points, choose the coordinate system and other projection parameters and then minimize residuals. Residuals are the difference between the actual coordinates of the control points and the coordinates predicted by the geographic model created using the control points. They provide a method of determining the level of accuracy of the georeferencing process.
In situations where data have been collected and assigned to postal or area codes, it is usually necessary to convert these to geographic coordinates by use of a definitive directory or gazetteer
Gazetteer
A gazetteer is a geographical dictionary or directory, an important reference for information about places and place names , used in conjunction with a map or a full atlas. It typically contains information concerning the geographical makeup of a country, region, or continent as well as the social...
file. Such gazetteers are often produced by census agencies, national mapping organizations or postal service providers. At their simplest, these may simply comprise a list of area codes or place names and another list of corresponding codes, names or coordinate locations. The range and purpose of the codes available is country-specific. An example is the UK's National Statistics Postcode Directory which shows each postcode's membership of census, administrative, electoral and other geographical areas. In this case, the directory also provides dates of creation and deletion, address counts and an Ordnance Survey
Ordnance Survey
Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
grid reference
Grid reference
Grid references define locations on maps using Cartesian coordinates. Grid lines on maps define the coordinate system, and are numbered to provide a unique reference to features....
for each postcode, allowing it to be mapped directly. Such gazetteer files support many web-based mapping systems which will place a symbol on a map or undertaken analysis such as route-finding, on the basis of postal codes, addresses or place names input by the user.
See also
- Altimetry
- Cadaster
- GeocodingGeocodingGeocoding is the process of finding associated geographic coordinates from other geographic data, such as street addresses, or zip codes...
- GeoportalGeoportalA geoportal is a type of web portal used to find and access geographic information and associated geographic services via the Internet...
- GeotaggingGeoTaggingGeotagging is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS feeds and is a form of geospatial metadata...
- HydrographyHydrographyHydrography is the measurement of the depths, the tides and currents of a body of water and establishment of the sea, river or lake bed topography and morphology. Normally and historically for the purpose of charting a body of water for the safe navigation of shipping...
- Image registrationImage registrationImage registration is the process of transforming different sets of data into one coordinate system. Data may be multiple photographs, data from different sensors, from different times, or from different viewpoints. It is used in computer vision, medical imaging, military automatic target...
- Nomenclator
- Reverse geocodingReverse geocodingReverse geocoding is the process of back coding of a point location to a readable address or place name. This permits the identification of nearby street addresses, places, and/or areal subdivisions such as neighbourhoods, county, state, or country...
- Reference ellipsoidReference ellipsoidIn geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body....
- Transport infrastructure
- World Geodetic SystemWorld Geodetic SystemThe World Geodetic System is a standard for use in cartography, geodesy, and navigation. It comprises a standard coordinate frame for the Earth, a standard spheroidal reference surface for raw altitude data, and a gravitational equipotential surface that defines the nominal sea level.The latest...
External links
- Discovering Location Indicators of Toponyms from News to Improve Gazetteer-Based Geo-Referencing - paper presented at Geoinfo 2008
- Geographical referencing resources for social scientists online tutorial material from the University of Southampton, UK