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Georeference

The term georeference, used as both a noun and a verb, has many and varied definitions in geographic information science. Most simply, to georeference is to specify the geographic location of some object, entity, phenomenon, image, concept, data, or information. A georeference, therefore, is the description of the location of something relative to the earth. All data used within GIS must be georeferenced. The term geocode is sometimes used synonymously with georeference, either as a noun or verb, though it is more often used with a more specific meaning relating to the transformation of street addresses to point locations on a map or in a GIS layer. More specifically, this transformation is referred to as address matching. This entry provides a basic framework of definitions for a number of related concepts, some of which are explained in more depth elsewhere in this volume.

A georeference can be stated mathematically using a geospatial referencing system, such as longitude and latitude, Universal Transverse Mercator (UTM) coordinates, national grid coordinates, or Universal Address. This form of georeferencing is variously known as direct, absolute, or formal georeferencing. Alternatively, the georeference may be stated using a placename for a city, country, or river, for example, or a place code such as a postal code, administrative district ID, or an address. This form is known as indirect, relative, or informal georeferencing.

Direct Georeferences

Assuming the spatial referencing system is defined as a formal geometrical system and a GIS has access to this system's mathematical definition, an object with a direct georeference can be displayed and analyzed on the GIS without further transformation. For example, if the spatial referencing system is defined on a specific datum (e.g., WGS84), projection (e.g., Transverse Mercator), and coordinate system (e.g., UTM), then mathematical coordinates assigned to all records in the database can be used to draw points, lines, or polygons on a map showing the locations of each item.

Indirect Georeferences

Indirect georeferences must be translated through some transformation process that links the code or placename used to identify the location in the data record and the actual location of that place on the earth's surface. If the indirect georeference is a placename, then a gazetteer can be used to determine the specified location.

If a place code of some sort is used as the indirect georeference, then it is necessary to have access to a map or GIS layer that indicates where these coded places fall on the earth's surface and then a means to link each data item to the spatial reference file. A spatial reference file may be similar to a gazetteer—a database or table linking place codes to geographic locations—or it may be a map.

Census data provide a simple example of how indirect georeferences work. Consider a spreadsheet containing the census data for all the census zones in a city. Each row in the spreadsheet shows the census data for a single census zone, and somewhere in this row will be a column that lists a unique place code or ID. This code is associated with a specific area on the earth's surface, which is represented on a GIS layer of the census zones as a polygon. Each polygon in this layer will have corresponding unique IDs. The transformation process between the map of the census zones and the spreadsheet of the data will be completed by relating the two data sets through the unique IDs.

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