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Geographical Referencing Learning Resources There are very many different ways of describing geographical locations but we here make an important distinction between direct and indirect geographical referencing.
Direct referencing refers specifically to the use of values in some coordinate referencing system, such as Ordnance Survey Grid References or Latitude/Longitude. All computer-based mapping relies on direct geographical referencing in some way, as coordinate information is at the heart of knowing which information should be placed where on the screen or paper. Direct references for geographical objects are generally obtained from physical surveys, remote sensing, digitising of documentary sources or direct capture by Global Positioning System (GPS) receivers. In reality, most social science researchers do not go out and capture directly referenced locational information as part of their studies.
Indirect referencing refers to the use of other geographical codes or names relating to a known location but not including explicit coordinates. Frequently encountered examples include postcodes, zip codes, county codes or names, census output area/enumeration district/tract codes; local government districts; health authorities; road names; place names; etc. Many social science research applications involve the collection of indirectly referenced information, for example by asking respondents for their addresses or postcodes. It is entirely possible to work with geographically referenced data without having direct geographical references, for example by matching two sets of indirect references together (e.g. a postcode and a census ward code) in order to link records from different datasets. However, it is not possible to produce digital maps without using direct references.
The link between direct and indirect referencing usually takes the form of a standard reference dataset, such as a lookup table or set of digital boundary data. These contain names or codes representing the indirect referencing and coordinates representing the equivalent direct reference. In the case of a lookup table such as the National Statistics Postcode Directory, an indirect reference such as a postcode can be associated with other indirect references such as ward and local authority district codes, or with direct references such as grid coordinates. In the case of a digital boundary dataset, area names or codes are associated with sequences of coordinates which define the locations of the boundaries, allowing data associated with the areas to be mapped.
Standard lookup tables are usually created using geographical information systems (GIS) software, which can identify relationships between any data which are directly referenced, for example by calculating which crime locations fall within a police beat area or the areas of overlap between historical and contemporary counties. The National Statistics Postcode Directory is created in this way, by calculating which postcode locations fall within a series of administrative area boundaries. Once the lookup table has been completed, it can be used by others who do not need the original GIS software. Where no such lookup tables exist, as most often applies to historical or study-specific datasets, it may be necessary for the user to create their own directly referenced data (for example, by scanning or digitizing maps) as a starting point for their analysis.
