4 Digital Representations of Geographies: Digitised Boundary Datasets
A digitised boundary dataset is a digital representation of a set of geographical boundaries. Digital representations of geographical information is the domain of a branch of information technology known as Geographical Information Systems (GIS). GIS is a broad term that includes the capture, storage, analysis, management and visualisation of geographical information. An introduction to GIS is beyond the scope of this module. For the purpose of this module - digital boundary representation of geographic regions at the simplest level the geometry that defines the extent of the geographical region (the spatial data) is stored together with other data about that region (the aspatial data).
When dealing with a digital boundary representation of a particular geography, each instance of that geography, that is each unique region is held as a record that contains the geometry of the region plus a geographic identifier that uniquely identifies the region within the dataset as whole plus other information related to the region.
Image 11: How does the GIS software know to draw a polygon for each feature within the digital boundary dataset? Let`s focus on the polygon labelled above as 35UCFS0001
This shows a sample of 2001 census Output Areas. The map consists of a set of polygons. A polygon is an area of enclosed space bound by a boundary. Each Output Area has assigned to it a geographic identifier. In this case 00FANE0018. This geographic identifier uniquely identifies the specific Output Area within the complete 2001 census output area dataset. The geographic identifier is the means by which we are able to link data to the census output areas.
Let`s look in more detail at how the geometry of geographical features is stored within the GIS. Here we have another image that shows some more geographical boundaries inside a GIS software application.