Global Positioning System Reference
In-Depth Information
8.3.5
National-Scale GIS
Mapping agencies used advanced and networked GIS to sell digital maps on
demand. Mapmakers might use a digital landscape model as the underlying
information space with a cartographic model created on demand to fit specific
visual requirements. For example, the Ordnance Survey's (OS) national
geographic database model for Great Britain (population 60 million) consists of
around 500 million entities, each of which is uniquely identified by a 16-figure
topographic identifier (known as a TOID). There are around 5000 changes per day
to this massive vector database.
GIS are constructed as an information hierarchy. The foundations are the
spatial vectors forming referenced shapes or polygons. These polygons will then
have attributes to show various themes (road, rail, water, parkland, farmland,
contours, etc.). Although the map is essentially seamless, it will need to be tiled to
cover useful areas. There may be other referencing systems associated with the
map, for example, postal code areas and the addresses of buildings and cross
references to information spaces held by other organizations. National GIS are
controlled by governments and their agencies and are of strategic significance
both economically and militarily. The obscure name for the Ordnance Survey
illustrates its military origins in 1791.
Today, the importance of national mapping is growing as it provides the raw
material for the availability of Web-based maps offered via Web services by the
various Web 2.0 map service providers and vehicle navigation mapping
organizations.
