Understanding Digital Map Data

Digital mapping is the collection of information in computer systems (often called Geographical Information Systems, or GIS for short) that represent, in digital form, information that can be shown on a map.

The information that goes to make up the maps has to be represented in terms that computers can understand. There are basically two types of digital map data models: raster data models and vector data models (see the diagram below).

Raster data models represent spatial features in grid-cells or dots known as pixels. Raster digital map data are created by scanning paper maps (such as the Ordnance Survey Landranger® 1:50000 series) or aerial photography into a computer file.

Vector data models represent spatial features in three ways, known as entities or features:

• Points, which are the basic building block and, depending on the map, may represent individual features such as trees or lampposts. The location of a point is described by an X and Y coordinate, which fix the object to some reference system such as the Ordnance Survey's National Grid. Map data often does not include a Z coordinate (or height);
• Lines, which are sets of points that represent linear features such as roads and rivers;
• Polygons or areas, which are sets of lines used to represent closed areas such as woodland, bodies of water or cities and also administrative areas such as parliamentary constituencies.

Note that the same spatial feature can be represented by different entities on different maps. For example, Edinburgh might be a point on a small-scale map of the UK or a polygon on a larger-scale map of the surrounding area. And if the map is of Edinburgh itself, all three entities can be used. Points can represent post offices or railway stations; lines can represent roads or railways; and polygons can represent parkland or buildings.