GIS Files 2: Geographical data

2.1: Data capture from maps (2)

Digitising

Digitising requires the use of special equipment. The source map is laid flat on a table (tablet) and an electronic cursor is passed over the features of the map. In this way, each of the coordinate points which make up the different shapes can be identified. By clicking the cursor when it is held over a point, digitising captures map data in vector form.

For digitising to work, the tablet must have a magnetic field embedded in the flat surface, so as the cursor is moved around the map, its location can be identified.

Digitising can be very time consuming because every single point or vertex must be captured individually. Ordnance Survey's National Topographic Database currently contains more than 230 million features. You can imagine what a time consuming task it was to digitise it originally. Fortunately, the database is maintained by surveying methods that generate digital data directly.

When a cartographer is capturing information by digitising, it is possible to attach attribute information to features. Often, the digitising tablet has some kind of menu of feature types. Once a particular feature is digitised the resulting data contains information about its type and shape.

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