GIS Files 1: Getting to grips with GIS

1.5: The significance of scale (3)

Generalisation

Very detailed mapping, showing the outline of individual objects such as walls and fences, is known as large-scale data. The positional accuracy of features shown on this type of mapping is very high but there is so much detail that if you zoom out the view becomes very cluttered.

The mapping that most of us recognise has been deliberately simplified. A cartographer creates these simple, readable maps by selecting information from a larger-scale source. Not all the detail from the source map can be shown. For example, a road atlas that attempted to show every building in the country would become far too cluttered, so some features are aggregated, smoothed out or omitted altogether.

The illustration shows how large-scale data (shown on the left of the image) when viewed at a small scale (zoomed out), appears cluttered, whereas small-scale data (shown on the right) when viewed at a large scale (zoomed in), appears very sparse.

Sometimes it may be necessary to alter a feature's true survey position slightly to make space for the map symbols. Furthermore, the thick red lines of an A road are shown much wider on the map than the actual road is on the ground. This science of small-scale map production is known as generalisation.