Ordnance Survey – Great Britain's national mapping agency
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6.4: Derived mapping (1)
Generalisation
Map generalisation is the process of reducing the scale and complexity of map detail whilst maintaining the important elements and characteristics of the location. When creating a map using traditional manual techniques, a cartographer aims to achieve a balance between the amount of real-world information required to make the map useful and avoiding confusion for the user. This is a time-consuming and expensive process.
GIS has led to the realisation that the efficiency of the cartographer could be increased through the automation of some of the more time-consuming techniques such as line and polygon simplification. Current off-the-shelf GIS software packages contain tools that allow basic generalisation to be performed. An example of polygon generalisation is shown here. Merging and simplification have been used to produce a cartographic representation of the original data.
Probably the most famous line generalisation algorithm was developed by Douglas and Peucker in 1973. The Douglas-Peucker algorithm simply filters the number of vertices along a digitised line to create a representation suitable for the specified depiction scale.
Although these algorithms go some way to help in the automated production of smaller-scale maps, generalisation technologies are very much in their infancy. The challenge of replacing an experienced cartographer with a computer that can make the same decisions to produce a map is significant.
The main problem that needs to be addressed in generalisation is how to resolve the conflict between different map features when they are displayed at smaller scales. As there is not enough space to display all of the information in an uncluttered manner, methods to typify the data in an intelligent, consistent and coherent way at smaller scales need to be developed. It is for this reason that the move from map generalisation as a manual art to a computerised scientific process is a distant dream. For the foreseeable future the process will be a semi-automated collaboration between cartographer and machine.