Cartographic Generalization as Seen from the Point of View of Topographic Maps

Osmo Niemelä

Abstract

The purpose of generalization
Factors which influence on generalization
Methods of generalization
Selection
Graphic generalization

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This presentation deals primarily with General Topographic Maps and partly with Special Topographic Maps.

The topic is limited to practical cartography only and seen from the point of view of a cartographer performing his work by traditional methods.

Cartographic generalization belongs to the preparation of derived maps. This means that smaller scale maps are derived from larger scale maps.

Generalization is a total task, which touches all the items to be shown on a map. When a specific feature group of a map is to be generalized, the cartographer must know the degree of generalization intended to be made for the other feature groups. The cartographic quality or poorness of a map is judged on the basis how harmoniously the different layers fit together and of course also on the basis how well the generalization by selection has been made.

Map’s instructions of cartographic presentation contain the main rules for generalization. The specifications indicate what is shown on a map and how. This means that both the designer of a map and the cartographer preparing it are responsible for the quality. In order to achieve good results the both persons have to post themselves thoroughly up on the role of map user and his/her expectations.

The terrain must be depicted almost always in generalized form because the area of a map is much smaller than the actual area in the field to be shown. This causes a conflict between the perfectness of a map and the readability of a map. The clearness and readability of a map must be maintained at the cost of perfectness.

The purpose of generalization

At the different stages of map preparation the cartographers must take into consideration the laws of human vision and psychology. Professor Eduard Imhof has said that the purpose of generalization is to prepare a clear and easily readable map from base material prepared by photographic reduction. This means that the clarity of a map is more important than perfectness.

The purpose of generalization – to create a clear and easily readable map – is carried out by 1. selecting what is shown – generalization by selection – and by using 2. suitable methods of cartographic presentation – graphic generalization.

Factors which influence on generalization

The generalization depends on many factors. These are among others the scale of a map, map type, character of the landscape, feature or features to be shown, purpose of the map as well as the accuracy of mapping work or source maps. When a 1:100 000 scale topographic map is derived from a 1:20 000 scale basic map the work must be done so, that there is no need to use the basic maps again when a 1:1 mill. map is derived from the 1:100 000 scale maps. To this point belongs the question how the names are generalized. The names on a basic map are arranged according to the font type and size which means that the problems are not very big.

Methods of generalization

The purpose of generalization – to make the map clear and easily readable – is carried out by using different methods of generalization parallel. The cartographer must create a harmonic entireness using selection and graphic generalization.

Selection

The cartographic specifications of the map to be prepared define mainly the features to be shown; generalization by selection means that the feature is selected on the map or it is left out. The characteristic feature of cartographic presentation is to show the important, characteristic features as well as the landforms and if necessary to exaggerate them or to delete unimportant features.

Graphic generalization

The graphic generalization is an important type of generalization if we examine the clarity of a map. The task demands good experience and professional skill from the cartographer.

The methods used to achieve the necessary clarity and clearness are simplification, combination, displacement and exaggeration

The computer assisted cartography together with accurate digital data will bring new ideas and problems to the map making. Generally speaking the operator/cartographer must now decide if the map is corrected using displacement and exaggeration as has been the practice in traditional cartographic generalization. Or if the features will remain on their true location and partly overlap each other. The basic cartographer – I think – must accept it.