Basic Classification Rules

How to Define the Number of Classes

The number of classes is to be connected to the objectives assigned to the cartographic representation.
If the purpose of the map is to show simply progress, a continuous pressure gradient (as for example the increase of the height on a topographic map) then the number of classes can be relatively important. On the opposite, if it is to put classes in an obvious place for having a meaning for the author, then it is indispensable that the consequential classes and the graphic symbols have a correspondence without ambiguity. In this case, the reader has to have the possibility of differentiating well beaches. Then the number from 5 to 6 appears as a maximum.

How many classes per map?

Generally, it is difficult to visually perceive information, which divides up into several classes or categories. This is why the number of classes must be big enough to be useful, but limited to allow the map to be readable without any trouble.
Generally, a distribution between 4 and 8 classes is correct and 5-6 classes is the standard.

How to Define the Classes

There are several ways to define classes. You should still choose the method which suits best and for the data distribution and for the aspect of the data you want to emphasise. In any case, with the method you choose:

• The classes have to cover all the data values in order that the limits are contiguous to the data.
• All classes should be homogeneous.

Further Classification Rules

• A value has to appear in a class and in only one class. The class limits must not overlap.
• You should not present the values of the limits of classes with a significant number of decimals superior to the one that allows the exactness of the data.