Thematic classification is the assignment of data, or observations, into groups in order to simplify the mapped distribution to the map reader.

These groups or classes are so designed to reveal the pattern of the distribution and the thematic implications.

The number of classes depends upon several factors; viz: the pattern of the distribution and also whether greyscale/ monochromatic tonal values are used.

• should greyscale/ monochromatic tonal values be employed then it is advisable to use no more than six classes due to the difficulty in perceiving visual differences.
• The use of spectral adjacency can extend the number of classes - a typical example being the portrayal of a negative range (blue/ green) to a positive range (yellow/ red).
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