ABSTRACT
In this chapter we display data from a grid of sensors belonging to the Integrated Air Quality system of the Madrid City Council (Section 13.1) with circles as the proportional symbol, and range grading as the scaling method. The objective when using range grading is to discriminate between classes instead of estimating an exact value from a perceived symbol size. However, because human perception of symbol size is limited, it is always recommended to add a second perception channel to improve the discrimination task. Colors from a sequential palette will complement symbol size to encode the groups.
