Empirical evaluation of encoding multi-temporal quantitative data using spatial unit sizes

The concept for visually encoding multi-temporal quantitative data, which is based on adjusting sizes of spatial units according to change values, is briefly introduced and empirically tested. This concept (called ‘Sponge Maps’) includes three optional extensions of the underlying Value-by-area Map concept–considering the total map size, different scaling function, and dual coding. All in all, the empirical study confirmed the expected intuitiveness and effectiveness of the concept. The detection rates (roughly between 60% and 80%) are lower compared to conventional choropleth maps using colour lightness encoding. It is assumed that this is due to the unfamiliarity of the new concept and missing explanations for test users. Since the choice of design parameters is made by the map author, there is also a certain degree of subjectivity in the creation of Sponge Maps–therefore, corresponding countermeasures are sketched out.