We may wish to compare maps collected at different scales. We may wish to convert the geometry of the map base. We may wish or need to change the map data structure. Robinson's Classification Origins of the approach date from Robinson's Classification of symbolization methods and map types. Levels of Measurement Robinson's Classification was based on dimension and level of measurement.
Nominal data assume only existance and type.
An example is a text label on a map. Ordinal data assume only ranking. Interval data have an arbitrary numerical value, with relative value. Ratio data have an absolute zero and scale. Transformations as Stages in Map Production Transformation of level can be shown in making a choropleth map.