nominal data: showing differences in kind
ordinal data: showing relative differences in quantities as well as kind
interval data: is information that can be arranged using a standard scale along which operations of addition and subtraction have meaning. Temperature is an example of an interval measure.
is information that, like interval data, can be arranged along a scale but, in addition, the scale begins at a non-arbitrary zero point. At the zero point, no features are present. The operations of multiplication and division can be employed with ratio data to consider proportions and magnitudes. Elevation above sea level, precipitation, and population are all examples of ratio data.
Graphics compiled from the University of Texas Cartographic Communication website.
See The Cartographer's Palette: The Semiotics of Cartography