Isograms

Isograms or isolines are lines of equal value on a geographic map or graph. Isograms indicate continuous changes in a quantity as a function of two other variables, for example, of geographic latitude and longitude on maps, and quantities that change gradually in space (e.g. air temperature). However, they also are used on maps to show changes in quantitative characteristics of phenomena with time (e.g. the earth's magnetism), the rate of change of phenomena (.e.g the first frosts), the duration of phenomena (e.g. the number of days with neap tides), the orientation of some physical quantities, and the frequency or probalibity of phenomena. Their ease of application sometimes leads to over-use, with mappings, vertical cross sections, graphs and other planar information surfaces crowded with data. This tendency is aggravated with CAO/CAM.