Isobars

http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Closed_Isobars/Closed_Isobars.html

Below is a map of a fictitious piece of land that shows closed isobars around a low pressure system, and is a good example of how, much like a topographic map, isobars can represent the change in a value over space, such as here, as it shows barometric pressure increasing as it gets further from the center of low pressure.