isopleth [key], line drawn on a map through all points of equal value of some measurable quantity. In many meteorologic, oceanographic, or geologic studies some physical or chemical property is examined that varies from place to place on a map. Isopleths showing the quantity of the property being studied can be drawn on the map to highlight regional trends of high or low abundance of that property. For example, topographic maps showing contours of equal elevation are probably the most common type of isopleth maps. Relief “highs” (hills) are shown by concentric contour isopleths, and depressions such as volcanic craters are shown by concentric contours with hachures pointing toward the center of the depression. Isopleths are drawn on weather maps to indicate lines of equal air pressure (isobars) and equal temperature (isotherms). Isobaths are lines connecting points of equal depth in lakes and oceans. Isopach maps show distribution of thickness of a given rock unit. Gravity maps are drawn showing isogals (lines of equal gravitational acceleration). Variations in the strength of the earth's magnetic field are shown by isogams (after gamma, the common unit for measuring magnetic strength).
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