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Column density. A special type of area density is called column (mass) density (also columnar mass density), denoted ρ A or σ.It is the mass of substance per unit area integrated along a path; It is obtained integrating volumetric density over a column:
Population density (in agriculture: standing stock or plant density) is a measurement of population per unit area, or exceptionally unit volume; it is a quantity of type number density. It is frequently applied to living organisms , most of the time to humans .