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The following map that I have included is showing how isopleth maps are constructed using points 1 through 4.
Here is a featured breakdown of each point.
"1. The point-values are located
2. the critical isopleth of value 70 is interpolated, with the aid of crosses placed between pairs of values at a distance proportional to the value of each
3. other isopleths are similarly interpolated
4. a system of density shading is applied for clarity between the isopleths."
Map Url: www.fao.org/DOCREP/003/T0446E/T0446E06.htm