Isopach

http://www.geo.utexas.edu/faculty/barker/kempter/rbtephra.html

This Isopach map shows the Tephra deposits of the Rio Blanco Crater.  The red contours are Tephra deposits that are more than 20 meters thick while the whitish contours are tephra that are greater than 20 cm but less than 50 cm thick.   It also has contours of elevation for the crater.  Isopach maps measure thickness.