"By drawing a diagram, a ground plan of a house, a street plan to the location of a site, or a topographic map, one draws a "logical two dimensional picture." A "logical picture" differs from a natural or realistic picture in that it rarely looks like the thing it stands for. It is a two dimensional analogy or metaphor - A is Z."

from Unpublished Writings in Robert Smithson: The Collected Writings, edited by Jack Flam, published University of California Press, Berkeley, California, 2nd Edition 1996