Consider these cases:–
     (1) Someone asks “How did you estimate the height of this
     building?” I answer: “It has four storeys; I suppose
     each storey is about fifteen feet high; so it must be about
     sixty feet.”
     (2) In another case: “I roughly know what a yard at that
     distance looks like; so it must be about four yards long.”
     (3) Or again: “I can imagine a tall man reaching to about
     this point; so it must be about six feet above the ground.”
     (4) Or: “I don't know; it just looks like a yard.”