"The testing of the steel tapes used by the engineer and surveyor is done at the Bureau of Standards by comparison with a steel bench standard described in this paper. For the test of base-line tapes (usually made of invar with a length of 50 m) a much more elaborate apparatus is employed. This geodetic comparator is also described in detail. Results have shown that the length of 50 m invar base-line tapes can be determined with this apparatus to an accuracy such that the absolute errors of the certified result do not exceed 1 part in 1,000,000.