The datum line or base from which the grades of the several streets and avenues of Three Forks shall be determined, defined, fixed, and designated, shall be by reference to the following described benchmarks: first, the top of the cement work at the bottom of the small hole cut in the surface of the sidewalk at the northeast corner of the square formed by the intersection of Main and Cedar Streets, is sixty six and twenty five one- hundredths feet (66.25') above datum and in line A of diagram. Datum is still further fixed by reference to the top of the square stone set on the south side of the track of the N.P.R.Y. near the present station for Three Forks, State of Montana, in the SW1/4 of Section 30, in Township 2N Range 2E, the elevation of which is four thousand sixty one feet (4,061') above sea level as published by authority of the U.S. Geological Survey in the bulletin entitled "A Dictionary of Altitudes in the United States". The elevation of the point on the datum or base line is, therefore, referenced to sea level and is four thousand sixty six and twenty five one-hundredths feet (4,066.25'), and for convenience in the use of the figures referring to the elevations of points on the datum or base line for Main Street, the figures in excess of four thousand feet (4,000') are used. (1977 Code § 10.02.010)