The datum plane provided for in § 12.04.010 shall be defined to be a horizontal plane lying 171.2 feet below the top of a certain metal bolt set in the foundation of the cement sidewalk at the northwesterly corner of Main and Sixth Streets. The metal bolt is situated on the southeasterly corner of Lot 1, Block 144. The top of the bolt is about one-half inch below the surface of the cement sidewalk.
(`78 Code, § 12.04.020.)