There is hereby established a city datum to which the elevations of points used in connection with public improvements in the city shall be referred as follows:
The datum shall be a place 570.60 feet below the mark or cross (X) on the south end of the stone doorstep at the foot of the stairway at the west entrance of the building at the northeast intersection of State and Washington Streets, the mark or cross being on the east line of State Street, and 24 feet north of the north line of Washington Street.