"Official grade" means the grade or curblines. Grades shall be established at street intersections and at such points between as may be desirable, and grade lines shall be unbroken lines between adjacent established points. Elevations for grades shall be taken at the four corners of each street intersection (and at such other points, if any, as the City Engineer may seem desirable). The points at which such elevations shall be taken shall be defined to be the points of intersection of the curblines of the street upon which the grade is proposed to be established, with the property lines extended, of cross streets.
(`78 Code, § 12.04.030.)