Determining reference plane for surface plate inspection

Claim 1

THE METHOD OF CHECKING THE VARIATION FROM TRUE FLATNESS OF THE GAGING SURFACE OF A SURFACE PLATE BY MEANS OF A CALIBRATED STRAIGHTEDGE, WHICH METHOD IS CHARACTERIZED BY THE STEPS OF: A. MOUNTING THE STRAIGHTEDGE ON A PAIR OF FOOTBLOCKS OF EQUAL HEIGHT RESTING ON SAID SURFACE OF THE SURFACE PLATE AT SPACED APART LOCATIONS ALONG ONE OF A PAIR OF INTERSECTING LINES OF MEASUREMENTS ACROSS SAID SURFACE; B. TAKING MEASUREMENTS OF THE DISTANCES BETWEEN THE STRAIGHTEDGE AND SAID SURFACE AT DEFINED STATIONS ALONG SAID ONE LINE OF MEASUREMENTS, INCLUDING THE STATION DEFINED BY THE INTERSECTION OF SAID TWO LINES OF MEASUREMENTS; C. SIMILARLY (1) MOUNTING THE STRAIGHTEDGE AND (2) TAKING MEASUREMENTS AT DEFINED STATIONS ALONG THE OTHER LINE OF MEASUREMENTS, INCLUDING THE STATION DEFINED BY THE INTERSECTION OF SAID LINES OF MEASUREMENTS; D. SEPARATELY GRAPHICALLY REPRESENTING THE MEASUREMENTS TAKEN ALONG EACH LINE OF MEASUREMENTS, CORRECTED FOR CALIBRATED DEVIATION OF THE STRAIGHTEDGE, WITH THE REPRESENTATIONS OF THE MEASUREMENTS ALONG EACH LINE SPACED APART AT INTERVALS CORRESPONDING TO THE SPACES BETWEEN SAID STATIONS TO THUS PROVIDE PORTRAYALS OF THE PROFILES OF SAID SURFACE ALONG SAID LINES; E. DELINEATING A STRAIGHT LINE IN SUCH RELATION TO ONE OF SAID PORTRAYALS AS TO CORRESPOND TO THE LOCATION OF A LINE LYING IN A DESIRED REFERENCE PLANE; F. LOCATING THE POINT RELATIVE TO THE OTHER OF SAID PORTRAYALS THAT CORRESPONDS TO THE LOCATION OF SAID STRAIGHT LINE RELATIVE TO THE SURFACE PROFILE AT SAID STATION; AND G. DELINEATING A STRAIGHT LINE THROUGH SAID LOCATED POINT THAT BEARS A PREDETERMINED RELATIONSHIP TO THE SURFACE PROFILE DEPICTED BY SAID OTHER PORTRAYAL AND WHICH COOPERATES WITH THE FIRST DELINEATED STRAIGHT LINE IN DEFINING A REFERENCE PLANE TO WHICH MEASUREMENTS TAKEN ON OTHER LINES OF MEASUREMENTS ACROSS SAID SURFACE, EACH INTERSECTING SAID TWO LINES OF MEASUREMENTS, CAN BE RELATED.