An instrument and method for measuring the invert of a sewer pipe relative to a manhole employs a horizontal bar which spans the manhole and rests upon its rim. A sight rod is slideably mounted on a sleeve fixed to the bar. The sleeve has adjustments for mounting the bar in true vertical and plumb position with a spirit level. Removably connected to the lower end of the rod is an arm which extends laterally and below the rod at its distal end. The rod is lowered until the distal end of the arm contacts the invert of the pipe. The vertical distance between the bottom of the arm at contact with the invert and the intersection of the rod with the bottom of the horizontal bar yields the desired elevation. It may be read directly off the sight rod when the rod is specially marked with corrected graduations.