At SLAC (Stanford Linear Accelerator Center) a fully automated vertical comparator for the calibration of digital levels and invar staffs was developed by the Metrology Department in cooperation with the Institute of Engineering Geodesy and Measurement Systems at the Graz University of Technology. This vertical comparator is the first in the US. With the vertical comparator it is possible to perform system calibration and CCD camera measurements of rods. System calibration uses the height readings of the digital level at different positions of the rod and compares them with the reference readings obtained by the interferometer. In the case of CCD camera measurements, the positions of the edges in the image is determined and again compared with the interferometer readings. This document gives an overview of the current set-up of the SLAC vertical comparator and experimental results of critical applications including measurements at the end sections of the rod, at critical sighting distances, with unfocused optics and under illumination with the digital levels in use at SLAC.