Comparator, Geodetic

an instrument used to measure the length of measuring wires and tapes. The length of the wires and tapes is determined by comparing them to the known length of the geodetic comparator. Opticomechanical geodetic comparators are a series of concrete posts on which microscope micrometers are installed plumb and in the same vertical plane. The distances between the axes of neighboring microscope micrometers are measured by means of a rod of known length, which is moved along the geodetic comparator in a cart that travels along a railway installed beneath them. The total of these distances is the length of the geodetic comparator. The lengths of the measuring instruments are then compared to the total using the extremal microscope micrometers. A geodetic comparator for low-precision measurements of length is a bench with the necessary distances marked on it with a standard tape, and the lengths of measuring instruments are compared to them.

An interference geodetic comparator consists of two mirrors mounted on special attachments parallel to one another and two microscope micrometers placed above them. A quartz rod whose length (usually 1.2 m) has been measured on a metrological interference comparator is installed between the mirrors. The distance between the extremal mirrors (usually 24 m) is determined from the known length of the rod by means of light interference, and the distance between the axes of the microscope micrometers and the extremal mirrors is determined by micro-metric measurements. The lengths of measuring instruments are compared with the distance between the axes of the microscope micrometers, as in the case of opticomechanical geodetic comparators. The most precise measuring instruments (invar wires 24 m long) have a precision of length determination of 5 X 10^˗7 for opticomechanical comparators and 2.5 × 10^˗7 for interference comparators.