BOREHOLE INCLINOMETER MONUMENT FOR MILLIMETER HORIZONTAL GEODETIC CONTROL ACCURACY

A geodetic monument is described whose horizontal position may be monitored relative to points 30 m deep to an accuracy of 0.5 mm, and to points mom than 2 km to a potential accuracy of 2 mm. The monument incorporates a 7 cm diameter borehole lining equipped with orthogonal alignment grooves that guide a portable inclinometer during measurements. The inclinometer is raised incrementally from the base of the hole to measure the tilt of contiguous 0.5-m-long segments of the borehole prior to geodetic occupation. Integrating the results yields a measure of the surface monument's position. Random errors increase with the square root of the number of downhole measurements N as 0.03 square-root N mm (N=2d for d-m-deep hole). Systematic errors are approximately 0.02% of the lateral offset of the base of the hole relative to the surface monument. Unlike previous monuments the inclinometer-based system permits surface and subsurface instability to be characterized, and the effectiveness of the monument to be monitored.