Precise Dam Surveys—Los Angeles County Flood Control District

The Los Angeles County Flood Control District performs precise control surveys on eight concrete arch and gravity dams, five earthfill dams, and 12 major earthfill debris basins. Probable error in position of surveillance points varies generally from ±0.5 to ±0.2 mm with extreme values ranging to ±6 mm for unfavorable geometric or refractive conditions, or both. Techniques are considered excellent and optimal with base-line precisions generally at 0.5 to 1 ppm in probable error, and angular direction probable error on the order of ±0.5 sec. Standards for precision should be based on reasonable effort consistent with that magnitude of movement with adequate reliability that would induce response action if exceeded. The standards should, in addition, be dependent on the nature of the structure, associated risks, and the limitations of the site geometry comprising the precise systems.