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Memo to Susan McCannon Tatum Letter of May 11, 1995 page 3 information earlier we would have provided a copy to the Tatums upon request. The two monuments installed adjacent to the Railroad tracks, for which BRI has recently provided survey monitoring results indicate a rise of point "B" by 0.005 foot, a drop of point "IN-2" by 0.036 foot, and a rise of point "IN- 1 " by 0.155 foot. In accordance with the "Classification, Standards of Accuracy, and General Specifications of Geodetic Control Surveys", published by the U.S. Department of Commerce in February,1974, Second Order, Class II standards of accuracy are recommended for "rapid subsidence studies". The Division has imposed this standard for accuracy on subsidence monitoring surveys completed by Coal Mine operators. Second Order, Class II accuracy standards specify 1 part in 20,000 for horizontal accuracy and 1.3 mm IKI'~Z for vertical accuracy, where K is the transect distance in kilometers. Over the transect length of approximately 6640 feet to point "IN-1 ", the allowable horizontal variance would be 10.332 foot and the allowable vertical variance would be ±0.006 foot. Using this accuracy threshold, the movement at point "B" is insignificant and would be considered anon-detection. The movements at "IN-1 " and IN-2" would be considered a potential detect. When requiring the installation of grid line monuments for subsidence monitoring and movement detection the Division requires the installation of monuments, preferably at a spacing equivalent to approximately 1 /10 the maximum overburden thickness. This provides two benefits: (1) increased accuracy in determining the angle of draw; and (2) a sufficient number of monuments to allow comparative verification of monument stability. The Division prefers the installation of a collared and drained monument, intended to minimize freeze-thaw heave. Isolated driven monuments, such as those installed at BRI's discretion adjacent to the tracks, are prone to instability. In addition, monument installations in the vicinity of a facility such as a railroad track or highway are prone to vibrational and loading instability which can also cause non-subsidence induced movement of the monuments. It was with consideration of all of these factors that I have observed in my February 8th memorandum: "I also examined the railroad right-of-way which runs east-west, approximately 400 feet south of the Tatum structure. The mine workings have undermined portions of the railroad right-of-way. The rails are straight and level. No vertical or horizontal distortions could be detected with the naked eye." I continue to hold this opinion.