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retreat mined during June 1985. 'The trarisec-t was carefully <br />• surveyed and studied to determine draw angles. 'The 1986 <br />Annual Hydrology and SZil~sidence Report contains the results of <br />this analysis and calculates the angle of draw at 16 degrees. <br />Total elevation changes for this transect average -1.07 feet <br />and range from -1.65 feet to -0.38 feet. Recent elevation <br />changes have been minimal and the area is believed to be <br />stabilized. Some motion was detected during the 1989 survey <br />and is thouc~t to be related to the establishment of new <br />control station coordinates. Most surveyed sites were found <br />to be approximately 0.35 feet la/er as a result of the 1989 <br />survey. <br />Stations 201-217: Stations 201-217 comprise a high resolution transect of survey <br />points bisecting the 1st North, 8 Fast, 5 North, 1 We:,t panel. <br />'This panel underlies 1250 to 1550 feet of Overburden in the <br />vicinity of the transect. The panel was retreat mined during <br />February 1986. Surveys were initiated in 1981 and continued <br />thrax~ 1990. Elevation changes for this tran_sect. average <br />0.30 feet and range fr®n 0.73 feet to -0.10 feet. Recent <br />elevation changes have been minimal and the area is believed <br />to have never sham any discernable subsidence effects. Ne„.t <br />control station coordinates established in 1989 suggest a <br />survey error had occurred in the initial establishment of the <br />control for this transec.-t as new elevations were calculated to <br />be approximately 35 feet lower wen all sites. <br />• Stations 218-224: Stations 218-224 c~ng~rise a high resolution transec,-t of survey <br />points which lie perpendicular to stations 201-217 anti Overlay <br />950 to 1250 feet of Overburden in the vicinity of the <br />transec°t. 'This transact lies adjacent to the 1st North, 8 <br />East, 5 North, 1 West panel. The panel was retreat mined <br />during February 1986. Surveys were initiated in 1981 and <br />continued through 1990. Elevation changes for this transec-t <br />average 0.66 feet and range from 2.24 feet to 0.19 feet. 'The <br />validity of the 2.24 foot elevation gain is questionable and <br />likely attributable to survey error. If this figure is <br />ignored, the average elevation change becomes 0.40 feet with <br />a maximum of 0.78 feet. Recent elevation changes have been <br />minimal and the area is believed to have never shown any <br />discernable subsidence effects. New control station <br />coordinates established in 1989 suggest a survey error had <br />oceurrea•1 in the initial establishment of the control for this <br />transact as new elevations were calculated to be approximately <br />35 feet lower Over all sites. <br />Visual Monitoring 1990 <br />Visual subsidence monitoring is accortq~lishexi by periodic "walker/ers" of <br />• mined areas within the present permit area. Walkovers are generally done <br />in conjunction with survey monitoring due to the seasonal accessibility of <br />the area. Visual monitoring is concentrated Over areas of retreat mining <br />8 <br />