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III Third -West Section Geological characteristics of the third -west section were similar to both previous sections, with increased overbur- den over some areas. The section was traversed by Cot- tonwood Creek. Three monitoring lines were installed mer the section. The network layout is shown in figure 7. The cast -west L -line monitored the centerline of the panel, and the A- and C -lines provided subsidence information across the panel. The A-linc was oriented to provide in- formatinn regarding the cffettivcness of the barrier pillars beneath Cottonwood Creek. MONITORING PROCEDURES AND EQUIPMENT The Roadside Mine subsidence monitoring program was designed primarily to measure vertical movement of the subsidence monuments. The initial and final monu- ment elevations showing the subsidence profile were of prime importance. Another initial objective of the study was to determine the timing and rate of subsidence dc- vclopmcni. Because of a limited number of surveys, the actual subsidence rates were not evaluated. Traverse surreys were used to position the monitoring network over the mine workings at predetermined lova• tions and to monitor vertical and horizontal displacements. The traverse surveys were run using a total station ge- odimctcr and reflecting prism assemblies (fig. 8). Hori- zontal and vertical angles and slope distances were Figwo O.—ToW ata*m ga0d1m9W and rMacdon prtam as- ae"Y measured to each subsidence monument from instrument stations with known coordinates. The elevations and coor- dinatcs of the instrument stations were determined from stablc control points beyond the influence of mining, and a closed traverse survey was performed on all instrument stations and control points as part of each survey to ensure accuracy. The target unit for the traverse surveys (fig. 8) held a prism for distance measurement and a target for angle measurement and was clamped securely onto the subsidence monument and then surveyed. The inter- changeable bottom clamp could be attached to circular monuments ranging in diameter from 9/16 to 1.1/2 in. If additional height was required for improved visibility, 1 -ft aluminum extensions were attached between the clamp and the target. Third -order direct level surveys were used to monitor elevation in terrain with moderate relief. These surveys were performed using an automatic, self -leveling level and a level rod with 0.01 -ft graduations. The accuracy of CIC• vations from direct level surveys is greater than that from traverse surveys. Thud -order level surveys have a maxi- mum allowable closure error of :0.05 ft times the square root of the length of the level line in milcs(0.05 Vmi ), which resulted in a standard error calculated from several levet surveys of 0.02 ft. The elevations computed from the traverse surveys yielded a standard error of 0.08 ft. DATA PROCESSING Following each traverse survey, field data were down- loaded from the total station memory into computer files. Data from the total station geodimeter included station names, horizontal and vertical angles, slope distances, and target and instrument heights. The casting, northing, and elevation of each monument were then stored in a mass storage file; these data were filed by survey location, survey type, and date. Raw data from the level surveys were manually typed into the computer, and elevations for each subsidence monument were calculated and stored in the same mass storage file. A computer program calcu- lated point elevations and closure errors and then adjusted the monument elevations accordingly. In this form, the information was readily accessed and used as input for programs that performed calculations such as coordinate and elevation differences between any two surveys. The data were also used to map monument locations and plot suhsidcncc profiles. Several graphics software programs were used to generate subsidence profiles and maps with subsidence monument coordinates.