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• Powerline Monitoring Power structures in areas of active subsidence will be monitored periodically for operational purposes until all residual subsidence is complete. The frequency of monitoring will be determined by operational experience. III.A.7.1 Subsidence Monitoring Program. A program was designed to yield subsidence data that would be used to verify the accuracy of initial predictions under actual ground conditions, and which could be used to predict future effects after monitoring the first longwall and the first room and pillar panel. BME monitored subsidence over the first longwall pane{ (LW1) using state-of-the-art analytical photogrammetry. LW1 was mined in calendar year 1987 and the first two months in 1988. Four flights were flown to record and monitor the surface over LW1. The first flight was flown during August, 1988 to establish baseline conditions and base drawings for both LW1 and RPS for comparison with the future flights. The second and third flights occurred in May 1987 and September 1987. Data and drawings from these flights were presented to Colorado Mined Land Reclamation • Division (CMLRD), now DMG, in a report in December, 1987 for review. The fourth flight occurred in June, 1988. The fourth flight information and drawings were assembled into a final report and presented to DMG in October, 1988. Residual subsidence may occur after June, 1988, above panel LW1. It is important, for purposes of verifying the accuracy of subsidence projections contained within the permit document, to accurately document the magnitude of subsidence which occurs above this test panel. Conventional semi-annual survey monitoring of the five control points located within the zone of subsidence (see description below) continued until cessation of subsidence was accurately documented. In the event that any of these conventional survey monuments experienced more than 1.0 foot of residual subsidence, an additional aerial photographic flight, subsidence map and report would have been prepared for presentation to CMLRD within one year following cessation of subsidence. This reporting will no longer be required. The following is a description of the various phases of the subsidence monitoring program. (1) Ground Survey Eight control monuments were established outside the zone of subsidence to provide horizontal and vertical control for the area. Before each flight, the monuments were panelled and re-levelled to determine their elevations by differential levelling or by using reciprocal Permit Renewal #3 (Rev, 8/99) III-23