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at its eastern boundary. It is anticipated that as the aquifer approaches its eastern <br />boundary the tops and bottoms should nearly converge and the thickness would <br />approach zero. To verify this, a difference was computed for logs that contained top and <br />bottom picks within a mile of the aquifer's perimeter, excluding the western perimeter. <br />The difference ranged from 0 to over 100 feet. The average difference varied between <br />aquifers, but 50 feet was a justifiable difference for all aquifers to specify for the <br />thickness at the perimeter. <br />A final set of outliers was determined by comparing the pick elevations to ground <br />surface. If a pick elevation was greater than ground surface using a USGS 30 meter NED, <br />then the pick was removed from the final round of gridding. <br />1.2.5 Estimation of Control Picks <br />To correct the negative and anomalous thickness areas, additional bull's-eye picks were <br />removed or estimated control points were added to the data sets. Control points were <br />developed by reviewing the top and bottom surface that defined the thickness. If one <br />surface contained data in an area of concern and the other surface did not, then a <br />thickness consistent with surrounding areas was estimated at that location and the value <br />was added to or subtracted from the existing pick to be used as a data point for the <br />surface of sparse control. <br />A second method of adding control picks was used in areas where there was no top or <br />bottom pick for an aquifer along its outcrop boundary. Near the aquifer's boundary the <br />top of the aquifer is at or near ground surface. However, it is not appropriate to assume <br />that the top of the aquifer is always at ground surface due to the presence of alluvial <br />paleochannels and soils overlying the bedrock formations. Additionally, as indicated in <br />the discussion above, the aquifer's thickness at its defined extent was not found to be <br />zero at many locations. Because of these reasons, ground surface elevation were <br />extracted from a USGS 30-meter NED along the perimeter of each aquifer's boundary, <br />outside of the SPDSS alluvial extent and away from existing data; and the top was given <br />a control pick equal to ground surface and the bottom was given a pick equal to ground <br />surface minus 50 feet. This method was not used along the western margin because of <br />the larger elevation changes and the faulted structure in this area. <br />The estimated control picks derived from existing picks or from ground surface were <br />added to the list of database picks and gridded for the final development of each <br />surface. A list of control points used for each surface can be found in Appendix C. <br />Sections 3.1 through 3.4 present the contoured surfaces for the Dawson, Denver, <br />Arapahoe, and Laramie-Fox Hills Aquifers, respectively. <br />For the sand thickness maps, control points representing a minimal net sand thickness of <br />10 feet were added along the northern, eastern, and southern boundaries of each aquifer. <br />Along these areas of the aquifer boundaries, the aquifer tops have been eroded and the <br />tops and bottoms begin to converge at the boundary. Therefore, the estimated control <br />points of 10 feet help constrain the grid of the net sand thickness as an aquifer pinches <br />out near its perimeter. Along the western boundary of the aquifers, the top was not <br />eroded in a similar fashion and many net sand thickness picks much greater than 10 feet <br />SPDSS Phase 2 Task 42.2 TM -Final 9 <br />2/13/06 <br />