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I I I I I I I' I I I I I I I I I I I I MAPPING INFORMATION The community of Silverthorne, Colorado provided mapping of topography, streets, and other cultural information in electronic format (Auto CAD drawing files). Simons & Associates conducted additional bathymetric survey at 105 transects. The surveyed transects were supplemented with sections from the existing FIS per the scope of work for the update. This n:sulted in complete mapping within the community of Silverthorne. For areas of the Blue River floodplain that are adjacent to Silverthorne but within Summit County, sections from the existing FIS for Summit County were used to describe the floodplain. Summit County does not have detailed topographic mapping of the floodplain. The mapping for the Town of Silverthorne and for the existing FrS is on the NA VD 1929 datum. The new transect survey for the FIS update was tied to a monument that is common to the cadastral survey for the Town of Silverthorne and the existing FrS. The USGS identification of this point is KL0044. The monument is 6.9 miles north from Dillon along State Highway 9 from the junction with Highway 6. KL0044 is a First Order horizontal monument and second order vertical monument adjusted by the National Geodetic Survey in 1995 and 1991, respectively. Simons & Associates used two additional second order benchmarks that are located along Highway 9, KL0041 and KL0040. Table 9 summarizes the elevation adjustment at each monument for 1929 and 1988 NA VD datum. Table 9. NA VD Elevations - 1929 vs. 1988 USGS ID NA VD 1929 (feet) KL0044 8505.80 KL0041 8591.99 KL0040 8639.64 NA VD 1988 (feet) 8510.98 8597.21 8644.86 /). (feet) 5.18 5.22 5.22 A vertical adjustment of 5.20 feet was used to adjust existing FIS and Town of Silverthorne elevations. Positional information for surveyed transects was determined by GPS. Simons & Associates used a Trimble ProXR GPS receiver to locate each transect in the field. Elevation at a tempora.ry monument at the end of each transect was determined by trigonometric leveling from one of the three benchmarks. Transect data (distance and elevation data) were determined using a total station instrument. Mapping information was compiled into two AutoCAD (version 14) files: Workmap.dwg and Ba.semap.dwg. Both files are included on Iomega Zip Disk with this report. A photo log of the bridges in the Blue River study reach was prepared.