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<br />INVESTIGATIONS, ANALYSES, AND TECHNICAL DATA <br /> <br />Surveys <br /> <br />(scale 1:24,000). Flood hazard areas are plotted on photogrammetric <br /> <br />Engineering field surveys for horizontal and vertical control were <br /> <br /> <br />conducted by Nelson, Haley, Patterson and Quirk, Engineering Consultants. <br /> <br /> <br />Horizontal control is based on 1927 North American datum, Colorado <br /> <br /> <br />coordinate system south zone. The vertical control is based on U. S. <br /> <br /> <br />Coast and Geodetic Survey, 1929 sea level datum. Field survey was <br /> <br /> <br />conducted in October 1975, prior to the aerial photography. <br /> <br />Maps <br /> <br />Basin maps were developed from U. S. Geological Survey topographic maps <br /> <br />maps prepared especially for this study. Topography, planimetry, and <br /> <br /> <br />cross-section data were compiled by photogrammetric methods from low <br /> <br />level aerial photography flown in October 1975. The photogrammetry is <br /> <br /> <br />by Nelson, Haley, Patterson and Quirk, Engineering Consultants, 2021 <br /> <br /> <br />Clubhouse Drive, Greeley, Colorado. Funds for the mapping were provided <br /> <br />Most of the data for the 158 cross-sections in the study area were <br /> <br /> <br />obtained photogrammetrically. Cross-section data at road and bridge <br /> <br /> <br />crossings and field control in reach No.6 were surveyed by tne Soil <br /> <br /> <br />Conservation Service. Survey crew assistance was provided by the <br /> <br /> <br />Colorado Water Conservation Board and the San Juan Soil Conservation <br /> <br />by the Upper San Juan Regional Planning Commission (USJRPC) ,and the <br />contracting for services was through the Colorado Water' Conservation <br /> <br />Board (CWCB). The maps were reproduced on a scale of 1" = 200' and <br /> <br /> <br />reduced to I" = 400' for report purposes. Mylars on a scale of I" = <br /> <br />District. <br /> <br />These surveys were conducted during the period June 1976' <br /> <br />200' are on file. Contour interval is 2-feet. Map topography compiles <br /> <br />through February 1977. Ingress and egress rights for the survey were <br />provided by the town of Pagosa Springs and Archuleta County. <br /> <br />with national map accuracy requirements. <br /> <br />Elevation Reference Marks <br /> <br />In February 1977, the town of Pagosa Springs, requested the lower limits <br /> <br />of the study area be extended an additional 3,000' on the San Juan <br /> <br /> <br />RiVer. The purpose for this study extension was to provide flood plain <br /> <br /> <br />information for potential developments in an area which had, not been <br /> <br />During June 1976, several of the temporary reference marks used for <br /> <br /> <br />photogrammetric mapping were replaced with permanent markers consisting <br /> <br /> <br />of brass caps imbedded in concrete. This work was done by survey crews <br /> <br /> <br />employed by the San Juan Soil Conservation District. Survey notes of <br /> <br /> <br />the original horizontal and vertical control are on file with Colorado <br /> <br /> <br />Water Conservation Board. The following are a tabulation of the per- <br /> <br /> <br />manent reference marks used for this study. <br /> <br />anticipated in the original plan of study. Since this area was not <br /> <br />included in the original aerial photography, the mapping was compiled by <br /> <br />, ground control and field survey methods. The mapping was done by the <br /> <br /> <br />Soil Conservation Service (SCS) and is included as study reach No.6 on <br /> <br />Flood Hazard Area Map, Supplement 2. <br /> <br />C - <br /> <br />C - 2 <br />