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<br />. <br /> <br />BaS8 Mapping <br />The 1 OO'year floodplain information and data must ultimately be delineated <br />on a map of suitable scale and detail. The map may be an existing map <br />(USGS quadrangle map. town. county. etc.) or a large scale topographic map. <br />Floodplain outlines shown on large scale topographic (," = '00' or <br />," = 200' with 2 foot contours) maDS with cultural features are desirable <br />because the floodplain outlines can be defined more accurately. and physi- <br />cal features can be shown more clearly. (Accuracy is important for effective <br />floodplain zoning and regulation, Administrative prOblems and the need for <br />costly and time-consuming field surveys increase as map accuracy de- <br />creases. Maps which are enlarged from small scale maps have only the <br />accuracy equivalent to that of the anginal small scale map.) <br />Rectified photographic contour maps with flood outlines must be used with <br />great caution when establishing regulatory elevations or floodplain boun- <br />daries. These maps may be distorted. Contours drawn to scale may not <br />match the photo background because of this distortion. On the other hand. <br />line maps showing planimetric features and ortho'photograhlc maps are true <br />to scale within their stated accuracy. <br />The CWCB encourages local governments to obtain large.scale planimetric <br />or ortho-photo contour maps for floodplain administration purposes. Large' <br />scale maps can also be used for other community purposes such as planning <br />and design of highways and streets. water and sewer lines. and land use <br />planning. and for planning and deSign of pnvate developments. <br /> <br />. <br /> <br />Field Surveys <br />Preparation of accurate f/eld surveys is a prerequisite to any topographic <br />mapping prolect. The field survey ties speCific pOints on the map to known <br />points on the ground and assures anyone using the maps of their <br />relationship to condilions In the field. The prOVision of surveYing monuments <br />In the field ensures that the maps will b easier for local officials and property <br />owners to use. The field surveys for base maps must be performed at a stan. <br />dard of third'order control as established by the NatIOnal GeodetiC Survey, <br />These standards are stated in a document entitled "ClaSSification. Standards <br />of Accuracy, and General Specifications of Geodetic Control Surveys:' dated <br />June. 1980. In addition, the CWCB has prepared speCifications for obtaining <br />both field surveys and large'scaie topographiC mapping which are available <br />for local governments to use. The speCifications are entitled. "Specifications <br />for Photogrammetrlc Services for Floodplain Studies In Colorado" dated <br />August. 1982. <br /> <br />. <br /> <br />Datum Consistency <br /> <br />Developers of land In the floodplain who use flood elevations established In <br />a floodplain study as a guide must use that information accurately, The <br />establishment of field survey datum continuity between the study results and <br />the on.the-ground conditions IS a major factor to accomplish thiS. ThiS can be <br />done through: <br />. Uniform Datum Standards n The base levels. elevations. and mapping <br />may be tied to a Single local datum; It is preferrable however. that they <br />be tied to the Nat/onal Datum I MSL for vertical datum and State Plane <br />Coordinates for hOrIZontal datum). By tying to the National Datum. <br />Uniformity IS maintained within the region. the state. and the nation. <br />. Benchmarks n Permanent elevations can be established in the field <br />through a program of monumentat/on In the field. This can be accom. <br />plished easily by uSing Federally established benchmarks where avail. <br />able, Where Federal benchmarks do not exist within a community, it IS <br />beneficial to construct a control marker network. By having control <br />markers with known elevations. property owners can more eaSily and <br />accurately determine their own flood hazard risks through conventional <br />field surveYing procedures. <br />