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<br />2-6 <br /> <br />areas. This will facilitate survey work and avoid the necessity of establishing <br />a new baseline at the time of construction. This is usually done by <br />staking a series of tangent lines offset a convenient distance from the <br />centerline of the existing channel. It is recommended that all points <br />of intersection (prs) of the tangent lines be staked with metal pins, <br />when practicable, for later use in final design and construction layout <br />surveys. A closed traverse survey should be made on this baseline. <br />For closed traverse surveys, the error of angular closure should not <br />be greater than I.S...rN minutes, N being the number of angles in the <br />traverse. Horizontal closure of chained distances should not exceed <br />1.0 foot per 1000 feet of traverse length. All traverse angles should <br />be doubled and checked by comparing computed bearings with observed <br />magnetic bearings. <br /> <br />e <br /> <br />The use of a coordinate system will often facili tate the drafting and <br />layout of complex horizontal controls. The coordinates should be related <br />to existing systems in the area, where established, or to an assumed <br />origin located so that all traverse points will plot in the same quadrant. <br /> <br />Vertical control <br /> <br />Vertical datum. - - Mean sea level datum is recommended for all channel <br />improvement work. <br /> <br />Bench marks. - - Reasonably permanent bench marks should be established <br />in the work plan stage at tributary junctions, at or near bridges or culverts <br />where channels intersect roads, and at the beginning and ejld of channels. <br />In addition, bench marks are suggested at approximately one-half mile <br />intervals along the valley to facilitate the survey of valley cross sections <br />and the preparation of an adequate profile. <br /> <br />e <br /> <br />All bench marks should be set by making turns through each bench mark; <br />they should never be set by side shots from the level circuit. Level <br />circuit surveys should be closed within the required degree of accuracy. <br />The bench mark may be a plate or cap set in concrete, a plate on a <br />concrete post similar to those used by USGS, or other similar means <br />that will give the same degree of permanency, Manufactured survey <br />markers, consisting of metal stakes with bronze caps are sometimes <br />used because they are relatively inexpensive and can be set quickly <br />and easily. Where manufactured markers are not used, it is advantageous <br />to tag each bench mark with an aluminum tag showing the bench mark <br />number and elevation. <br /> <br />Permissible error of closure. - - For most channel work, vertical control <br />is of paramount importance. Channel designs are dependent upon the <br />amount of fall between hydraulic control points. Therefore, "third <br />order" leveling (i.e., .05 times the square root of the length of level <br />circui t in miles) should be maintained for all bench level circuits, including <br />subcircuits within larger circuits. These bench marks should be established <br />to the specified degree of accuracy in the planning stage for use in <br />both the planning and final design surveys. <br /> <br />e <br /> <br />. <br />