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GROUND POINT SYSTEMS
Cross  Refiles - 14071_167

Engineering Aid 2 - Intermediate Structural engineering guide book
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vertical distance from your eye level to the ground is 5.7 feet. ‘Then with the hand level at your eye and with you standing on station  A, the HI is 112.5 + 5.7= 118.2 feet. If a level rod is set up anywhere on the 112.0-foot contour, the reading you would get from station  A would be 118.2 – 112.0= 6.2 feet. Therefore, to determine the point where the 112.0 foot contour  crosses  AB, you only need to have the rodman back out from point A along AB until he comes to the point  where  you  read  6.2  feet  on  the  rod.  You  can determine  the  point  where  the  112.0-foot  contour crosses AD in the same reamer as  AB. You can measure the distance from  A to each point and then record the distance from  A to the 112.0-foot contour on  AB and AD. When all of the contours have been located on  AB and AD, you can shift to station C and carry out the same procedure to locate the contours along  BC and CD. You have now located all the points where contours at a l-foot interval intersect the traverse lines. If the slope of the ground is uniform (as it is presumed to be in fig. 8-13), you can plot the contour lines by simply drawing lines between points of equal elevation, as shown in that figure. If there were irregularities in the slope, you would send the rodman out along one or more lines laid across the irregular ground, locating the contours on these lines as you located them on the traverse lines. Grid  Coordinate  System In the grid coordinate system, the area is laid out in squares of convenient size, and the elevation of each comer point is determined. While this method lends itself  to  use  on  relatively  level  ground,  ridge  or  valley lines must be located by spot elevations taken along the lines. The locations of the desired contours are then determined on the ridge and valley lines and on the sides of the squares by interpolation. This gives a series of points through which the contour lines may be drawn Figure 8-14 illustrates this method. Assume that the squares here measure 200.0 feet on each side. Points a, b, and c are points on a ridge line, also 200.0 feet apart. You need to locate and draw the 260.0-foot contour line. By  inspection,  you  can  see  that  the  260.0-foot  contour must cross AD since the elevation of A is 255.2 feet and the elevation of D is 263.3 feet. However, at what point does  the  260.0-foot  contour  cross  AD?  This  can  be determined  by  using  a  proportional  equation  as  follows. Figure 8-14.-Grid system of ground points Assume that the slope from A to D is uniform. The difference in elevation is 8.1 feet (263.3 – 255.2) for 200.0 feet. The difference in elevation between 255.2 and 260.0 feet (elevation of the desired contour) is 4.8 feet.   The   distance   from   A  to   the   point   where the  260.0-foot  contour  crosses  AD is the value of  x in   the   proportional   equation:   8.1:200   =   4.8:x  or x = 118.5 feet. Lay off 118.5 feet from  A on AD and make a mark. In the same manner, you locate and mark the points where  the  260.0-foot  contour  crosses  BE,EF, EH,  and GH.  The   260.0-foot   contour   crosses   the   ridge, obviously, between point b (elevation 266.1 feet) and point  c (elevation  258.3  feet).  The  distance  between  b and c is again 200.0 feet. Therefore, you obtain the location of the point of crossing by the same procedure just  described. You now have six plotted points: one on the ridge line between  b and c and the others on AD, BE, EF, EH, and GH. A line sketched by hand through these points is the 260.0-foot contour line. Note that the line is, in effect, the line that would be formed by a horizontal plane that passed through the ridge at an elevation of 260.0  feet.  Note,  too,  that  a  contour  line  changes direction at a ridge summit. Control Points This explanation illustrates the fact that any contour line may be located by interpolation on a uniform slope between  two  points  of  known  elevation  a  known distance apart. We, also, demonstrated how a ridge line is located in the same manner. If you locate and plot all the important irregularities in an area (ridges, valleys, and any other points where







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