Grid Coordinate System

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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