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Example |
Ex.32-1 Figure 32.2 shows a set of coordinate grid lines for plotting the stations of a traverse. For a small traverse such as this one, it is desirable to locate the grid lines so as to center the traverse lies approximately at the centre on the map sheet. This is done by examining the adjusted coordinates, noting the maximum and minimum values for X and Y coordinates, and then calculating the maximum changes in X and Y coordinates. These distances reduced to map scale, subtracted from the dimensions of the map sheet and divided by 2, yield the distances that should be measured from the edges of the map sheet to the control points having maximum and minimum X and Y coordinates. For the example, let us consider the following data are available:
With a grid interval of 200 m and in view of the values of coordinates for the westernmost and northernmost stations, it is necessary to determine the locations for the western most Y grid line at X = 4200 m and for the northernmost X grid line at Y = 6400 m. It is possible to work either at map scale or in metres on the ground. Assume that the best metric scale available for the work is. graduated only in millimetres, so that working at map scale is more convenient. Since the change in X exceeds that in Y, the X axis is aligned with the long dimension of the map sheet. The location of the Y grid line for X = 4200 m is determined from the left or western edge of the sheet. Since the maximum change in X coordinates is 129.8 mm at map scale, the distance from the left edge of the sheet to the grid line is (282 -129.8)/2 -(4321.4 -4200)/10 = 64.0 mm at map scale. This distance is measured from the left edge of the map sheet and the Y grid line for X = 4200 m is drawn parallel to ,the edge of the sheet. The location of the X grid line for Y = 6400 m is determined from the top of the map sheet next. The maximum change in Y is 42.2 mm at map scale, so that the distance from the top of the sheet to the X grid line for Y = 6400 m is (218 -42.2)/2 -(6405.3 -6400)/10 = 88.4 mm at map scale. This distance is then scaled from the top of the map sheet and the X grid line for Y = 6400 m is drawn through the point and perpendicular to the Y grid line. The balance of the grid lines are then drawn parallel to these two controlling lines at intervals of 20 mm at map scale or 200 m on the ground. After the grid lines are labeled, the position of station 1 is plotted by subtracting X = 4200 and Y = 6200 from the X and Y coordinates for 1 to yield increments in the X and Y directions of 121.4 m and 40.6 m, respectively. These distances are scaled along the grid lines and perpendiculars are erected, the intersection of which defines the control point. The remaining stations are plotted in similar fashion. When all points have been plotted, the distances between points should be scaled as a check against the adjusted distances between the stations of the traverse. In this way the accuracy of the plotting is verified analytically Further, verification of the plotted positions are done during plotting of feild details. |