Figure 9-1.—Primary traverse and secondary traverse.edge of the blade. A more in-depth discussion oforienting the plane table will follow later in this chap-ter.Next, carefully measure the vertical distance be-tween the horizontal line of sight through the telescopeand the ground level at D1. Let’s say this distance is4.5 feet. This means that, whenever you sight on a rod,you will line up the horizontal cross hair with the4.5-foot graduation on the rod.Figure 9-2 is a sketch of the detail points that weare plotting. Point D1 and point A in this figure corre-spond to the same points in figure 9-1. Assuming thatyour alidade is equipped with a Beaman stadia arc(some alidades are not), plot point 1 of figure 9-2 inthe following way. With the edge of the alidade bladeexactly on D1 on the paper, train the telescope on a rodheld on point 1, and line up the horizontal cross hairwith the 4.5-foot mark on the rod.You read a rod intercept of 6.23 feet. This meansthe slope distance is 623.0 feet. On the H-scale of theBeaman arc, you read three-tenths of one percent; youwill have to estimate this less than one-percent read-ing. The horizontal distance, then, is three-tenths ofone-percent less than the slope distance, or623.0 feet - (623.0 x 0.003 feet) = 623.0-1.87.This rounds off to the nearest foot at 621 feet. Adda focal distance of 1 foot, and the result is 622 feet.Figure 9-2.-Sketch of topographic detail points.On the V-scale, you read 44. You know that thevalue you use is the difference between what you readand 50. In this case, it is 6. Therefore, the differencein elevation is 6 percent of the slope distance, or623.0 x 0.06 = 37.4 feet.Then, the elevation of point 1 is the elevation ofD1 minus the difference in elevation, or532.4 -37.4 = 495.0 feet.As you know, the difference in elevation wassubtracted because the vertical angle was negative.Finally, with the edge of your alidade blade stillon D] and your telescope still trained on point 1, youcan draw a light line and measure off 622 feet from D1along the line to locate point 1. At that distance alongthe line, mark and label the point and write in theelevation. Many topographers use the decimal point inthe elevation to mark the point.ORIENTATION METHODSAs you learned from the above example, plottingof detail points cannot begin until the plane-table9-2
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