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AREA BY TRAPEZOIDAL FORMULA
PARCELS THAT INCLUDE CURVES - 14070_141

Engineering Aid 1 - Advanced Structural engineering guide book
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Figure  7-29.—Computing  area  by  counting  the  squares. However, this works out to And this, in turn, reduces to Substituting in the formula the data from figure 7-26, you have If you work this out, you will find that the result is 25,950 square feet or approximately 0.6 acre. AREA   BY   COUNTING   THE   SQUARES.— Another method of computing the area of an irregular figure is to plot the figure on a sheet of graph paper (plotting  is  explained  later  in  this  chapter).  Then  you determine the area by counting the squares within the figure outline and multiplying the result by the area represented by each square. Figure 7-29 shows the same figure shown in figure 7-28 but plotted to scale on a sheet of graph paper on which each of the small squares is 5 feet x 5 feet or 25 square feet. When you count the squares within the outline, you will find that they total 1,038 squares which means 1,038  x  25  =  25,950 square  feet. AREA BY PLANIMETER.—  A planimeter is a mechanical device that you can use to compute the area of an irregular figure after tracing the perimeter of a scale drawing of the figure with the tracing point on the planimeter. The most commonly used instrument is called the polar planimeter. Figure 7-30 shows a polar planimeter. Its parts include an anchor point, P; a tracing point, T, with a guide, G; a vernier, V; and a roller, R. An adjustable arm, A, is graduated to permit adjustment to conform to the scale of the drawing. This adjustment provides a direct ratio between the area traced by the tracing point and the revolutions of the roller. As the tracing point is moved over the paper, the drum, D, and the disk F, revolve. The disk records the revolutions of the roller in units and Figure  7-30.—Polar  planimeter. 7-20







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