Figure 7-29.—Computing area by counting the squares.However, this works out toAnd this, in turn, reduces toSubstituting in the formula the data from figure7-26, you haveIf you work this out, you will find that the result is25,950 square feet or approximately 0.6 acre.AREA BY COUNTING THE SQUARES.—Another method of computing the area of an irregularfigure is to plot the figure on a sheet of graph paper(plotting is explained later in this chapter). Then youdetermine the area by counting the squares within thefigure outline and multiplying the result by the arearepresented by each square.Figure 7-29 shows the same figure shown in figure7-28 but plotted to scale on a sheet of graph paper onwhich each of the small squares is 5 feet x 5 feet or 25square feet. When you count the squares within theoutline, you will find that they total 1,038 squares whichmeans1,038 x 25 = 25,950 square feet.AREA BY PLANIMETER.— A planimeter is amechanical device that you can use to compute the areaof an irregular figure after tracing the perimeter of ascale drawing of the figure with the tracing point on theplanimeter. The most commonly used instrument iscalled the polar planimeter.Figure 7-30 shows a polar planimeter. Its partsinclude an anchor point, P; a tracing point, T, with aguide, G; a vernier, V; and a roller, R. An adjustable arm,A, is graduated to permit adjustment to conform to thescale of the drawing. This adjustment provides a directratio between the area traced by the tracing point and therevolutions of the roller. As the tracing point is movedover the paper, the drum, D, and the disk F, revolve.The disk records the revolutions of the roller in units andFigure 7-30.—Polar planimeter.7-20

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