Figure 7-31.—Area within straight-line and curved-line boundaries (curved segments).tenths; the drum, in hundredths; and the vernier, inthousandths.Specific instructions for using the polar planimeterare found in the instruction booklet that is provided withthe instrument. With minimal practice, you will find thatthe planimeter is a simple instrument to operate. Youshould remember, though, that the accuracy obtainedwith the planimeter depends mostly on the skill of theoperator in accurately tracing the boundary lines of thefigure with the tracing point of the planimeter.If the instruction booklet has been lost, do not worry.The planimeter can still be used. Simply determine howmany revolutions of the roller it takes to trace a figureof known area (drawn to the same scale as the figure youwish to determine the area of). Then trace the figure youare working with and read the number of revolutionstaken to trace the unknown area. You now know threevalues as follows: (1) the area of the figure of knownsize, (2) the number of revolutions taken to trace thefigure of known size, and (3) the number of revolutionstaken to trace the figure of unknown size. B y ratio andproportion, you can then determine the unknown area.PARCELS THAT INCLUDE CURVES.— Not allparcels of land are bounded entirely by straight lines.You may have to compute the area of a construction sitethat is bounded in part by the center lines or edges ofcurved roads or the right-of-way lines of curved roads.Figure 7-31 shows a construction site with a shapesimilar to the traverse you have been studying inprevious examples. In this site, however, the traverselines AB and CD are the chords of circular curves, andthe boundary lines AB and CD are the arcs interceptedby the chords. The following sections explain themethod of determining the area lying within thestraight-line and curved-line boundaries.The data for each of the curves is inscribed on figure7-31; that is, the radius R, the central angle A, the arclength A (to be discussed in chapter 11 of this7-21

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