Figure 8-32.—Interpolating contour lines with a scale.between 11 and 12 feet. Select the scale on theengineer’s scale that has 12 graduations for a distanceand comes close to matching the distance between A andB on the map. In figure 8-32,this is the 20 scale. Let the0 mark on the 20 scale represent 530.0 feet. Then the 0.2mark on the scale will represent 530.2 feet, the elevationof A. Place this mark on A, as shown.If the 0 mark on the scale represents 530.0 feet, thenthe 11.7 mark represents530.0 + 11.7, or 541.7 feet,the elevation of B. Place the scale at a convenient angleto the line from A to B, as shown, and draw a line fromthe 11.7 mark to B. You can now project the desiredcontour line locations from the scale to the line from Ato B by drawing lines from the appropriate scalegraduations (2, 4, 6, and so on) parallel to the line fromthe 11.7 mark to B.Figure 8-33 shows a graphic method ofinterpolating contour lines. On a transparent sheet, drawa succession of equidistant parallel lines. Number thelines as shown in the left margin. The 10th line is number1; the 20th, number 2, and so on. Then the intervalbetween each pair of adjacent lines represents 0.1 feet.Figure 8-33 shows how you can use this sheet tointerpolate contour lines at a 1-foot interval betweenpoint A and point B. Place the sheet on the map so thatthe line representing 1.7 feet (elevation of A is500.0 + 1.7, or 501.7 feet) is on A, and the linerepresenting 6.2 feet (elevation of B is 500.0 + 1.7, or506.2 feet) is on B. You can see how you can then locatethe l-foot contours between A and B.Figure 8-33.—Graphic method of interpolating contour lines.For a steeper slope, the contour lines would becloser together. If the contour lines were too close, youmight find it advisable to give the numbers on thegraphic sheet different values, as indicated by thenumerals in the right-hand margin. Here the spacebetween each pair of lines represents not 0.1 foot, but0.2 foot. Points A´ and B´ have the same elevations aspoints A and B, but the fact that the horizontal distancebetween them is much shorter shows that the slopebetween them is much steeper. You can see how the1-foot contours between A´ and B´ can be located, usingthe line values shown in the right margin.A third method of rapid interpolation involves theuse of a rubber band, marked with the correct, equaldecimal intervals. The band is stretched tocorrect graduations on the points.GENERAL REQUIREMENTS FORTOPOGRAPHIC MAPSbring theThe scale and contour interval of a map that you arepreparing will be specified according to the purpose forwhich the map will be used. Obviously, a map that willbe used for rough design planning of a rural dirt roadwill be on a smaller scale and have a larger contourinterval than one to be used by builders to erect astructure on a small tract in a built-up area.The extent to which details must be shown may alsobe specified; if not, it is usually inferred from the8-23