verticalknownFigure 8-12.-Methods of indicating relief.distance between adjacent contour lines isas the contour interval. Starting at zeroelevation the topographer draws every fifth contour linewith a heavier line. These are known as index contours.At some place along each index contour, the line isbroken and its elevation is given. The contour linesfalling between index contours are called intermediatecontours. They are drawn with a finer line than theindex contours and, usually, do not have their elevationsgiven. Examples of index contours and intermediatecontours are shown in figure 8-13.GROUND POINT SYSTEMSThe essential data for showing relief by contourlines consists of the elevation of a sufficient number ofground points in the area. Methods of determining theFigure 8-13.—Traverse with contour lines.horizontal and vertical locations of these ground pointsare called ground point systems. The systems mostfrequently used are (1) tracing contours, (2) grids, (3)control points, and (4) cross profiles. In practice,combinations of these methods may be used in onesurvey.Tracing ContoursIn the tracing contours system, the ground pointslocated are points on the actual contours. Points on agiven contour are plotted on the map, and the contourline is drawn through the plotted points. The methodmay be illustrated by the following simple example.Refer again to the traverse shown in figure 8-13. Inthis figure, assume that the traverse runs around theperimeter of a small field. The elevations at comers A,B, C, and D are as shown. Obviously the ground slopesdownward from AB toward DC and from AD towardBC.You want to locate contours at a contour interval of1 fret; that is, you want to plot the 112-foot contour line,the 110-contour line, the 110-foot contour line, and soforth. In this example, we will assume that the requiredorder of precision is low, such as you may encounter ina reconnaissance survey, and because of this you areusing a hand level.You stand at station A with a hand level. Theelevation of this station is 112.5 feet. Assume that the8-13