The essential data for showing relief by contour lines consists of the elevation of a sufficient number of ground points in the area. Methods of determining the

horizontal and vertical locations of these ground points are called ground point systems. The systems most frequently used are (1) tracing contours, (2) grids, (3) control points, and (4) cross profiles. In practice, combinations of these methods may be used in one survey.

In the tracing contours system, the ground points located are points on the actual contours. Points on a given contour are plotted on the map, and the contour line is drawn through the plotted points. The method may be illustrated by the following simple example. Refer again to the traverse shown in figure 8-13. In this figure, assume that the traverse runs around the perimeter of a small field. The elevations at comers A, B, C, and D are as shown. Obviously the ground slopes downward from AB toward DC and from AD toward BC.

You want to locate contours at a contour interval of 1 fret; that is, you want to plot the 112-foot contour line, the 110-contour line, the 110-foot contour line, and so forth. In this example, we will assume that the required order of precision is low, such as you may encounter in a reconnaissance survey, and because of this you are using a hand level.

You stand at station A with a hand level. The elevation of this station is 112.5 feet. Assume that the vertical distance from your eye level to the ground is 5.7 feet. Then with the hand level at your eye and with you standing on station A, the HI is

If a level rod is set up anywhere on the 112.0-foot contour, the reading you would get from station A would be

Therefore, to determine the point where the 112.0 foot contour crosses AB, you only need to have the rodman back out from point A along AB until he comes to the point where you read 6.2 feet on the rod. You can determine the point where the 112.0-foot contour crosses AD in the same reamer as AB. You can measure the distance from A to each point and then record the distance from A to the 112.0-foot contour on AB and AD. When all of the contours have been located on AB and AD, you can shift to station C and carry out the same procedure to locate the contours along BC and CD. You have now located all the points where contours at a l-foot interval intersect the traverse lines. If the slope of the ground is uniform (as it is presumed to be in fig. 8-13), you can plot the contour lines by simply drawing lines between points of equal elevation, as shown in that figure. If there were irregularities in the slope, you would send the rodman out along one or more lines laid across the irregular ground, locating the contours on these lines as you located them on the traverse lines.