Another method of deciding visibility is by proportion. Measure the distance between the three points A, B, and C, and obtain their elevations above the datum (lowest of the 3) and using similar triangles. Take the same case as above, letting X represent the point above which the view is clear at 1,260 yards from point A, the line of sight passes through this point.
1760 (A—B) : 500 (B—C) :: 50 (elev. A) : X
solving, X = 14.2
Now, since the ground at point C is 25 feet above the base and the line of sight passes within 14.2 feet of the base at this place, an observer at A is unable to see B.
The matter of profiling is very simple. Merely mark where the contours cut the edge of a piece of co-ordinate paper and extend the proper elevations, then pass a line through these points, remembering that the surface of the ground has a natural curve.
Lesson 6. (Field Work—five Hours.)
Problem.—By use of the slope scale on the sketch board and the contour interval scale on the alidade, each man will secure vertical data on the flat sketch made in the fourth lesson. Certain critical elevations will be determined and marked with red flags before hand. The elevations of two points on the ground will be furnished, one as the datum and the other as a check. Draw in contours of this sketch with the help of drainage lines and elevations already secured.
The chief points to be considered are to take slopes from points established on the sketch; to take several sights and average the angle of slope; to properly lay off the elevation by using the slope scale on the alidade; and finally to put in the contours along these lines of sight on the spot thus allowing for difference in topography between the point of sight and the station from which the elevation is taken. Careful note must be made of the drainage systems as these are the keynotes to the sketch and finally the contours are connected together, keeping in mind always that no contour stops unless it makes a closed curve or goes off the map. Remember also that contours make fingers pointing up stream and are blunt around hill sides. Contours cross streams to opposite points and break at roads, continuing on the other side. Uniform slopes have equally-spaced contours. Do not try to measure every slope, two intersecting elevation sights on a hill will check the height. Put the intervening contours in by eye.
Lesson 7. (Classroom—four Hours.)
Problem.—Completing the map sketch previously made and making a landscape sketch.