EXAMINATION OF COUNTRY.
For the purpose of obtaining the requisite data upon which to base the location of a road, it is neces sary that a careful examination be made of the topo graphical features of the country through which the line is to pass. The relative elevations of the termini of the line and of intermediate points should be obtained, and the directions and steepnesses of the various natural slopes determined.
If a line were to be located connecting points at long distances from each other, as sometimes occurs in railway location, it would be necessary to study the general configuration of the country, noticing the direction of flow of the streams, and the location and elevations of the various passes in the ridges through which it might be possible to carry the line. Usually it would be found that the country is composed of a series of valleys, separated by ridges, branching in a systematic manner from the main watercourse of the region, and that the passes in the ridges occur at the head of side streams, and especially where streams flowing into valleys on opposite sides of the ridge have their sources near each other.
In the location of common roads, however, the prob lem is ordinarily of a less extended nature, and may consist in joining two points lying in the same valley, or in joining points in adjacent valleys by a line pass ing over a ridge. In these cases it is only necessary to take into account the slope of the valleys in question, the positions and elevations of available passes, and the side slope of the ridges.
The slope of the bed of a valley, in hilly country, usually forms a concave curve, the rate of slope gradu ally increasing from the lower to the upper end. In a valley of considerable length this increase in the rate of slope may be very gradual or in short valleys rising to a considerable height it may be more sudden. The profile ABCD in Fig. i i shows the slope of a short valley which decreases in slope from about ten feet per hundred at the upper end to about two feet per hundred at the lower end.
When a map of the country to be traversed is avail able, showing the positions and elevations of the points controlling the location, the work is very much simpli fied, the reconnaissance may for the most part be limited to a study of the map, and the routes may be sketched upon the map to be tried in the field. If the map at hand is an accurate contour map on a sufficiently large scale, the entire location may be worked out in detail upon the map, leaving only the work of staking out the line to be clone upon the ground.
Maps may be obtained, in most parts of this country, upon which the horizontal positions of points may be readily fixed with sufficient accuracy for the purposes of the preliminary examination. Where such maps are not obtainable, the positions of points must be ascer tained and a rough map prepared. For this purpose directions may be measured with a pocket compass, and distances estimated or obtained by the use of an odometer or pedometer, as may be most convenient.
Differences of elevation are easily obtained with a fair degree of accuracy by the use of an aneroid barometer, and slopes may be measured with a hand level, Where the rough means ordinarily employed in the reconnaissance are not sufficiently accurate to deter mine the controlling points of the lines to be adopted, a more complete examination of the country may often be made by a rapid topographical survey by means of the transit and stadia method.
Whatever means may be adopted for doing the work, the preliminary examination should determine a map showing the approximate positions of the con trolling points through which the road must pass, and enable a rough sketch to be made of the slopes of the country through which the line is to be run.