Cross Levels. Wherever considered necessary levels at right angle to the center line should be taken. These will be found useful in showing what effect a deviation to the right or left of the surveyed line would have. Cross levels should be taken at the intersection of all roads and railroads to show to what extent, if any, these levels will have to be altered to suit the levels of the proposed road.
Profile. A profile is a longitudinal section of the route, made from the levels. Its horizontal scale should be the same as that of the map; the vertical scale should be such as will show with distinct ness the inequalities of the ground.' Fig. 5 shows the manner in which a profile is drawn and the nature of the information to be given upon it.
. Bridge Sites. The question of choosing the site of bridges is an important one. If the selection is not restricted to a particular point, the river should be examined for a considerable distance above and below what would be the most convenient point for crossing; and if a better site is found, the line of the road must be made subordinate to it. If several practicable crossings exist, they must. be carefully compared in order to select the one most advantageous. The follow ing are controlling conditions: (1) Good character of the river bed, affording a firm foundation. If rock is present near the surface of the river bed, the foundation will be easy of execution and stability and economy will be insured. (2) Stability of river banks, thus securing a permanent concentration of the waters in the same bed. (3) The axis of the bridge should be at right angles to the direction of the current. (4) Bends in rivers are not suitable localities and should be avoided if possible. A straight reach above the bridge should be "secured if possible.
Final Selection. In making the final selection the following principles should be observed as far as practicable.
(a) To follow that route which affords the easiest grades. The easiest grade for a given road will depend on the kind of covering adopted for its surface.
(b) To connect the places by the shortest and most direct route commensurate with easy grades.
(c) To avoid all unnecessary ascents and descents. When a road is encumbered with useless ascents, the wasteful expenditure of power is considerable.
(d) To give the center line such a position, with reference to the natural surface of the ground, that the cost of construction shall be reduced to the smallest possible amount.
(e) To cross all obstacles (where structures are necessary) as nearly as possible at right angles. The post of skew structures increases nearly as the square of the secant of the obliquity.
(f) To cross ridges through the lowest pass which occurs.
(g) To cross either under or over railroads; for grade crossings mean danger to every user of the highway.
Examples of Cases to be Treated. In laying out the line of a road, there are three cases which may have to be treated, and each of these is exemplified in the contour map, Fig. 4. First, the two places to be connected, as the towns A and B on the plan, may be both situated in the same valley, and upon the same side of it; that is, they are not separated from each other by the main stream which drains the valley. This is the simplest case. Secondly, although both in the same valley, the two places may be on opposite sides of the valley, as at A and C, being separated by the main river. 'Thirdly, they may be situated in different valleys, separated by an intervening ridge of ground more or less elevated, as at A and D. In laying out an extensive line of road, it frequently happens that all these cases have to be dealt with.
The most perfect road is that of which the course is perfectly straight and the surface practically level; and, all other things being the same, the ')est road is that which answers nearest to this de scription.
Now, in the first case, that of the two towns situated on the same side of the main valley, there are two methods which may be pursued in forming a communication between them. A road follow ing the direct line between them, shown by the thick dotted line A B, may be made, or a line may be adopted which will gradually and equally incline from one town to another, supposing them to be at different levels; or, if they are on the same level, the line should keep at that level throughout its entire course, following all the sinuosities and curves which the irregular formation of the country may render necessary for the fulfillment of these conditions. According to the first method, a level or uniformly inclined road might be made from one to the other; this line would cross all the valleys and streams which run down to the main river, thus necessitating deep cuttings, heavy embankments, and numerous bridges; or these expensive works might be avoided by following the sinuosities of the valley. When the sides of the main valley are pierced by numerous ravines with projecting spurs and ridges intervening, instead of following the sinuosities, it will be found better to make a nearly straight line cutting through the projecting points in such a way that the material excavated should be just sufficient to fill the hollows.