PLACING THE LINE.
After the preliminary examination of the locality is complete and the positions and elevations of the con trolling points of the line are known with reference to each other, the line must, be selected and run in upon the ground, or, if the reconnaissance is not conclusive as to the position of the best line, it is advisable to run in two or more lines and make a more detailed comparison between them.
The controlling points of a line are those points at which the position of the road is restricted within narrow limits, and is not subject to change. These may be points where the location is governed by the necessity of providing for traffic, or points where the position of the line is restricted by topographical con siderations, such as a summit over which the line is to pass a ridge or a favorable location for a bridge.
Where the line is to be located to a uniform gradi ent, it should be started from the controlling point at the end of the grade, which is usually the summit. It is then laid off along the slope in such manner as to cause it to have continuously the rate of grade decided upon. Taking D (Fig. II) at the summit of the valley as the controlling point, it is seen that the distance from C to D is sufficient to give a gradient of Ito in zoo by following directly down the valley, and the line with that gradient may be run in that manner.
The maximum gradient from A to C is, however, only 5 in zoo, and if thought advisable the same maxi mum gradient may be used between C and D by run ning the line DHC diagonally down the slope, as shown. This line, having one-half the gradient, will have about twice the length of the line CD.
In running this line it is started from the highest point of maximum grade, and points at the surface of the ground are continually selected, in advance of the placing of the line, which are at the proper elevation to permit the grade to pass through them. This may be accomplished by setting off the angle of the gradi ent upon the vertical circle of the transit, or upon a gradienter, and sighting upon a rod which is moved until the line of sight strikes it at the same height from the ground that the instrument is setting. The points
for the line may also be found by running a line of levels ahead of the transit line (a hand level is conven ient for this purpose) and pacing distances upon which to reckon the gradient, the distances and elevations being frequently checked upon those of the measured line.
The location of a gradient upon a common road differs from that upon a railroad only in that steeper gradients are used, sharper curves or angles may be employed, and the gradients need not be lessened on ordinary bends or curves. If the line is to make a turn upon the slope as at H, the grade should be flattened at the turn, and a curve of as large radius as possible, without too great expense for grading, be introduced.
In a manner similar to the above a line might be run from D on the other side of the valley, which using a 5 per cent gradient would give the line DML, reaching the bed of the valley at the point L. A lighter gradient may be obtained from A to D by starting from D and going down by a continuous gradient of 4 in I(x) on the line DFGA, and greater or less rates of descent may be adopted and lines corresponding to them located, as may be considered advisable.
The center-line for a final location should be care fully run, and points permanently marked from which it may be relocated when necessary. An accurate line of levels should also be run over the centerline and a profile drawn, upon which the grades may be estab lished and earthwork estimated.
After placing the center-line, topography should be taken carefully upon each side of the line for some distance, and a map drawn showing the topography and giving elevations by means of contours. This will serve to show whether the line is placed to the best advantage, and whether any changes are desirable. This is especially necessary over rough ground or where the line is on maximum gradient, as frequently, and perhaps usually, the first line run will be useful only as a preliminary line, which with its accompany ing topography will permit a proper location to be made.