RAILWAY SURVEYS. The surveying operations requisite to and preceding the construction of a railway are in general a reconnaissance, a pre liminary survey, and a locating survey or loca tion. The reconnaissance is a general and some what hasty examination of the country through which the proposed road is to pass for the pur pose of noting its more prominent features and acquiring a general knowledge of its topography with reference to the selection of a suitable route. A preliminary survey consists of an in strumental examination of the country along the several available routes for the purpose of ob taining such details of distances, elevations, and topography as may be necessary to prepare a map and profile of each, make an approximate estimate of the cost of the road, and furnish the data from which definitely to locate the line. The locating survey consists specifically in estab lishing the centre line of the road on the ground in the position which it is finally to occupy. De fined more broadly, the location consists first in choosing the best route from the several which are available, and second in selecting for the chosen route the best combination of grades and curves. In determining the best combination of grades and curves for the route chosen the engi neer has to take into account both the cost of construction and the cost of operation. On one hand he has the annual interest upon the origi nal cost, and on the other the annual expense of operating the road. But the best combination of grades and curves is that which will render the sum of these two a minimum. To select the best line from several available lines, the engineer de termines the most economical combinations of grades and curves for each one, calculates the in terest on the entire cost of constructing the line with this combination and also the annual ex pense of operating the line, and takes the sum of the two amounts. That route is best in re spect to which this sum is the least. In this last statement it is assumed that so far as their ability to command traffic is concerned all of the available routes are on a parity. This is not always the case. Sometimes one route is su perior to any of the others in its ability to com mand traffic while being inferior in its ability to present the most economical combination of grades and curves. In such a case it often be comes the duty of the engineer to select the more expensive route for the sake of securing the greater amount of available traffic. It plain
upon very little thought that to answer each of these broad general questions a multitude of minor factors have to be carefully integrated, and that altogether the location of a railway is a task which, if it be well performed, calls for skill, experience, and good judgment on the part of the engineer. Beyond the statement of this fact it is impossible to proceed within the limits of the space available in this article, but the reader who wishes to study the problem of rail way location in detail will find it presented at great length in Wellington, Economic Theory of . Railway Location (New York, 1000).
When the engineer has his route and has selected a combination of grades and curves for this route his next task is to establish its centre line on the ground with all the grades and curves properly indicated. In plan the centre line consists of a combination of straight lines or tangents and of curves. The curves may be simple curves, that is, plain circular curves; or compound. that is. consisting of two or more circular arcs of different radii; or reverse curves, that is, two simple curves so joined as to form a curve like a flat letter S. Curves are further designated by their degrees of curvature. The degree of a curve is determined by the angle at the centre subtended by a chord 100 feet long. For example, if on any curve a chord 100 feet long subtends an angle of 5° at the centre, that curve is known as a five-degree curve. lu profile the centre line is composed of a combination of level or horizontal lines and of inclined lines or grades ascending or descending from the hori zontal. Grades are designated either by stating the number of feet of rise or fall in a horizontal length of line of 100 feet or of one mile. For example, a grade having a rise of one foot in a horizontal length of line of 100 feet is known as a 1 per cent. grade. The same grade defined in terms of feet rise per mile of length would be known as a grade ..of 52.8 feet per mile. When two grade lines meet or when a grade line and a level meet the junction is marked by an angle more or less abrupt. This angle is always re placed by a vertical curve which is convex up ward at a summit and concave at a valley.