The surveys for the construction of railroads, applicable also to canals, graded pikes, dikes, etc., are made in the following order.

1. The reconnoissance, to locate the route. The termini being agreed upon, sometimes several routes are examined, so that an approximate judgment can be formed in reference to the economy of construction and purchasing the right of way, the amount of stock taken. at different towns along the route, and the profits from local business.

2. The transit survey, to determine definitely the

middle line along the surface, after the route has been decided upon by the preliminary reconnoissance.

3. The section leveling, to determine the profile of the middle line along the surface.

4. The cross-section work, to determine the position and slopes of the sides, so that the amount of earth to be removed or filled can be estimated.

345. Section Leveling.

Section leveling is simply an application, with slight modifications, of leveling for section, before described.

The first bench-mark is assumed at some convenient point near the beginning of the line, and its location described in the column of remarks.

The datum line is generally assumed at such a depth below the first bench-mark-for example, at mean high-tide water, in case one end of the route is in the vicinity of tide-water- that its whole length shall be below the section line at the surface.

The engineer's chain, 100 feet in length, is usually employed in taking the horizontal distance.

A turning-point is a hard point chosen as far in advance as possible, but not necessarily in exact line,. upon which the rod rests while a careful reading is taken just before it is necessary to change the position of the instrument, whose exact height above the datum line thus becomes known in the new position.

The difference between a turning-point and a bench is this:

A turning-point is merely a temporary point, neither marked nor recorded, used to determine the height of

the instrument in a new position. A bench is both marked and noted, and thus made permanent.

If, however, it is thought best to make a turningpoint permanent, it is marked and recorded, and becomes a bench.

In order that a bench be not destroyed in constructing the road, it should be a little removed from the line surveyed. The location of the benches should be carefully noted, so that they may be readily found from the field notes.

The plus sights are the first readings of the rod, made after each new position of the instrument, as the rod stands on a bench or turning-point, and are taken to thousandths of a foot.

The minus sights are the other readings, and are taken to tenths, except the last minus sight, before the position of the instrument is changed, which, being taken as the rod stands on a turning-point or bench, is taken to thousandths.

The height of the instrument above the datum line is equal to a plus sight, plus the height of the corresponding bench or turning-point.

The height of the surface above the datum line, at any position of the rod, is equal to the height of the instrument, minus the corresponding backsight.

These heights are taken at intervals of 1 chain, and at intermediate points where the irregularity of the surface is deemed sufficient to render it important.

In the following field notes D. denotes distance; B., bench; T. P., turning-point; S., plus sight; H. I., height of instrument; S, minus sight; S. H., surface height; G. H., grade height; C., cut; F., fill; R., remarks.

S. N. 30.

-

The numbers in the horizontal column, T. P., are found thus: The S., 11.399, is obtained from the first position of the instrument by the reading of the rod on T. P. 21.512 32.911 - 11.399. The + S., 2.243, is

=

the reading of the rod from the new position of the instrument. 23.775 = 21.512 +2.243. The cutting or filling is the difference of S. H. and G. H.

347. Profile of Section and Grade.

5-6

11.5

348. Remarks.

1. The grade height at 0, minus the grade at 6, which

2. The establishment of the grade is influenced by the object of the work, economy, the balance of cuttings and fillings, the points desirable for termini, etc.

3. The method exhibited above may be extended to any distance.

349. Example.

Fill out the notes of the following table, and make a profile of section and grade from S. H. at 0 to S. H. at 5.

Excavations and embankments are constructed with sloping sides, in order to prevent the sliding of earth down the surface.

The ratio of slope is the vertical distance divided by the horizontal, and is therefore the tangent of the angle which the sloping surface makes with a horizontal plane.

The usual ratio of slope is 3, and the angle 33° 41'.