SECTION IV.

DEFINITIONS.

54. Surveying comprises all the operations necessary for finding the lengths and directions of the bounding lines of any portion of the earth's surface, the area of such portion, and for making on paper an accurate delineation, or map, of the surface and the boundaries.

55. Plane Surveying is that branch of surveying in which the curvature of the earth is neglected, as it may be when the survey is limited to small portions of the surface.

56. Geodesy, or Geodetic Surveying, is that branch in which the curvature of the earth is taken into account, as it must be in all extensive surveys.

57. A Horizontal Plane at any point is a plane perpendicular to the radius of the earth drawn to that point.

58. A Vertical Plane is a plane perpendicular to a horizontal plane.

59. A Horizontal Line is any line of a horizontal plane.

60. A Vertical Line is a line perpendicular to a horizontal plane.

61. An Oblique Line is a line inclined; i. e., neither parallel nor perpendicular to a horizontal plane.

Thus (Fig. 24), AB and DC

are horizontal lines; BC and

D

FIG. 24.

B

AD are vertical lines; and AC and BD are oblique lines.

62. The Horizontal Distance between two points is the horizontal line intercepted between the two vertical lines passing through those points. Thus, DC or AB (Fig. 24), is the horizontal distance between the two points A and C, or between. the points B and D.

63. A Horizontal Angle is an angle whose sides are horizontal; the plane of its sides is also horizontal.

64. A Vertical Angle is an angle the plane of whose sides is vertical.

65. An Angle of Elevation is a vertical angle having one of its sides horizontal and the other oblique, the oblique side being above the horizontal side. Thus, BAC (Fig. 24) is the angle of elevation from A to C.

66. An Angle of Depression is a vertical angle having one of its sides horizontal and the other oblique, the oblique side being below the horizontal side. Thus, DCA (Fig. 24) is the angle of depression from C to A.

67. An Oblique Angle is an angle the plane of whose sides is inclined to a horizontal plane.

BOOK II.

CHAIN

SURVEYING.

SECTION I.

MEASUREMENT OF DISTANCES.

68. Any tape, rod, or chain, divided into equal parts, may be used as a measure for finding the distance between two points.

The measure in general use for land surveying is a chain of four rods, or sixty-six feet in length, called Gunter's chain (Fig. 25), from the name of the inventor. It is composed of 100 links, each joined to the other by two or three rings. Every tenth link from either end, is marked by a small attached brass pendant

or tag, which is notched to designate its number from the end. The tag at the middle, or fifty-link, point is distinguished by being rounded, or by some other peculiarity of make. As the tags at equal distances from the two ends of the chain are marked the same, care must be taken not to mistake forty links for sixty, &c., and the reverse. To avoid such error, it would be better to have the tags marked in regular order from the beginning to the end of the chain, rather than from both ends to the middle.

A link in measure includes a bar, with its connecting ring at each end; when there are three connecting rings, a ring and a half at each end is included.

The handles are of brass, and each forms part of the end link, to which it is connected by a nut, by which also the length of the chain is adjusted.

To determine whether to measure from the inside of the brass handle, or from the outside, double back the last two or three links upon the preceding links and compare.

The division of the chain into 100 equal parts is very convenient, since the divisions, or links, are decimals of the whole chain, and in the calculations are treated as such.

An excellent chain for accurate measurements is Grumman's patent "suspended chain," which is made of very light steel wire, is fitted with spring-balance, thermometer and spiritlevel attachments, and is held above the surface when in use, the ends of the chain being marked upon the ground by the points of plummets let fall from the end notches.

Instead of a chain, which is liable to error because of the bars and rings becoming worn by frequent contacts, a steel ribbon or tape is often used.

69. Besides the chain or tape, the surveyor needs ten (or better, eleven) marking pins (Fig. 26), made of iron or steel wire, about an eighth of an inch in thickness and a foot long, sharpened at one end and bent into a ring at the other, for marking chain lengths on the ground; a plumbob (Fig. 27) and line for referring, when necessary, points in the chain. held horizontally to the inclined surface of the ground; and a set of flag-poles, or ranging rods, for marking stations and ranging out lines. The marking pins should be strung upon an iron ring with a spring-catch, and this ring should be attached to a strap to be passed over the right shoulder suspending the pins at the left side; or, better, the pins may be carried in a leather quiver strapped to the waist.

FIG. 26.

FIG. 27.

The pins should be tagged with white cloth to enable the surveyor to find them again readily, when they have been left to mark a point.

70. To Measure a Horizontal Line. The point where the measurement is to begin is located by a staff temporarily placed for the purpose, or by some one of the many permanent marks by which the angular points in a boundary are fixed.

The other extremity of the line must be provided with a staff or flag which can be easily seen.

Two chainmen are required, a fore-chainman, or leader, and