Measurement of angles

UNIT – II

Chapter - 3 (B)

MEASUREMENT OF ANGLES

To plot a survey line on paper, its length and direction must be known. The direction of a survey line may be defined either (i) by the horizontal angle between the line adjacent to it, or (ii) by the angle called the Meridian and the line. Meridian is the horizontal angle measured between a reference line and a survey line. It is denoted in degrees.

In the field work, angles are measured in degrees (^o), minutes (') and seconds ("). 1 rotation = 360°; 1° = 60'; and l' = 60”.

TRAVERSE SURVEYING or TRAVERSING

Two methods of surveying are: Triangulation and Traversing. Triangulation Method is used in chain surveying, when the area is small and flat. For large areas, Traversing is adopted.

Traverse Surveying is a survey consisting of a set of connected lines whose Lengths and Directions are measured. The area to be surveyed is connected by a number of lines forming the figure other than triangle. The lengths of survey lines are measured using chain or tape. The directions of survey lines are measured using angular measuring instruments such as Prismatic Compass, Surveyor's Compass and Theodolite and fixed by measuring the Bearings. The details are filled by taking offsets as in the case of chain surveying.

Traverse surveying is similar to chain surveying except the following:

• The bearings of survey lines are observed to fix their directions.

• The framework formed by survey lines need not be divided into number of triangles.

1. TYPES OF TRAVERSE SURVEYING

1. Closed Traverse: See Fig. 20(a).

A traverse is said to be closed, if the survey lines close at the starting point forming a closed polygon.

Closed traverse is suitable for surveying, the boundaries of large area of land such as a pond, lake, a village layout, forest, etc.

Wherever possible, all traverses should be (b) closed. If not closed, there can be no rigid check on the accuracy of the work.

Also, since the lines form a closed polygon, we can check the sum of the interior angles.

2. Open Traverse: See Fig. 20(b). A traverse is said to be open, if the survey lines do not close at the starting point to form a closed polygon. This traverse consists of series of lines running in the same direction. Open traverse is suitable for long narrow strip of works like highway roads, rivers and for laying water pipe lines, etc.

2.  METHODS OF MEASUREMENT OF ANGLES

Angular Measurement: The direction of survey lines in a traverse surveying are the measurement of an angle from the preceding survey line or from any reference line called Meridian. This type of measurements is called Angular Measurement.

Horizontal Angles are measured in different ways. In a closed traverse along the perimeter of an area of land, interior angles are measured. In open traverse, angles are measured in the following two ways:

1. Deflection Angle Method

Deflection Angle is the angle that any line makes with the extension of the preceding line. Such angles should be clearly identified as measured to the right (clockwise) or to the left (anti-clockwise) from the extension of the preceding line. ϕ₁, ϕ₂ and ϕ₃ are the deflection angles between successive lines. See Fig. 21.

2. Included Angle Method

Fig. 22 shows the included angles θ₁, θ₂ and θ₃  measured clockwise from the back line to a forward line.

Instruments used for the measurement of angles are: Compass, Theodolite and Box Sextant.