To set out the base line and Perpendicular lines / Offsets in the field.

Object: To set out a base line and perpendicular lines/offsets in the field.

Theory:

Lateral measurements to chain lines for locating ground features are known as offsets. For measuring offsets tapes are commonly used. Offset which can be judged by naked eye or offset less than 15m is called short offset and offset greater than 15m is called long offset. Most commonly short offsets are preferred.

Offset may also be classified as perpendicular offsets and oblique offsets. The offsets which are taken perpendicular to the chain line are termed as perpendicular offsets. All the offsets which are not taken at the right angle to chain line are known as oblique offsets.

There are three methods to measure the perpendicular offsets:

1. By optical square
2. 3-4-5 method
3. Swinging the tape

1. BY OPTICAL SQUARE

Instruments required:

1. Optical square
2. 4 Rods
3. Plumb bob
4. Arrows

Theory:

Optical squares are simple sighting instruments used to set out right angles. They can be provided either with mirrors or with one or two prisms. Because of practical difficulties in using squares with mirrors, they have been replaced by squares with prisms: “prismatic squares”. There are two major types of prismatic squares: single prismatic squares and double prismatic squares. In our practical, we used double prismatic square. The double prismatic square, also called double prism, has two prisms. The two prisms are placed in such a way that it is possible to look at the same time at a right angle to the left and to the right; in addition the observer can look straight ahead of the instrument through openings above and below the prisms. It is thus possible to see the base line and the perpendicular line at the same time.

Procedure:

• The base line is defined by setting stations A and B, distance between them being about 100 feet.
• Station D is set at some distance within the boundary of stations A and B.
• The observer holds the optical square vertically on the base line. This can be checked with the plumb bob. Looking through the instrument the observer moves slowly trying to find a position on the base line. The instrument is slowly rotated until the image of rod A is in line with the image of rod B. The instrument is then in line with stations A and B of the base line.
• The observer moves along the base line towards rods A or B. He stops when rod D can be seen through the instrument and forms one line with the images of rods A and B. He drops the plumb bob to the ground at that point.
• Arrow is fixed under the plumb bob; this being the station C. Station C and station D form the line perpendicular to the base line i.e. perpendicular offset of base line AB.

Observation in the field:

1. 3-4-5 METHOD

Instruments required:

1. Measuring Tape
2. 2 ranging rods
3. Arrows

Procedure:

• The base line is defined by setting stations A and B.
• The first person holds together, between thumb and finger, the zero mark of the tape on the base line.
• The second person holds between thumb and finger the 3 meter mark of the tape on the base line and places an arrow; the point being station C.
• The third person holds the 7 meter mark on the ground and places an arrow; the point being station D.
• When all sides of the tape are stretched, the angle between the line connecting stations C and D and the base line is a right angle and a triangle is formed with lengths of 3 m, 4 m and 5 m.

NOTE: Instead of 3 m, 4 m and 5 m a multiple can be chosen: e.g. 6 m, 8 m and 10 m or e.g. 9 m, 12 m and 15 m.

Observation in the field:

https://youtu.be/wx_CNT7kuJI

Results (3-4-5 Method):

1. SWINGING THE TAPE

Instruments required:

1. Measuring Tape
2. 2 ranging rods
3. Arrows

Procedure:

• The base line is defined by two stations.
• One end of the tape is placed around the arrow at point ‘A’ established. A certain length of the tape is stretched to the base line and an arrow is placed at that point; this being point ‘B’.
• The length of the tape established at ‘B’ is moved in an arc of circle on the other side of the base line while keeping the tape straight.
• Another arrow is placed at the point where the length of the tape intersects the base line while moving it in the arc; this being point ‘C’.
• The distance BC is measured with the tape and the midpoint ‘D’ is established which is taken as the foot of the offset.
• The line AD i.e. the offset is perpendicular to the base line. This is checked by putting the zero of the tape at ‘A’ and stretching it at different points on the base line. The point ‘D’ must give the shortest measurement.

Observation in the field:

Results (Swinging the Tape):

Verification by Pythagoras’ Theorem:

AB^2 = CD^2 + AD^2

AD = √ (AB^2 – CD^2)

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