Perpendicular Offset

1. Offsets
• Lateral measurements to chain lines for
locating ground features are known as offsets.
For this purpose
• perpendicular or oblique offsets may be taken .
If the object to be located (say road) is curved
more number of offsets should be taken. For
measuring offsets tapes are commonly used.

2. Offsets
Perpendicular Offsets
• The offsets which are taken perpendicular to
the chain line are termed as perpendicular
offsets. These offsets are taken by holding zero
end of the tape at the object and swinging the
tape on the chain line. The shortest distance
measured from object to the chain line is
usually the perpendicular offset.

3. Perpendicular Offsets

4. Offsets
Oblique Offset
• Oblique distance is always greater than
perpendicular distance. All the offsets which
are not taken at the right angle to chain line are
known as oblique offsets.

5. Oblique Offset

6. Offsets

7. Offsets
• For setting perpendicular offsets any one of the
following methods are used:
• (i) Swinging
• (ii) Using cross staffs
• (iii) Using optical or prism square.

8. Perpendicular Offset by
Swinging
• Chain is stretched along the survey line. An
assistant holds the end of tape on the object.
Surveyor swings the tape on chain line and
selects the point on chain where offset distance
is the least.
• and notes chain reading as well as offset
reading in a field book on a neat sketch of the
object

9. Perpendicular Offset by
Swinging

10. Perpendicular Offset by
Swinging

11. Perpendicular Offset by
Swinging

12. Perpendicular Offset by
Swinging

13. Perpendicular Offsets Using Cross
Staffs
• Three different types of cross staffs used for setting
perpendicular offsets. All cross staffs are having two
perpendicular lines of sights. The cross staffs are
mounted on stand. First line of sight is set along the
chain line and without disturbing setting right angle line
of sight is checked to locate the object.
• With open cross staff (Fig(a)) it is possible to set
perpendicular only, while with French cross staff
• (Fig(b)), even 45º angle can be set. Adjustable cross
staff can be used to set any angle also, since there are
graduations and upper drum can be rotated over lower
drum.

14. Perpendicular Offsets Using Cross
Staffs

15. Perpendicular Offsets Using Cross
Staffs

16. Perpendicular Offsets Using Optical
Square and Prism Square
• These instruments are based on the optical
principle that if two mirrors are at angle ‘θ’ to
each other,
• they reflect a ray at angle ‘2θ’. Figure shows a
typical optical square.

17. Perpendicular Offsets Using Optical
Square and Prism Square

18. Optical Square and Prism
Square

19. Optical Square and Prism
Square

20. Perpendicular Offsets Using Optical
Square and Prism Square

21. Perpendicular Offsets Using Optical
Square and Prism Square

22. Perpendicular Offsets Using Optical
Square and Prism Square

23. Perpendicular Offsets Using Optical
Square and Prism Square

24. Perpendicular Offsets Using Optical
Square and Prism Square

25. Perpendicular Offsets Using Optical
Square and Prism Square

26. 3-4-5 Method
• AB is the chain line it is required to erect a
perpendicular at C of the chain line. Establish a pt
D at 3 m distance. 5m & 6m marks are brought to
gather to form a loop .The tape is stretched tight
by fastening the end D and C. The point D is
established such that DE = 5 m and CE= 4 m and
CD = 3 m and DE2 = CD2 + DE2
• i.e. 5 2 = 3 2 + 4 2 = 25 thus Angle DCE = 90 0
• i.e. CE is perpendicular to chain line at C

27. 3-4-5 Method

28. 3-4-5 method

29. 3-4-5 method