Agriculture Engineering 101Theory Class 7.pptx

Agriculture Engineering

1. 1 7. OFFSETS – PERPENDICULAR AND OBLIQUE OFFSET. INSTRUMENTS USED FOR SETTING OUT RIGHT ANGLES – OPEN CROSS STAFF, FRENCH CROSS STAFF, ADJUSTABLE CROSS STAFF AND OPTICAL SQUARE .

2. 2 Offsets Offsets are lateral distances measured from the survey lines to the objects or features which are to be plotted. They can be on either side in the survey line. There are two types of Offsets 1. Perpendicular offsets 2. Oblique offsets

3. 25 November 08 Unit 2 :Chain Surveying 2.4 Locating the ground features Offsets- lateral distance of an object from survey line. Offsets should be as small as possible Types Perpendicular – less measuring on ground Oblique Method of ties P Q A C P B 90 B A P C ≠ 90

4. 25 November 08 Unit 2 :Chain Surveying 2.4 Locating the ground features Offsets are taken in the order of their chaninage. Number of offsets depends upon the shape of objects Straight wall, zigzag wall, road with constant width Constant width road Boundary wall Contd….

5. 5 Perpendicular offsets The measurements are taken at right angle to the survey line called perpendicular or right angled offsets. Oblique offsets The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.

6. Setting the Perpendicular by the Chain and Tape 6 To erect a perpendicular to a chain line from a point on it Triangle , whose sides are in proportion of 3,4 and 5 2. To drop a perpendicular to a given chain line froma point outside

7. 7

8. 25 November 08 Unit 2 :Chain Surveying 2.4 Locating the ground features Swing of the tape on chain line Shortest distance is perpendicular offset P Contd….

9. 9 Instruments for Setting out Right Angles Cross staff: The cross staffs are used for a) Finding out foot of the perpendicular from a given point to a line. b) Setting right angle at a given point on a line

10. 10 There are three forms of cross staff 1 . Open cross staff 2. French cross staff 3. Adjustable cross staff 4. Optical Square 5. Prism Square

11. 11 1. Open Cross Staff It is a simple. It consists of two parts 1 ) The head. 2) The leg. The common type of cross staff consists of 4 metal arms with vertical slits for sighting.

12. 12 The head is fixed to the top of an iron stand about 1.2 to 1.5 m long. this is driven in to the ground. For setting perpendicular lines, one pair of opposite arms is aligned with the chain line. It is specially used for setting off and marking contour lines.

13. 13 To find the foot of perpendicular from a given point to a given lines. To take offset, the cross staff is held vertically on chain where the offset is likely to occur, and turn until one pair of opposite slits directed to a ranging rod at the forward end of the chain line, the offset is taken is to be bisected.

14. 14 In setting out a right angle at point on the chain line: The cross staff is held vertically over the given point on the chain and turn until the ranging road at either end of the chain line is bisected the line of slight through 1 pair of the opposite slits. Then the line through the other pair of slits which is at right angles to the chain line through the other pair of slits which is at right angles to the chain line.

15. 15 2. French Cross Staff The octagonal form cross staff is the French cross staff. It consists of an octagonal brass tube with slits on all eight sides.

16. 16 It has an alternate vertical slit and an opposite vertical window with a vertical horse hair on each of the four sides. These are used for setting right angles. The base carries a socket so that it may be fitted on the pointed staff when the instrument is to be used.

17. 17 3. Adjustable Cross Staff It consists of a brass cylindrical tube about 8 cm in diameter and 1 cm in deep and is divided in the centre. The upper cylinder can be rotated relatively to the lower one by a circular rack. Both are provided with sighting slits.

18. 18 The lower part is graduated to degrees and sub divisions, while the upper one carries a vernier . Thus it may be use for setting out angles of any magnitude. It has a magnetic compass at the top, which may serve to take the bearing of the line.

19. 19 4. Optical Square An optical square is an compact hand instrument used in setting out right angles with greater accuracy than a cross staff. It consists of a circular box about 5 cm in diameter and 1.25 cm deep in which two mirrors are fitted at right angles to the plane of instruments .

20. 20 The mirror ‘h’ called the horizontal glass which is half silvered and half uncovered. The mirror ‘I’ is known as index glass and wholly silvered. The surveyor simply turns the optical square upside down which throws the aperture for the object on that side.

21. 21 5. Prism Square It is a modern instrument and is a very use full for setting out right angles. It is based on the same principles as the optical square and is used in same manner. It requires no adjustment, since the angle between reflecting surface of the prism (45 degree) is fixed.

22. 22 It is unaffected by dust & can be used in poor light. For taking offset to an object the observer holds the instrument in his hand & sights directly over the prism at ranging of station.

23. 23 Cross Staff Survey   This type of survey is undertaken to locate boundaries of a field and determinations of its area. Principle: The principle of the survey is to divide the given area in to number of right angled triangles and trapezoids and to calculate and plot the areas of triangles and trapezoids. . Instruments/Material Required: Two Chains, arrows taps, ranging rods, cross-staff and drawing materials.

24. 24 Procedure: Two chains are usually provided, one for measuring distance along the chain line and other for measuring the offsets. The cross staff is used to set out the perpendicular directions for offsets. In this survey, the base line runs through the center of the area, so that the offsets are left or right side of base line are fairly equal.

25. 25 To check accuracy, length of the boundary lines may also be measured. After the field work is over, the survey is plotted to a suitable scale.

26. Calculations of Field Area By this method of survey, the field as divided in to right angled triangles and trapezoids are calculated as under: 1 . Area of right-angled triangle = ½ base x Height. 2. Area of trapezoid = sum of parallel sides/2 x Height. Add the areas of all the triangles & trapezoids and sum is equal to the total area of a field. The computations for area should be written in a tabular from as given below 26

27. Cross staff surveying

28. Area is divided into triangles, rectangles, squares or trapeziums Area of the one figure (e.g. triangles, rectangles, squares or trapeziums) is calculated and multiplied by total number of figures. Area along the boundaries is calculated as Total area of the filed=area of geometrical figure + boundary areas

29. Problem 1-Result

30. 30 S.No . Figure Chainage (m) Base (m) Offset (m) Mean Offset (m) Aera (m 2 ) 1 2 3 4 5 6 7 Total Add all the areas of figures m 2 TABLE

31. Questions

32. Thank You