Introduction to Compass Traversing, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

1. INTRODCTION AND PURPOSE

In chain surveying, the area to be surveyed is divided into a number of triangles. This method is suitable for fairly level ground covering small areas. But when the area is large, undulating and crowded with many details, triangulation (which is the principle of chain survey) is not possible. In such an area, the method of traversing is adopted.

In traversing, the framework consists of a number of connected lines. The lengths are measured by chain or tape and the directions identified by angle measuring instruments. In one of the methods, the angle measuring instrument used is the compass. Hence, the process is known as compass traversing.

Note: Consideration of the traverse in an anticlockwise direction is always convenient in running the survey lines.

2. DEFINITIONS

1.True meridian  The line or plane passing through the geographical north pole, geographical south pole and any point  on the surface of the earth, is known as the ‘true meridian’ or ‘geographical merdian’. The true meridian at a station is constant. The true meridians passing through different points on the earth’s surface are not parallel, but converge towards the poles.

But for surveys is small areas, the true meridians passing through different points are assumed parallel.

The angle between the true meridian and a line is known as ‘true bearing’ of the line. It is also known as the ‘azimuth’.

2. Magnetic meridian When a magnetic needle is suspended freely and balanced properly, unaffected by magnetic substances, it indicates a direction. This direction is known as the ‘magnetic meridian’.

The angle between the magnetic meridian and a line is known as the ‘magnetic bearing’ or simply the ‘bearing’ of the line

3. Arbitrary meridian Sometimes for the survey of small area, a convenient direction is assumed as a meridian, known as the ‘arbitrary meridian’. Sometimes the starting line of a survey is taken as the arbitrary meridian.

The angle between the arbitrary meridian and a line is known as the ‘arbitrary bearing’ of the line.

4. Grid meridian Sometimes, for preparing a map some state agencies assume several lines parallel to the true meridian for a particular zone. These lines are termed as ‘grid lines’ and the central line the ‘grid meridian’. The bearing of a line with respect to the grid meridian is known as the ‘grid bearing’ of the line.

5. Designation of magnetic bearing Magnetic bearings are designated by two systems :

            (i) Whole circle bearing (WCB), and

            (ii) Quadrantal bearing (QB).

(a) Whole Circle Bearing (WCB) The magnetic bearing of a line measured clockwise from the north pole towards the line, is known as the ‘whole circle bearing’, of that line. Such a bearing may have any value between 0⁰ and 360⁰. The whole circle bearing of a line is obtained by prismatic compass

For example,

            WCB of AB = θ₁

            WCB of AC = θ₂

            WCB of AD = θ₃

            WCB of AE = θ₄

(b) Quadrantal Bearing (QB)The magnetic bearing of a line measured clockwise or  counterclockwise from the North Pole or South Pole (whichever is nearer the line) towards the East or West, is known as the ‘quadrantal bearing’ of the line. This system consists of four quadrant)Quardrantal Bearing (QB s – NE, SE, SW and NW. The value of a quadrantal bearing lies between 0⁰ and 90⁰, but the quadrants should always be mentioned. Quadrantal bearings are obtained by the surveyor’s compass

For example,  QB of AB = N

6. Reduced bearing (RB) When the whole circle bearing of a line is converted to quadrantal bearing. It is termed the ‘reduced bearing’. Thus, the reduced bearing is similar to the quadrantal bearing. Its value lies between 0⁰ and 90⁰, but the quadrants should be mentioned for proper designation.

7. Fore and back bearing The bearing of a line measured  in the direction of the progress of survey is called the ‘fore bearing’ (FB) of the line.

The bearing of a line measured in the direction opposite to the survey is called the ‘back bearing’ (BB) of  the line

            For example,                           FB of AB = θ

                                                            BB of AB = θ₁

Remember the following:

(a) In the WCB system, the difference between the FB and  BB should be exactly 180⁰, and the negative sign when it is more than 180⁰. Remember the following relation:

                                                BB = FB  ± 180⁰

Use the positive sign when FB is less than 180⁰, and the negative sign when it is more than 180⁰.

(b)   In the quandrantal bearing (i.e. reduced bearing) system, the FB and B3 are numerically equal but the quadrants are just opposite.

For example, if the FB of AB is N 30⁰ E, then its BB is S 30⁰ W.

8. Magnetic declination The horizontal angle between the magnetic meridian and true meridian is known as ‘magnetic declination’.

When the north end of the magnetic needle is pointed towards the west side of the true meridian, the position is termed  ‘Declination West’ ().

When the north end of the magnetic needle is pointed towards the east side of the true meridian, the position is termed  ‘Declination East’

9. Isogonic and agonic lines Lines passing through points of equal declination are known as ‘isogonic’ lines.

The Survey of India Department has prepared a map of India in which the isogonic and agonic lines are shown properly as a guideline to conduct the compass survey in different parts of the country.

10. Variation of magnetic declination The magnetic declination at a place is not constant. It varies due to the following reasons:

(a) Secular Variation The magnetic meridian behaves like a pendulum with respect to the true meridian. After every 100 years or so, it swings from one direction to the opposite direction, and hence the declination varies. This variation is known as ‘secular variation’.

(b) Annual Variation The magnetic declination varies due to the rotation of the earth, with its axis inclined, in an elliptical path around the sun during a year. This variation is known as ‘annual variation. The amount of variation is about 1 to 2 minutes.

(c) Diurnal Variation The magnetic declination varies due to the rotation of the earth on its own axis in 24 hours. This variation is known as ‘dirunal variation’. The amount of variation is found to be about 3 to 12 minutes.

(d) Irregular Variation The magnetic declination is found to vary suddenly due to some natural causes, such as earthequakes, volcanic eruptions and so on. This variation is known as ‘irregular variation’.

11. Dip of the magnetic needle If a needle is perfectly balanced before magnetisation, it does not remain in the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is found to be inclined towards the pole. This inclination of the needle with the horizontal is known as the ‘dip of the magnetic needle’.

It is found that the north end of the needle is deflected downwards in the northern hemisphere and that is south end is deflected downwards in the southern hemisphere. The needle is just horizontal at the equator. To balance the dip of the needle, a rider (brass or silver coil) is provided along with it. The rider is placed over the needle at a suitable position to make it horizontal.

12. Local attraction A magnetic needle indicates the north direction when freely suspended or pivoted. But if the needle comes near some magnetic substances, such as iron ore, steel structures, electric cables conveying current; etc. it is found to be deflected from its true direction, and does not show the actual north. This disturbing influence of magnetic substances is known as ‘local attraction’.

To detect the presence of local attraction, the fore and back bearings of a line should be taken. If the difference of the fore and back bearings of the line is exactly 180⁰, then there is no local attraction.

If the FB and BB of a line do not  differ by 180⁰, then the needle is said to be affected by local attraction, provided there is no instrumental error.

To compensate for the effect of local attraction, the amount of error is found out and is equally distributed between the fore and back bearings of the line.

For example, consider the case when

                        Observed FB of AB = 60⁰30’

                        Observed BB of AB = 240⁰0’

                        Calculated BB of AB = 60⁰30⁰ + 180⁰0’ = 240⁰30’

                      Corrected BB of AB = 1/2 (240⁰0’ + 240⁰30’) = 240⁰15’

            Hence,             Corrected FB of AB = 240⁰15’ – 180⁰0’ = 60⁰15’

13.  Method of application of correction

(a) First Method The interior angles of a traverse are calculated from the observed bearings. Then an angular check is applied. The sum of the interior angles should be equal to (2n – 4) x 90⁰ (n being the number of sides of the traverse). If it is not so, the total error is equally distributed among all the angles of the traverse.

Then, starting from the unaffected line, the bearings of all the lines may be corrected by using the corrected interior angles. This method is very laborious and is not generally employed.

(b) Second Method In this method, the interior angles are not calculated. From the given table, the unaffected line is first detected. Then, commencing from the unaffected line, the bearings of the other affected lines are corrected by finding the amount of correction at each station.

This is an easy method, and one which is generally employed.

Note: If all the lines of a traverse are found to be affected by local attraction, the line with minimum error is identified. The FB and BB of this line are adjusted by distributing the error equally. Then, starting from this adjusted line, the fore and back bearing of other lines are corrected.

3. PRINCIPLE OF COMPASS SURVEYING

The principle of compass surveying is traversing, which involves a series of connected lines. The magnetic bearings of the lines are measured by prismatic compass and the distances of the lines are measured by chain. Such survey does not require the formation of a network of triangles.

Interior details are located by taking offsets from the main survey  lines. Sometimes subsidiary lines may be taken for locating these details.

Compass surveying is not recommended for areas where local attraction is suspected due to the presence of magnetic substances like steel structures, iron ore deposits, electric cables conveying current, and so on.