COMPASS SURVEY
In COMPASS SURVEY Method of traversing is used.
In TRAVERSING there are numbers of connected line are used and length of them is measured with the help of tape or chain and direction is measured by angle measuring instruments.
When we use compass for measuring the angle, we call it as compass survey or traversing.
TERMINOLOGY
MERIDIAN
Imaginary semi-circle joining the earth's poles, and crossing the equator and all latitudes (baselines) at right angles. All meridians traverse in north-south direction and their ends converge at north and south poles. Meridian lines are used as one of the reference points (coordinates) with baselines in land surveying grid system to locate any point on earth. Also called longitude.
TRUE MERIDIAN
The line on a plane passing through the geographical North Pole or geographical South Pole and any point on the surface of the earth is known as true meridian. It is also called as geographical meridian. The angle between true meridian and line is known as true bearing of the line. It is also known as azimuth.
MAGNETIC MERIDIAN
When magnetic needle is suspended freely and balance properly, unaffected by magnetic substance it indicates a direction this direction is known as magnetic meridian. The angle between magnetic meridian and line is known as magnetic bearing of the line.
ARBITRARY MERIDIAN - Sometime survey of a small area a convenient direction is assume as a meridian known as Arbitrary meridian.
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. Designation of magnetic bearing Magnetic bearings are designated by two systems-(i) 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 0 ̊ and 360 ̊ .
The whole circle bearing of a line is obtained by prismatic compass.
(ii) 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) Quadrantal Bearing (QBs – NE, SE, SW and NW).
The value of a quadrantal bearing lies between 0 ̊ and 90 ̊, but the quadrants should always be mentioned.
Quadrantal bearings are obtained by the surveyor’s compass.
WCB QB QUADRANT
0 ̊-90 ̊ RB= WCB NE
90 ̊-180 ̊ RB= 180 ̊ – WCB SE
180 ̊-270 ̊ RB= WCB – 180 ̊ SW
270 ̊-360 ̊ RB= 360 ̊ – WCB NW
FORE BEARING
Fore bearing the bearing of a line measured in the direction of the progress of survey is called the ‘Fore Bearing’ (FB) of the line.
BACK BEARING
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
(a) In the WCB system, the difference between the FB and BB should be exactly 180 ̊, and the negative sign when it is more than 180 ̊. Remember the following relation-
BB = FB ± 180 ̊
Use the positive sign when FB is less than 180 ̊, and the negative sign when it is more than 180 ̊.
(b) In the quadrantal bearing (i.e. reduced bearing) system, the FB and BB are numerically equal but the quadrants are just opposite. For example, if the FB of AB is N 30 ̊ E, then its BB is S 30 ̊ W.
MAGNETIC DECLINATION
The horizontal angle between the magnetic meridian and true meridian is known as ‘magnetic declination’.
ISOGONIC- Equal Declination
AGONIC – Zero Declination
Variation of magnetic declination
The magnetic declination at a place is not constant. It varies due to the following reasons:
(a) Secular Variation
The Earth's magnetic field is slowly changing on time scales that range from 100 years to millions of years. The meridian swings like a pendulum in one direction for about 150 years and gradually comes to a stop and then swings back in the opposite direction. It is a slow, gradual, but unexplainable shift.
(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 earthquakes, volcanic eruptions and so on. This variation is known as ‘irregular variation’.
DIP OF MAGNETIC NEEDLE
Dip of the magnetic needle If a needle is perfectly balanced before magnetisation, But 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 (compass needle) with respect to the horizontal (pole) is known as the ‘dip of the magnetic needle’.
DIP ZERO AT EQUATORS.
DIP IS 90 ̊ AT POLES.
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.
SAME DIP – ISOCLINIC
ZERO DIP LINE- ACILINIC
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’. If the difference of the fore and back bearings of the line is exactly 180 ̊, then there is no local attraction.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.
METHOD OF APPLICATION OF CORRECTION
1. 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 90 ̊ (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.
2. 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. 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.
COMPASS TRAVERSING
In this method, the fore and back bearings of the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary corrections for local attraction are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowditch’s rule’ (which is described later on).
CHECK ON CLOSED TRAVERSE
1. Check on angular measurements
(a) The sum of the measured interior angles should be equal to (2N – 4) x 90 ̊ where N is the number of sides of the traverse.
(b) The sum of the measured exterior angles should be equal to (2N + 4) x 90 ̊.
(c) The algebraic sum of the deflection angles should be equal to 360 ̊.
(d) Right-hand deflection is considered positive and left-hand deflection negative.
2. Check on linear measurement
(a) The lines should be measurement once each on two different days (along opposite directions). Both measurements should tally.
(b) Linear measurements should also be taken by the stadia method. The measurements by chaining and by the stadia method should tally.
CHECK ON OPEN TRAVERSE
In open traverse, the measurements cannot be checked directly. But some field measurements can be taken to check the accuracy of the work. The methods are discussed below.
1. Taking cut-off lines Cut-off lines are taken between some intermediate stations of the open traverse. Suppose ABCDEFG represents an open traverse. Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off lines are measured accurately. After plotting the traverse, the distances and bearings are noted from the map. These distances and bearings should tally with the actual records from the field.
2. Taking an auxiliary point Suppose ABCDEF is an open traverse. A permanent point P is selected on one side of it. The magnetic bearings of this point are taken from the traverse stations A, B, C, D, etc. If the survey is carried out accurately and so is the plotting, all the measured bearings of P when plotted should meet at the point P. The permanent point P is known as the ‘auxiliary point’.
TYPES OF COMPASS
PRISMATIC COMPASS
SURVEYORS COMPASS
The magnetic needle is BOARD type.The magnetic needle is EDGE type.
The magnetic needle is attached to a graduated aluminum ring and does not rotates with line of sight.
The magnetic needle is not attached to a graduated aluminum ring and rotates with line of sight.
The graduation marked Thus
0 ̊ is at the south,
90 ̊ at the west,
180 ̊ at north and
270 ̊ at the east.
Measured WCB The graduation marked Thus
0 ̊ NORTH & SOUTH
90 ̊ at EAST & WEST
Measured QB
The graduations are engraved INVERTED since the graduated ring is read through the prism.
The graduations are engraved ERECT since, graduated ring read directly.
Reading are taken with the help of prism.
Reading are directly taken by seeing on the ring.
Sighting and reading can be done simultaneously.
Sighting and reading cannot be done simultaneously.
Instrument can also be taken in hand.
Instrument can not be used without tripod.
Eye vane consists of metal vane with large slits. No mirror.
Eye vane consists of small metal vane with small slits. A mirror is provided with the sight vane Least Count of Prismatic Compass is 30 minutes Least count of Surveyors Compass is 15 minutes.
TEMPORARY ADJUSTMENT OF PRISMATIC COMPASS
(FIELD PROCEDURE OF OBSERVING BEARING)
1. Fixing the compass with tripod stand
2. Centering
3. Levelling
4. Adjustment of prism
5. Observation of bearing
