MEAN SEA LEVEL (M.S.L)
WHAT IS MEAN SEA LEVEL (M.S.L)?
Mean sea level is a measurement of the average height of the sea between high and low tide. It is not dependent on any one place, in India or anywhere else. All elevations are measured above mean sea level which is a common base line in the whole world.
Mean sea level is a universal measure and acts as a baseline to calculate heights and elevations. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years
Ex:- Height of mount Everest is 8850 mts. The height is measured from the base line of mean sea level so the height of Mount Everest is 8850 mts above mean sea level.
If you want to know the mean sea level of your place. Go to the railway station in your area and see on the boards at either end of the plat form to find the mean sea level of that place written in one corner of the board. Like that bench mark of that area.
HISTORY OF MEAN SEA LEVEL
BEFORE PARTITION OF INDIA AND PAKISTAN
Karachi was the reference point for mean sea level until we got independence.
AFTER PARTITION OF INDIA AND PAKISTAN
Mean sea level adopted by survey of India for reference, is located at Mumbai high (500–041):0.79+/-0.11 mm/yr
Mumbai was the reference point for mean sea level after independence.
India mean elevation is 160 m (525 ft) and is taken from Mumbai sea port
India highest elevation: — Kanchenjunga 8586 m (28,169 ft)
India lowest elevation: — kuttanad, Kerala -2 m (-7 ft)
Mean sea level is an average level of the surface of one or more of earth’s oceans form which height s such as elevation may be measured but few things need to be kept in mind
1) Earth is not a perfect sphere. It has uneven surface, mountains, valley s
2) Gravitational force is not same every where
3) It does not have uniform rock composition.
So, scientists came up with earth gravitational model which gave us a standard to measure elevation of a place above or below mean sea level. Our gps uses it to give the elevation.
We know the mean sea level along the east coast of India is higher than that along the west coast of which means mean sea level along the coast India is higher in the Bay of Bengal then in the Arabian sea. The difference in sea level between Vishakhapatnam and Mumbai being about 30 cm and that the mean sea level at Chennai is 17 cm higher than that at Mangalore.
How to calculate the levels in anywhere to need SOME POINTS kept in mind
LEVELLING
Levelling is a branch of surveying to be determining the relative height s of different point on or below the surface of the earth is known as levelling. Thus levelling deals with measurements taken in the vertical plane.
METHODS OF LEVELLING
Three principal methods are used for determining difference in elevation
1) Barometric levelling
2) Trigonometric levelling or (indirect levelling)
3) Spirit levelling or (direct levelling)
1)Barometric levelling:-
Barometric levelling makes use of the phenomenon that difference in elevation between two points is proportional to the difference in atmospheric pressure at these points. A barometric, may be used and the reading observed at difference points would yield a measure of the relative elevation of those point
2)Trigonometric levelling:-
Trigonometric or indirect levelling is the process of levelling in which the elevation of points are computed from the vertical angles horizontal distance measured in the field just as the length of any side in any triangle can be computed from proper trigonometric relation. It is commonly used in mapping, both the difference in elevation and the horizontal distance between the point are directly computed from the measured vertical angle and staff reading .In a modified from called stadia levelling .
3)Spirit levelling:-
Spirit level is a branch of levelling in which the vertical distance with respect to a horizontal line may be used to determine the relative difference in elevation between two adjacent point. A horizontal plane of sight tangent to level surface at any point is readily established by mean of a spirit level or a level vial. The method is also known as direct levelling. It is the most precise method of determining elevations and the one most commonly used by engineers. In this method generally used in levelling.
LEVELLING INSTRUMENT:-
The instrument commonly used in direct levelling are
1) A level
2) A levelling staff
1) Level:-
The purpose of a level is to provide a horizontal line sight. Essentially a level consists of the there are the following chief types of levels
i) Dumpy level ii) wye (or y) level
iii) Reversible level iv) Tilting level
i) Dumpy level:-
A levelling instrument, also called the level is used for the determination of levels. The dumpy level is commonly used for levelling is designed by William Gravatt in 1832. The essential features of the dumpy level are shown in fig it consists of a telescope which is rigidly fixed to its support. It can neither about its longitudinal axis nor can it be removed from its support it is very advantages when several observations are to be made with one set-up the instrument.
Components of Dumpy Level
Dumpy level consists the following parts or components
1 )Telescope
2) Bubble tubes
3) Compass
4) Vertical spindle
5) Tribrach screws
6) Foot screws
7) Leveling head
8) Tripod
Fig: Components of a Dumpy Level
- Telescope:-Telescope is used to observe the distant object through line of sight provided by its arrangement. In general, the telescope is fixed to the vertical spindle of dumpy level so that it can be rotated along with vertical spindle.
Parts of Telescope in Dumpy Level
The important parts of telescope are as follows
i) Eye piece
ii) Objective lens
iii) Diaphragm
iv) Focusing screw
v) Ray shade
i) Eye piece:-
Eye piece is used by the observer’s eye to view the distant object. It contains magnifying glass which magnify the observing image and also the cross hairs of diaphragm. So, accurate reading can be obtained. Erecting eyepiece is used to view the normal image which is generally inverted by objective lens.
ii) Objective lens:-
Objective lens are provided at the other end of the telescope. The objective lens consists of two parts, the front part consists convex type lens and the back part consists concave lens. So, the image obtained from the objective lens is always inverted
iii)Diaphragm:-
Diaphragm is provided in front of the eye piece. It contains cross hairs made of dark metal which are arranged in perfect perpendicular positions. These cross hairs are used by the eye piece to bisect the objective through objective lens.
iv)Focusing screw:-
Focusing screw is used to adjust the focus if cross hairs and the image clarity. The magnification of eye piece is managed by this focusing screw.
v)Ray shade:-
Ray shade is used to prevent the objective lens from sunlight or any other light rays which may cause disturbance to the line of sight.
2) Bubble tubes:-
Bubble tubes are provided to check the level of the instrument. Two bubble tubes are provided in a dumpy level which are arranged perpendicular to each other on the top of the telescope. One tube is called as longitudinal bubble tube and another is called as cross bubble tube. The instrument is said to be in perfect position when both the bubbles of the tubes are at center or middle of the tube.
3) Compass:-
Compass is used to determine the magnetic bearing of line. In case of dumpy level, circular compass is provided just under the telescope. The compass contains a pointer in it and readings are marked inside it. The pointer is set to zero when it faces the north line from which the magnetic bearings are measured.
4) Vertical spindle:-
Vertical spindle is located at the center of the whole instrument. The telescope can be rotated in horizontal direction with respect to vertical spindle. The instrument is connected to the tripod stand using vertical spindle.
5) Tribrach:-
Tribrach plate is parallel to the leveling head or trivet. It is connected to trivet by leveling screws or foot screws which can adjust the tribrach plate. The horizontal level of the instrument can be achieved by adjusting this tribrach plate.
6) Foot screws:-
Foot screws are provided to regulate the tribrach position and hence the instrument can be leveled which is known by observing the bubble tube. The tribrach plates can be lowered or raised using foot screws. The position of tribrach is said to be correct when the bubble in bubble tube is at center.
7) Leveling head:-
Leveling head is also called as trivet. It contains two triangular shaped plates which are arranged parallel to each other. Three groves are provided at the three corners of the plates in which foot screws are supported.
8) Tripod:-
Tripod is used to support the whole leveling instrument on its top. It consists three legs which can be adjustable to required position. The legs are of same height and they may be solid or hollow. Steel shoes are provided at the bottom of each leg to hold the ground in a fixed position.
ii)wye level:- A wye level consists of a telescope held in two vertical wye supports. The wye supports are two-curved clip which may be raised to enable the telescope to be rotated about its longitudinal axis. It may be removed and turned end to end shown in fig
When the clip are tightened the telescope is held in supports rigidly. A level tube is attached to the stage carrying the wyes. The levelling head is similar to that of a dumpy level. In some levels, a clamp and fine motion tangent screw are provided to control the movement in the horizontal in the plane.
Wye-level has an advantage over dumpy level is that its adjustment can be tested rapidly. The disadvantage is that it carries many loose and open parts, which are liable to frictional wear.
iii)Reversible level:-
In this level the features of both dumpy and wye level are combined, so that the advantages of each could be utilized. The telescope is supported on two rigid sockets. The telescope is introduced from either end and then fixed in position by means of screw. The socket are rigidly connected to the spindle through a stage. Once the telescope is introduced into socket and screws tightened, it act as a dumpy level.
For testing and adjusting the level,the screws of the socket s are loosened , the telescope is removed and reversed end to end. The chief advantage of this level is collimation error is eliminated. It also permits an easy permanent adjustment. The features of reversible level
iv)Tilting level:-
The tilting level is commonly used for precise and quick levelling. It is similar to the dumpy level in many respects, but the telescope of the tiliting level is not rigidly fixed to the vertical spindle. The telescope can be tilted on a pivot about horizontal axis in the vertical plane upwards or downwords through a small angle by means of a tilting screw. This tilting screw acts against a spring.
Tilting level is more accurate than dumpy level. It has a shorter telescope and lighter in weight. The tilting arrangement saves time required for temporary adjustment
In some tilting levels the conventional three foot screws arrangement is replaced by a ball and socket joint. This enables the level to be quickly leveled, approximately .Exact leveling is done by tilting screw.
2 ) A LEVELLING STAFF :-
A levelling staff is a straight rectangular , wooden or aluminium rod graduated into metres and smaller division .The levelling staves of three meters and four metres length are commonly used in practice . There is one brass cap at the bottom and on at the top .The brass cap at the bottom helps in reducing the wear and tear of the base . The bottom of the staff represents zero reading graduations on the staff give the height from the bottom of the brass cap .
Each metre is divided into 200 divisions . The thickness of each division is 5mm or 0.005m. The divisions are painted alternatively black and white. The metre numerals are generally written in red on one side, but decimeter numerals are in black and on the other side of division.
TYPES OF LEVELLING STAFF :
Based on the methods of taking readings, the levelling staffs are dividing into two types.
i)Self reading staff
ii)Target staff
i)Self reading staff :
A self reading staff is the one which can be read directly by the instrument man through the telescope.
Based on the method of construction the levelling staffs are divided into three types
a)Solid staff
b)Folding staff
c)Telescopic staff
a)Solid staff :-
The solid staff is in one length of three metres as there in no joints are hinges in between three is no possibility of relative movements of different sections and the staff reading are quite accurate. Hence this staff is used for precise levelling work as shown in figure.
b)Folding staff :-
The staff is made of well seasoned timber and is of width 75mm ,thickness 18mm and length 4m.It is divided into two parts of length 2m each, having a locking arrangement. The two pieces are hinged together , so that one piece can be folded over the other when not is used.The two pieces become rigid and straight when unfolding. The two pieces can be detached and only a two meters length can be used when 4m length is not required. The staff has two handings ,one on each section,for holing the staff as shown in figure.
c)Telescopic staf :-
A telescopic staff consists of a 3 pieces which can be extended to a full length of 4m.The upper piece of 1.20m length can be slide into central piece of 1.30m length.The central piece in turn can slide into the lower piece of 1.5m length.The overall length of the staff this become 1.5m when the staff is not in used.
ii)Tareget staff:-
A target staff is a solid staff provided with a movable target.It is used for long distance sighting when it become difficult for the instrument man to take the reading of the staff accurately.The target is a small metal piece of circular or oval shape, about 125mm in diameter.It is painted red and white in alternative quadrants.The target is fitted with a vernier. The bottom vernier is at the center of the target.The instrument man sights the staff directs the staff man to move the target upward or download as required .Then the staff man takes the vernier readings which is the required staff reading.
ADJUSTMENT OF THE LEVEL:-
Two types adjustment are carried out for the levelling instrument. Those are temporary and permanent adjustment are the adjustment which are done at every instrument setting. Before the observations are taken with the instrument permanent adjustment are done when the desired relationship between the fundamental line of an instrument is disturbed. Once these adjustment are made, they last for a long time.
TEMPORARY ADJUSTMENT OF A DUMPY LEVEL
The following temporary adjustment are done in the case of a dumpy leve.
1)SETTING UP:-
As the level is not to be set at any fixed point the setting up of a level is much simpler than compared to other instruments. However, while fixing the position of a levelling instrument. The following points can be remembered.
i)the instrument is not too low or too high to facilitate reading on a bench mark.
ii)the length of the back sight should preferably not more than 100m
iii)the fore sight and back sight distance should be equal and the fore sight should be so located that it advances the line of levels. The process of setting up includes fixing the instrument and approximate levelling by leg adjustment.
a)fixing the instrument over tripod:-
first the tripod is setup, so that its at a convenient height. The clamp screw of the instrument is released. The level is held in right hand. Then it is fixed on the tripod by turning the trivet stage with the left and is firmly screwed over the tripod.
b)Leg adjustment:-
first all the foot screws are bought to the centre of their run the tribrach plate is made as nearly horizontal as possible. Now, fix any two legs of the tripod firmly into the ground and move the third leg right or lift in a circumferential direction until the main bubble in approximate in the center. The third leg is then pushed into the ground.some instruments are provided with a small circula bubble tube on the tribrach or the tripod top, to check the approximate levelling of the instrument. When the bubble is in the center, the instrument is approximate leveled.
2)LEVELLING UP:-
The purpose of levelling is to make the vertical axis of the instrument truly vertical. The following procedure is used for levelling up of a dumpy level provided with three foot screws head and four screw head.
i)Levelling with a three screw head:-
a)the clamp screw of the instrument is released.
b)turn the telescope, so that the bubble tube is parallel to the line joining any two foot screws, say no 1 and no 2
c)turn both the foot screws (1&2) inwards, to bring the bubble to the center of bubble tube, when the bubble is to the left center
d)turn both the foot screw s(1&2)outwards to bring the bubble to thr center of bubble tube, when the bubble is to the right of the center shown in fig b. The telescope is rotated through 90˚,so that it lies over the third foot screw. Now turn the thid foot screw anti-clockwise, to bring the bubble to the center of the bubble tube, when the bubble is away from the instrument man shown in fig c
e) turn the third foot screw clock-wise, to bring the bubble to the center of the bubble tube, when the bubble is nearer to the instrument man shown in fig(d).the telescope is rotated through 90˚to its original position i.e parallel to foot screw 1&2and above procedure is repeated till the bubble remains central in both the positions. The telescope is now rotated through 180˚. The bubble should remain central, if the instrument is in proper adjustment.
ii)Levelling with four screw head:-
a)trun the upper plate until the longitudinal axis of the plate level is roughly parallel to the line joining two diagonally opposite screw such as D and B shown in fig
b)bring the bubble central exactly in the same manner as described in the three head screw but in this case the foot screw s are alternatively rotated in clock wise or anti clock wise direction in shown in fig.
3)ELIMINATION OF PARALLAX (FOCUSSING):-
If the image formed by the objective is not in the plane of cross hair an error called parallax occurs. Parallax error is eliminated by focusing the eye piece and objective of the level.
a)focussing the eyepiece:-
The operation is done to make the cross hair appear distinct and clearly visible. For focusing the eyepiece the telescope is directed towards the sky or hold a sheet of white paper in front of the objective. Turn the eye piece ring either in or out till the cross hair visible sharp and distinct.
b)focusing the objective:-
This operation is done to bring the image of the object in the plane of the cross hairs. For focusing the objective , the telescope is directed towards the object (staff). Turn the focusing screw until the image of the object appears clean and sharp
SPECIAL METHODS OF SPIRIT LEVELLING
a)differential levelling:- It is the method of direct levelling the object of which is solely to determine the difference in elevation of two points regardless of the horizontal positions of the points with respect of each other. When the point are apart, it may be necessary to set the instrument several times. This type of levelling is also known as fly levelling.
b)Profile levelling:-it is the method of direct levelling the object of which is to determine the elevation of points at measured intervals along a given line in order to obtain a profile of the surface along that line.
c)cross sectioning:- cross sectioning or cross levelling is the process of taking levels on each side of a main line at right angle to that line, in order to determine a vertical cross section of the surface of the ground, or of underlying strata, or of both.
d)Reciprocal levelling:-It is the method of levelling in which the difference in elevation between two points is accurately determined by two sets of reciprocal observation when it is not possible to set up the level between the two point.
e)precise levelling:- It is the levelling in which the degree of precision required is too great to be attained by ordinary method, and in which, therefore, special, equipment or special precautions or both are necessary to eliminate as far as possible, all sources of error.
Fore sight(F.S):-It is the last staff reading taken from any setup of the instrument, on a point whose elevation is to be determined.
Back sight(B.S):-It is the first staff reading taken from any set up of the instrument, on a point whose elevation is to be determined.
Intermediate point(I.S):- It is any staff reading taken on a point of unknown elevation between back sight and fore sight.
Changing point or Turning point (T.P):-It is a point denoting the shifting of the level, on this point, an F.S is taken from one setting and a B.S from the next setting station is a changing point.
Station:- A point whose elevation is to be determined is called station.
TYPES OF BENCH USED IN DIFFERENT CASES:-
Bench mark (BM):- It is a fixed reference point of known elevation. The reduced level (RL) of this point are determined with reference to the datum line. These are very important point, as the RL of new point are established with reference to the BM depending on the permanency and precision, bench mark may be the following types
a)Great trigonometric survey(GTS)Bench mark:- These are established by the survey of india department at an interval of about 100km all over the country, with respect to the mean sea level at Karachi as datum the values of reduced levels the relevant positions and the number of bench marks are given in a catalogue published by this department.
b)permanent bench mark:- These are established with respect to GTS bench mark by different Government department like railway, irrigation ect. On of a permanent point s like the plinth of the building, the top of a parapet wall of a bridge or culvert, kilometer stone, railway plat form ect.
c)Arbitrary bench mark:-These are reference point, whose elevation (i.e RL) are arbitrarily assumed in small levelling operation. The elevation of this BM do not refer to any fixed datum.
d)Temporary bench mark:-The bench mark established temporarily at the end of a day s’ work and from where levelling is continued in the next day, are called temporary bench mark. They are generally established on the root of a tree, the parapet of a nearby culvert, a kilometer stone ect.
BOOKING AND REDUCED LEVELS:-
There are two methods of booking and reducing the elevation of point from the observed staff reading
i)Collimation or height of instrument method
ii)Rise and fall method
i)collimation or height of instrument method:-
In this method the height of the instrument(HI) is calculated for each setting of the instrument by adding back sight (plus sight) to the elevation of the B.M (first point). the elevation of reduced level of the turning point is then calculate by subtracting from H.I the fore sight . for the next setting of the instrument, the H.I is obtain by adding the B.S taken on T.P 1 to its R.L . the process continues till the R.L of the last point (a fore sight) is obtain by subtracting the staff reading from height of the last setting of the instrument. If there are some intermediate point, the R.L of those point is calculated by subtracting the intermediate sight from the height of the instrument for that setting.
Arithmetic check:- The difference between the sum of back sight and the sum of fore sight be equal to the difference between the last and the first R.L. thus
∑B.S-∑F.S =Last R.L — First R.L
ii)Rise and fall method:-In rise and fall method the height of instrument is not at all calculated but the difference of level between consecutive point is found by comparing the staff reading on the two point for the same setting of the instrument. The different between their staff reading indicates a rise or fall according as the staff reading at the point is smaller or greater than that at the preceding point.
Arithmetic check:- The difference between the sum of back sight and sun of fore sights should be equal to the difference between the sum of rise and the sum of fall and should also be equal to the difference between the R.L of last and first point.
∑B.S-∑F.S =∑Rise -∑Fall= Last R.L — First R.L
1)Problem on height of instrument method:-
The following consecutive reading were taken with a dumpy level.
1.895,1.500,1.865,2.570,2.990,2.020,2.410,2.520,2.520,and 2.960. The level was shifted after fourth, and sixth reading. The R.L of the first point was 30.500. Rule out a page of level book, and fill all columns. Use collimation method and apply the usual checks.
Entering the staff reading into level field book:
1) Enter the first reading in first row of B.S column.
2) The second & third readings are entered in I.S column in second and third rows respectively.
3) The fourth reading is entered in F.S column in fourth row, and the instrument is shifted after this reading.
4) Enter the fifth reading in fourth row B.S column, as this reading is taken on turning point (change point) and enter T.P1 in the remarks column.
5) As the instrument was shifted after the sixth reading, enter sixth reading in fifth row in F.S column.
6) Again enter the next reading i.e 7th in fifth row B.S column as this reading was observed on T.P2 and enter T.P2 in the remarks column.
7) Enter the eight reading in 6th row in I.S column.
8) Enter the last reading i.e, 9th in F.S column of 7th row.
9) Enter the R.L of this point in the 1st row of R.L column.
Calculation:-
1)Height of instrument ( R.L of line of collimation) for the first set up
H.I1 R.L of 1st point +B.S on 1st point
= 30.500+1.895
= 321.395m
Enter this value in first row of H.I column
2) R.L of the 2nd point
= H.I1 — I.S on 2nd point.
= 32.395–1.500
= 30.895m.
3)similary R.L of 3rd point
= H.I — I.S on 3rd point.
= 32.395–1.865
= 30.530m.
4)R.L of 4th point
= H.I1 –F.S on 4th point
= 32.395–2.570
= 29.825m.
5)Heigth of instrument for the second set up.
H.I2 = R.L of 4th point +B.S on T.P1
= 29.825–2.990
= 32.815m.
6)R.L of 5th point
= H.I2 –F.S on 5th point.
=32.815–2.020
=30.795m
7)Height of instrument for the third set up
H.I3 = R.L of 5th point +B.S on T.P2
= 30.795+2.410 = 33.205m.
8)R.L of 6th point
= H.I3 — I.S on 6th point.
=33.205–2.520
= 30.685m.
9) R.L of 7th point
= H.I3 –F.S on 7th point
= 33.205–2.960
= 30.245m
Arithmetical check:-
Add the reading in B.S column and get ∑ B.S = 7.295m.
Add the reading in F.S column and get ∑ F.S = 7.550m.
∑ B.S — ∑ F.S = Last R.L — First R.L
7.295–7.550 = 30.245 -30.500
- 0.255 = — 0.255
The ‘-‘ sign indicates a net fall from the point to the last point.
2)Problem on Rise and Fall method
The following staff reading were observed successively with a level, the instrument was shifted after the second and fifth reading 0.675,1.230,0.750,2.565,2.225,1.935,1.835,3.220 the first staff reading was taken with the staff held an a bench mark of reduced level 125.325m. Enter the reading in the level book from and find the reduced levels of all points by rise and fall method and check the correctness of the reduction.
Entering the staff reading into level field book:-
1)Enter the first reading 1st of B.S column also enter the R.L of B.M value in R.L column of 1st row
2)Enter the second reading in 2nd row F.S column and 3rd reading in B.S column of 2nd row, as the instrument was shifted after 2nd reading. Also enter T.P1 in remarks column of 2nd row.
3)Enter the 4th reading in 3rd row I.S column.
4)Enter the 5th reading in 4th row F.S column and 6th reading in B.S column of 4th row, as the instrument was shifted after 5th reading. Also enter T.P2 in remarks column of 4th row.
5)Enter the 7th reading in I.S column and last reading in F.S column, in the subsequent rows.
Procedure of reduction of level
1)B.S on point A — F.S on point B.
0.675–1.230 =-0.555
The ‘-‘ sign indicates the fall, hence enter the value in 2nd row fall column to get the R.L of station ‘B’ subtract the fall from R.L of B.M. (i.e R.L of A)
2)B.S on point B — I.S on point C=0.750–2.565 =-1.815.
Enter the value in 3rd row fall column, substract this fall from R.L of station B, to get R.L of station ‘C’
3)I.S on point C –F.S on point D =2.565–2.225 =’+’0.340. the ‘+’ sign indicate the rise, hence enter the value in 4th row rise column.
To get R.L of station ‘D’ add the rise to the R.L of station ‘C’.
4)B.S on point D — I.S on point E = 1.935–1.835 =+0.100 enter the value in 5th row rise column, add this rise value to R.L of station ‘D’ to get R.L of station ‘E’.
5)I.S on point E — F.S on point F = 1.835 -3.220 = -1.385 the ‘-‘ sign indicate the fall , hence enter the value in the 6th row, fall column.
To get the R.L of station F subtract the fall from R.L of station E.
Arithmetical check:-
∑B.S -∑F.S = ∑Rise -∑fall = last R.L — first R.L
Substituting
3.36–6.675 = 0.440–3.775 = 122.010–125.325–3.315 = -3.315
F.S
(4)
H.I
(5)
R.L
(6)
Remarkes
(7)
A
1.895
32.395
30.500
B
1.500
30.895
C
1.865
30.530
D
2.990
2.570
32.815
29.825
T.P1
E
2.410
2.020
33.205
30795
T.P2
F
2.520
30.685
G
2.960
30.245
Sum
7.295
7.550