The Engineer: Surveying

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Wednesday, October 18, 2017

Parts of A Vernier Theodolite

Unknown October 18, 2017 Surveying , 0 Comments

Shows a sectional view of a typical Vernier theodolite and plate 16.1 shows photograph of such theodolite. Main parts of such a theodolite are:
1. Telescope:
A telescope is mounted on a horizontal axis (trunnion axis) hence it can rotate in vertical plane. Its length varies from 100 mm 175 mm and its diameter is 38 mm at objective end. Its functions is to provide a line of sight.
2. Vertical Circle:
A vertical circle graduated up to an accuracy of 20′ is rigidly connected to the telescope and hence moves with it when the telescope is rotated in vertical plane. The graduations are in quadrantile system, 0-0 line being horizontal
3. Vernier Frame:
It is a T-shaped frame consisting of a vertical arm and a horizontal
Arm. With the help of the clamping screws the vertical frame and hence the telescope can be clamped at desired angle. Vertical frame is also known as T-frame or index frame. The Vernier arm is known as index arm. At the ends it carries Vernier’s C and D so as to read graduations on vertical circle. They are provided with glass magnifiers. Altitude bubble tube is fitted over the horizontal arm.
4. Standards or A-Frame:
The frames supporting telescope are in the form of English letter ‘A’. This frame allows telescope to rotate on its trunnion axis in vertical frame. The T-frame and the clamps are also fixed to this frame.
5. Upper Plate:
Upper plate supports standards on its top surface. On lower side it
is attached to a inner spindle which rotates in the outer spindle of lower plate. Using upper clamp, upper plate can be clamped to lower plate. Using tangent screws, it is possible to give slight relative motion between the two plates, even after clamping. Two diametrically opposite Vernier’s A and B fixed to upper plate help in reading horizontal circle graduations. They are provided with magnifying glasses.
6. Lower Plate:
The lower plate, attached to the outer spindle carries a graduated circle at its bevelled edge. Graduations are up to an accuracy of 20′. It can be clamped at any desired position using lower clamps. If upper clamp is locked and the lower one is loosened the two plates rotate together. If the upper clamp is loosened and lower clamp locked, upper plate alone rotates. This mechanism is utilised in measuring horizontal angle.
7. Plate Level:
One or two plate level tubes are mounted on the upper plate. If the two level tubes are provided they will be at right angles to each other one of them being parallel to trunnion axis. These levels help in making the vertical axis of the instrument truely vertical.
8. Levelling Head:
It consists of two parallel triangular plates known as tribrach plates. The upper tribrach plate is provided with three levelling screws—each one carried by an arm of tribrach plate. By operating screws the levelling of upper plate and hence telescope can be ensured. The lower tribrach can be fitted into a tripod head.
9. Tripod:
Theodolite is always used by mounting it on a tripod. The legs of tripod may be solid or framed. At the lower end the legs are provided with steel shoes to get good grip with the ground. The top of tripod is provided with external screw to which the lower tribrach plate can be screwed. When not in use tripod head may be protected with a steel cap, provided for this purpose.
10. Plumb Bob:
A hook is provided at the middle of lower tribrach plate from which a plumb bob can be suspended. It facilitates exact centring of the theodolite on a station.
11. Shifting Head:
It is provided below the lower plate. In this, one plate slides over another over a small area of about 10 mm radius. The two plates can be tightened in the desired position. It facilitates exact cantering of the instruments.
12. Magnetic Compass:
In some theodolites a magnetic compass is fixed on one of the strands. It is useful if readings are to be recorded with magnetic north as meridian.

Saturday, October 14, 2017

OBJECT AND USES OF SURVEYING

Unknown October 14, 2017 Surveying , 0 Comments

As stated in the definition, object of surveying is to show relative positions of various objects of an area on paper and produce plan or map of that area.
Various uses of surveying are listed below:
(i) Plans prepared to record property lines of private, public and government lands help in avoiding unnecessary controversies.
(ii) Maps prepared for marking boundaries of countries, states, districts etc., avoid disputes.
(iii) Locality plans help in identifying location of houses and offices in the area.
(iv) Road maps help travellers and tourist.
(v) Topographic maps showing natural features like rivers, streams, hills, forests help in planning irrigation projects and flood control measures.
(vi) For planning and estimating project works like roads, bridges, railways, airports, water supply and waste water disposal surveying is required.
(vii) Marine and hydrographic survey helps in planning navigation routes and harbours.
(viii) Military survey is required for strategic planning.
(ix) Mine surveys are required for exploring minearl wealth.
(x) Geological surveys are necessary for determining different strata in the earth crust so that proper location is found for reservoirs.
(xi) Archeological surveys are useful for unearthing relics of antiquity.
(xii) Astronomical survey helps in the study of movements of planets and for calculating local and standard times.

Wednesday, October 11, 2017

Level and Levelling

Unknown October 11, 2017 Surveying , 0 Comments

Elevation measurements

Elevation measurements involve measurements in vertical plane. It is also known as levelling. It may be defined as the art of determining the elevations of given points above or below a datum line or establishing given points of required heights above or below the datum line.

OBJECT AND USES OF LEVELLING

As stated in the definition of levelling, the object is

(i) To determine the elevations of given points with respect to a datum

(ii) To establish the points of required height above or below the datum line.

Uses of levelling are

(i) To determine or to set the plinth level of a building.

(ii) To decide or set the road, railway, canal or sewage line alignment.

(iii) To determine or to set various levels of dams, towers, etc.

(iv) To determine the capacity of a reservoir.

TERMS USED IN LEVELLING

Before studying the art of levelling, it is necessary to clearly understand the following terms used in levelling:

1. Level Surface: A surface parallel to the mean spheroid of the earth is called a level surface and the line drawn on the level surface is known as a level line. Hence all points lying on a level surface are equidistant from the centre of the earth.

2. Horizontal Surface: A surface tangential to level surface at a given point is called horizontal surface at that point. Hence a horizontal line is at right angles to the plumb line at that point

3. Vertical Line: A vertical line at a point is the line connecting the point to the centre of the earth. It is the plumb line at that point. Vertical and horizontal lines at a point are at right angles to each other

4. Datum: The level of a point or the surface with respect to which levels of other points or planes are calculated, is called a datum or datum surface.

5. Mean Sea Level (MSL): MSL is the average height of the sea for all stages of the tides. At any particular place MSL is established by finding the mean sea level (free of tides) after averaging tide heights over a long period of at least 19 years. In India MSL used is that established at Karachi, presently, in Pakistan. In all important surveys this is used as datum.

6. Reduced Levels (RL): The level of a point taken as height above the datum surface is known as RL of that point.

7. Benchmarks: A benchmark is a relatively permanent reference point, the elevation of which is known (assumed or known w.r.t. MSL). It is used as a starting point for levelling or as a point upon which to close for a check. The following are the different types of benchmarks used in surveying:

(a) GTS benchmarks

(b) Permanent benchmarks

(d) Temporary benchmarks.

(a) GTS Benchmark: The long form of GTS benchmark is Great Trigonometrical Survey

Benchmark. These benchmarks are established by national agency. In India, the department of Survey of India is entrusted with such works. GTS benchmarks are established all over the country with highest precision survey, the datum being mean sea level. A bronze plate provided on the top of a concrete pedestal with elevation engraved on it serves as benchmark. It is well protected with masonry structure built around it so that its position is not disturbed by animals or by any unauthorised person. The position of GTS benchmarks are shown in the topo sheets published.

(b) Permanent Benchmark: These are the benchmarks established by state government agencies like PWD. They are established with reference to GTS benchmarks. They are usually on the corner of plinth of public buildings.

neighbouring points is more important than their reduced level with respect to mean sea level. In such cases a relatively permanent point, like plinth of a building or corner of a culvert, are taken as benchmarks, their level assumed arbitrarily such as 100.0 m, 300.0 m, etc.

(d) Temporary Benchmark: This type of benchmark is established at the end of the day’s work, so that the next day work may be continued from that point. Such point should be on a permanent object so that next day it is easily identified.

LEVELLING INSTRUMENTS

A level is an instrument giving horizontal line of sight and magnifying the reading at a far away distance. It consists of the following parts:

(i) A telescope to provide a line of sight

(ii) A level tube to make the line of sight horizontal and

(iii) A levelling head to level the instrument.

The following types of levels are available:

(i) Dumpy level

(ii) Wye (or, Y) level

(iii) Cooke’s reversible level

(iv) Cushing’s level

(v) Tilting level and

(vi) Auto level.

Dumpy Level

It is a short and stout instrument with telescope tube rigidly connected to the vertical spindle. Hence the level tube cannot move in vertical plane. It cannot be removed from its support. Hence it is named as dumpy level. The telescope rotates in horizontal plane in the socket of the levelling head. A bubble tube is attached to the top of the telescope. Telescope is a tube with object glass and eyepiece. Object glass can be adjusted using the focussing screw before sighting the graduated staff held on the object. Eyepiece can be adjusted by rotating it to see that parallel is removed and cross hairs appears distinctly.

Eyepiece once adjusted needs no change as long as the same person takes the readings. Level tube is a glass tube with slightly curved shape provided over the level tube. The tube is filled with ether or alcohol leaving a little air gap, which takes the shape of a bubble. The air bubble is always at the highest point. The level tube is fixed with its axis parallel to telescope tube, so that when bubble is centred, the telescope is horizontal. The tube is graduated on either side of its centre to estimate how much the bubble is out of centre. The glass tube is placed inside a brass tube which is open from top and on lower side it is fixed to telescope tube by means of capston headed nuts. The bubble tube is adjusted with these nuts, if it is out of order.

Levelling head consists of two parallel plates with three foot screws. The upper plate is known as tribratch plate and the lower one as the trivet. The lower plate can be screwed on to the tripod stand. By adjusting the screws the instrument can be levelled to get perfect horizontal line of sight. Dumpy level is to be fitted to a tripod stand to use it in the field. The tripod stand consists of three legs connected to a head to which the lower plate of level can be fitted. The lower side of the legs are provided with metal shoes to get good grip with ground.

Wye or Y-Level

In this type of level, the telescope is supported in two Y-shaped supports and can be fixed with the help of curved clips. Clips can be opened and telescope can be reversed end to end and fitted. The advantage of this level is some of the errors eliminated, if the readings are taken in both the direction of telescope.

Cooke’s Reversible Level

In this instrument the telescope is supported by two rigid sockets into which telescope can be introduced from either end and then screwed. For taking the readings in the reversed position of telescope, the screw is slackened and then the telescope is taken out and reversed end for end. Thus it combines the rigidity of dumpy level and reversibility of Y-level.

Cushing's Level

In this reversing of telescope end for end is achieved by interchanging the eyepiece and the objective piece since both collars are exactly the same.

Tilting Level

In this, telescope can be tilted through about four degrees with the help of a tilting screw. Hence bubble can be easily centered. But it needs centeringof the bubble before taking every reading. Hence it is useful, if at every setting of the instrument number of readings to be taken are few.

Auto Level

The auto-level or the automatic-level is a self aligninglevel. Within a certain range of tilt automatic levelling is achieved by an inclination compensating device. The operational comfort, high speed and precision are the advantages of this instrument.

LEVELLING STAFF

Along with a level, a levelling staff is also required for levelling. The levelling staff is a rectangular rod having graduations. The staff is provided with a metal shoes at its bottom to resist wear and tear. The foot of the shoe represents zero reading. Levelling staff may be divided into two groups:

(i) Self reading staff

(ii) Target staff.

(i) Self reading staff: This staff reading is directly read by the instrument man through telescope. In a metric system staff, one metre length is divided into 200 subdivisions, each of uniform thickness of 5 mm. All divisions are marked with black in a white background. Metres and decimetres are written in red colour. The following three types of self readingstaffs are available:

(a) Solid staff: It is a single piece of 3 m.

(b) Folding staff: A staff of two pieces each of 2 m which can be folded one over the other.

upper part can slide into the central one and the central part can go into the lower part. Each length can be pulled up and held in position by means of brass spring. The total length may be 4 m or 5 m

(ii) Target staff: If the sighting distance is more, instrument man finds it difficult to read self readingstaff. In such case a target staff shown in [Fig. 15.4 (c)] may be used. Target staff is similar to self reading staff, but provided with a movable target. Target is a circular or oval shape, painted red and white in alternate quadrant. It is fitted with a vernierat the centre. The instrument man directs the person holding target staff to move the target, till its centre is in the horizontal line of sight. Then target man reads the target and is recorded.

Monday, October 9, 2017

INSTRUMENTS USED IN CHAINING

Unknown October 09, 2017 Surveying , 0 Comments

The following instruments are required for measurements with chain and tape:
(i) Arrows
(ii) Pegs
(iii) Ranging rods and ranging poles
(iv) Offset rods
(v) Laths
(vi) Whites
(vii) Plumb bobs and
(viii) Line ranger.

Arrows
When the length of the line to be measured is more than a chain length, there is need to mark the end of the chain length. Arrows are used for this purpose. Arrows are made up of 4 mm diameter steel wire with one end sharpened and other end bent into a loop. Length of an arrow is approximately 400 mm.

Pegs
Wooden pegs are used in measuring a length of a line to mark the endpoints of the line. The pegs are made of hard wood of 25 mm × 25 mm section, 150 mm long with one end tapered When driven in ground to mark station points they project about 40 mm.

Ranging Rods and Ranging Poles
For ranging intermediate points along the line to be measured, ranging rods and ranging poles are used. Ranging rods are 2 to 3 m long and are made of hardwood. They are provided with iron shoe at one end They are usually circular in section with 30 mm diameter and are painted with 200 mm colour bands of red and white or with black and white. If distance is more than 200 m, for clear visibility they
may be provided with multicoloured flags at their top.
The ranging rods are occasionally used to measure short distances since they are painted with alternate colour of band 200 mm. Ranging poles are similar to ranging rods except that they are longer. Their length varies from 4 m to 8 m and diameter from 60mm to 100 mm. They are made of hard wood or steel. They are fixed
in the ground by making 0.5 m holes and then packed to keep them vertical.

Offset Rods
These rods are also similar to ranging rods and they are 3 m long. They are made up of hardwood and are provided with iron shoe at one end. A hook or a notch is provided at other end. At height of eye, two narrow slits at right angles to each other are also provided for using it for setting right angles.

Laths
Laths are 0.5 to 1.0 m long sticks of soft wood. They are sharpened at one end and are painted with white or light colours. They are used as intermediate points while ranging or while crossing depressions.

Whites
Whites are the pieces of sharpened thick sticks cut from the nearest place in
the field. One end of the stick is sharpened and the other end is split. White
papers are inserted in the split to improve the visibility. Whites are also
used for the same purpose as laths.

Plumb Bob
A typical plumb bob is shown in Fig. 12.9. In measuring horizontal distances
along sloping ground plumb bobs are used to transfer the position to ground.
They are also used to check the verticality of ranging poles.

Line Ranger
It is an optical instrument used for locating a point on a line and hence useful for ranging. It consists of two isosceless prisms placed one over the other and fixed in an instrument with handle. The diagonals of the prisms are silvered so as to reflect the rays. To locate point C on line AB the surveyor holds the instrument in hand and
stands near the approximate position of C. If he is not exactly on line AB, the ranging rods at A and B appear separated The surveyor moves to and fro at right angles to the line AB till the images of ranging rods at A and B appear in a single line . Ithappens only when the optical square is exactly on line AB. Thus the desired point C is located on the line AB. Its advantage is it needs only one person to range. The instrument should be occasionally tested by marking three points in a line and standing on middle point observing the coincidence of the ranging rods. If the images of the two ranging rods do not appear in the same line, one of the prism is adjusted by operating the screw provided for it.

Saturday, October 7, 2017

CHAIN SURVEYING

Unknown October 07, 2017 Surveying , 0 Comments

Chain survey is suitable in the following cases:
(i) Area to be surveyed is comparatively small
(ii) Ground is fairly level
(iii) Area is open and
(iv) Details to be filled up are simple and less. In chain surveying only linear measurements are made i.e. no angular measurements are made.
Since triangle is the only figure that can be plotted with measurement of sides only, in chain surveying the area to be surveyed should be covered with a network of triangles. scheme of covering an area with a network of triangles.
No angle of the network triangles should be less than 30º to precisely get plotted position of a station with respect to already plotted positions of other station. As far as possible angles should be close to 60º. However, the arrangements of triangles to be adopted depends on the shape, topography, natural and artificial obstacles in the field.