undefined | unit 1 linear measurement and chain survey

Unit - 1

Linear Measurements and Chain Survey

Q1) Explain Chain Survey in detail?

A1)

In traverse survey a number of survey lines are connected to form a framework and the lengths and directions of the survey lines are measured with the help of an angle measuring instrument and a chain respectively.

If the survey lines forming framework ends back on the starting point, it is known as closed traverse i.e. survey starts from station A, proceeds through stations B, C, D and E and back to station A.

If the survey lines forming framework ends elsewhere, it is said to be an open traverse i.e. survey starts from station A, and ends at station F.

The closed traverse is suitable for locating any existing structure, woods, boundaries of plots or lakes etc. Whereas the open traverse is suitable for locating details of long narrow strips like canal, roads or coast lines.

$H:\fig\IMG_20210604_104436.jpg$

Fig.1.1: Compass Survey

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Fig.1.2: Compass Survey

Q2) Explain uses of Chains for correct measurement of length?

A2) Metric chain :

Metric chains are available in lengths of 20m and 30m. The 20m chain is divided into 100 links, each of 0.2m. Tallies are provided at every 10 links (2m). This chain is suitable for measuring distances along fairly level ground. The arrangement of tallies is shown in Fig.

$H:\fig\IMG_20210604_104523.jpg$

Fig.1.3: 20 m chain

You may see from the arrangement of tallies that the central tally is round and that the other tallies have one, two, three or four teeth. So, each tooth may correspond to two different readings when considered from opposite ends. Therefore, during the measurement, the surveyor should bear in mind the position of the central tally.

As per ISI recommendations, tallies should be provided after every 5m and brass rings after every 1 m In Fig., the central tally has two teeth and the tallies on opposite sides of it have one tooth each.

The 30m chain is divided into 150 links. So, each link is of 0.2m. The tallies are provided after every 25 links (5m). A round brass ring is fixed after every metre. This chain is heavy and is also suitable for measuring distances along fairly level ground. Here the central tally has three teeth.

Steel Band:

It consists of a ribbon of steel of width 16mm and of length 20 or 30m. It has a brass handle at each end. It is graduated in meters, decameters, and centimeters on one side and has 0.2m links on the other. The steel band is used in projects where more accuracy is required.

Engineer's chain:

The engineer's chain is 100 ft long and is divided into 100 links. So, each link is of 1ft. Tallies are provided at every 10 links (10 ft), the central tally being round. Such chains were previously used for all engineering work.

Gunter's chain :

It is 66 ft long and divided into 100 links. So, each link is of 0.66. It was previously used for measuring distances in miles and furlongs.

Revenue chain :

The revenue chain is 33 4t long and divided into 16 links. It is mainly used in cadastral survey.

Q3) Explain use of Tapes for measurement of correct length?

A3) Cloth or linen tape:

It is made of closely woven linen and is varnished to resist moisture.

It is 15 mm wide and available in length of 10 and 15 m.

It is used for measuring offsets and for ordinary works.

Metallic tape:

When linen tape is reinforced with brass or copper wires to make it durable it is called as metallic tape.

This tape is available in length 15, 20 and 30 m.

It is mostly used for all survey works.

Steel tape:

It is made of steel ribbon of width varying from 6 to 16 mm.

This tape is available in length are 10, 15, 20, 30 and 50 m.

It is graduated in meters, decimeters and centimeters.

It is not used in the field but used for measurements in construction works.

Invar tape:

It is made of an alloy of steel (64%) and nickel (36%)

Its thermal coefficient is very low.

This tape is available in length 30, 50 and 100m.

It is used in the triangulation survey conducted by the survey of India department.

Q4) Explain chaining along sloping ground?

A4) There are strategies for figuring out horizontal distance on sloping floor.

Direct Method

Indirect Method

Direct Method of Chain Surveying:

This technique is likewise acknowledged as “Stepping Method”.

The horizontal distances are without delay measured with the aid of using the procedure of stepping.

Procedure:

A route of chain or tape is stretched out from ‘P’.

The route period of chain or tape relies upon at the steepness of the floor.

The follower holds the 0 give up of the chain at ‘P’ and directs the chief at P1 to be withinside the line of PQ and stretch the chain or tape above the floor in horizontal line.

The chief then transfers the factor ‘P1’ to P2 at the floor with the aid of plumb bob or losing a pebble or an arrow,

Now the fans take the brand new role ‘P2’ and directs the chief to transport ahead and stretch the tape or chain in a line of PQ

Now the fans take the brand new role ‘P2’ and directs the chief to transport ahead and stretch the tape or chain in a line of PQ and the brand new role is P3.

Again the chief transfers the factor P3 to P4 at the floor as completed earlier.

This procedure is repeated until the factor Q is reached.

Horizontal distance PQ = S1 + S2 + S3 + S4 + S5

Indirect Method of Chain Surveying:

In this technique, the sloping aspect is measured at the floor and later it's far transformed into the horizontal equal the usage of geometrical condition.

Q5) What are the obstacles in chaining?

A5)

The 3 fundamental limitations in chaining of a line are of the subsequent types:

1. Chaining Free, Vision Obstructed

2. Chaining Obstructed, Vision Free

3. Chaining and Vision Both Obstructed.

It every so often takes place that a survey line passes via a few item including a pond, a building, a river, a hedge etc. which prevents the direct dimension of that a part of the road which the item intersects. The interfering item in any such case is known as on obstacle.

It is vital to conquer limitations in order that chaining can be endured in a directly line. Special techniques are, therefore, hired in measuring distances throughout the limitations.

Q6) Explain Compass Surveying?

A6)

The procedure of chain and compass survey consists of the following procedure:

Detailed inspection of the site where the survey is to be done is made and main stations are selected. Pegs are driven to fix up the stations and ranging rods erected just behind the pegs, so that the stations are visible from far of distance. The existing building is to be located on plan. Stations ABCDE are fixed.

While selecting stations following criteria can be used:

The selected stations should be intervisible.

Obstacles between two chain stations should be minimum.

The survey lines should run through levelled ground as far as possible.

There should not be any source of local attraction like electricity poles near to the survey stations.

Location sketches of at least two stations are drawn say of stations A and E. In Fig. with respect to E.P. and M.H. Their locations are determined with references to some permanent features.

Location sketch is a rough sketch of the station, the location of which is determined with reference to some permanent features like building or poles etc. Location sketches are useful if the survey station is to be relocated after the survey work is over.

Chaining is done between AB, BC, CD, DE and EA for finding distance between the peg stations. While chaining is done, off setting is also done to locate the details of the structure inside the closed traverse ABCDEA.

After completion of chaining and offsetting, the compass is centred on stations A, B, C, D. E each time and fore bearing and back bearings are observed at all the stations of all the respective lines. e.g. when compass is centred on station A, the bearing of line AB is observed by bisecting ranging rod B with compass called fore bearing of line AB. Similarly ranging rod at E is also bisected with compass centred at A called backbearing of line EA.

At the end an observation table consisting of line, length, observed bearings and corrected bearings is prepared.

After the bearings of all the lines are corrected the traverse is plotted on the sheet.

$H:\fig\IMG_20210604_104541.jpg$

Fig.1.4: Compass Surveying

Q7) Explain Temporary adjustment of Prismatic compass?

A7)

To take a studying from a compass the subsequent temparory of adjustment is required-

Centering – the compass have to be constant to the stand and set over the station. To the middle, the compass legs of the tripod stand have to be moved inward- outward or withinside the circumference direction. To test centering plumb can be used a pebble from the middle of the compass.

Leveling – in compass surveying ideal leveling isn't always vital however it have to be enough to allow loose suspension of the magnetic needle. After centering bubble have to be ensured withinside the center of the circle supplied for it withinside the level.

Focusing the prism – in prismatic compass to cognizance the prism at the graduated circle, it’s attachment is slid up and down until the studying are absolutely visible.

Q8) Explain Whole Circle bearing?

A8)

When the bearing of a line is measured with respect to magnetic north in clockwise direction, it is called as magnetic bearing or whole circle bearing.

The value of W.C.B. varies from 0° to 360°.

Four quadrants I, II, III and IV are shown in Fig.

The W.C.B. of a line OA in the first quadrant is 0, (0, between 0° to 90°).

The W.C.B. of a line OB in the second quadrant with respect to north is

₂ (

, between 90° and 180°) W.C.B. of a line OC in the third quadrant with respect to north is

, (

, between 180° and 270°) and W.C.B. of a line OD in the fourth quadrant with respect to north is

, (

, between 270° and 360°). See Fig.

$H:\fig\IMG_20210604_104602.jpg$

Fig.1.5: Whole Circle Bearing

Q9) Explain Reduced Bearing?

A9)

In reduced bearing, the bearing of survey lines is measured with respect to north and south line either in clockwise or anticlockwise direction towards east or west. Fig. shows quadrantal bearing system.

Any line in any of the four quadrant is expressed by an angle which is measured from either north or south.

The directions are essentially written before and after the value of angle.

This helps to identify in which quadrant the lines lie. The maximum bearing in such case is 90°.

$H:\fig\IMG_20210604_104620.jpg$

Fig.1.6: Reduced Bearing

Q10) What is mean by Local attraction?

A10)

When the prismatic compass is located over any station by centering, then the freely suspended magnetic needle will point towards the north direction (magnetic meridian);

But if the compass is centered nearby iron or steel structure or electric cable line such as L.T. line or transmission tower, lamp post carrying high voltage, steel button of shirt or steel wrist watch, bunch of keys, steel tape etc.

Then it is observed that the magnetic needle of prismatic compass will not point exactly towards north but will deflect from its normal position.

In short, the magnetic needle is under the influence of some external forces causing the deflection of needle from its normal position and showing some error in bearings of lines.

Therefore, it is concluded that the deviation of the magnetic needle from its normal position (North direction) under the influence of external forces is called as local attraction.

Local attraction shows some error in bearings of lines, therefore while selecting the stations for compass survey, objects made up of steel shall possibly be avoided.

Detection of Local Attraction :

The presence of local attraction can be detected or identified by observing the difference between the observed fore bearing (F.B.) and back bearings (B.B.) of each line.

If the difference between F.B. and B.B. is not exactly 180°, then the stations on the sides of a line are affected by local attraction.

If the difference between F.B. and B.B. is exactly 180°, then the stations on both the sides of a line are free from local attraction, or the amount of local attraction at both stations is same,

Note: (1) If F.B.-B.B. =180°; free from local attraction / same local attraction.

(2) IF.B.-B.B.180": affected by local attraction.

Q11) Explain the methods for correction for bearing?

A11) There are two methods by which the effect of local attraction can eliminated or can be adjusted.

Method 1:

In this method, the difference in fore bearings (F.B.) and back bearings (B.B.) of all the survey lines of the traverse are observed.

If for the line, difference between F.B. and B.B. is exactly equal to 180°, then this line is selected and the process of applying correction is started from bearings of this line itself and then corrections are applied to the observed bearings of other lines.

It is noted that, if the observed bearing is less than the corrected bearing, the error is negative and correction applied to the bearing is positive.

If the observed bearing is more than the corrected bearing the error is positive and correction applied to the bearing is negative.

Method II :

In this method, the included angles between the lines are calculated at all the stations.

If the traverse is of closed type, the sum of the internal included angles (2n-4) x 90° where n is number of sides of the closed traverse.

If there is any discrepancy in the total of the included angles, then discrepancy or error is distributed equally to all the angles.

When all the angles are corrected then, difference between F.B. and B.B. of each line is found to be 180°. This method is based on the fact that the included angles calculated from incorrect bearings will be correct, since the amount of error is same for all the bearings at the station.

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