Offsets and tie lines PDF

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Lecture notes on Offsets and tie lines...

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Module 2 LINEAR MEASUREMENTS Learning Outcomes On the completion of this module you should be able to:  

Demonstrate measuring offsets and ties by optical square and tape Describe and recording of measurements takes in a field by recognized booking method. Explain the identification and correction of field work errors Describe the factors which govern chain survey framework Demonstrate the application of chain survey principles to a small practical situation Demonstrate the plotting of survey lines including all detail.

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Offsets and tie lines

Offset lines These are the lateral measurements from the base line to fix the positions of the different objects of the work with respect to base line. These are generally set at right angle offsets. It can also be drawn with the help of a tape. There are two kinds of offsets: 1. Perpendicular offsets The measurements are taken at right angle to the survey line called perpendicular or right angled offsets. Perpendicular offsets may be taken in the following ways: (a) By setting a perpendicular by swinging a tape from the object to the chain line. The point of minimum reading on the tape will be the base of the perpendicular (b) By setting a right angle in the ratio 3 : 4 : 5 (c) By setting a right angle with the help of builder’s square or tri-square (d) By setting a right angle by cross-staff or optical square.

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Offset Method 1. Set up two control points on site, through the middle of the area to be recorded ensuring a clear line of sight between them. 2. Lay a tape measure between the two points, ensure that the tape is at the correct tension. 3. Ensure each point is securely fixed, as it will be pulled on by the tape measure. 4. One diver visits each feature and holds the free end of the tape measure. The other diver records the distance from the feature to the baseline and the distance along the baseline, where the tape crosses the baseline at right angles. 5. Record the depth of each feature and seabed type at the same time.

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The right angle point can be found by swinging the tape measure and finding where the distance is the shortest Note which side of the baseline the feature is located The distance from the control point should be limited to 10m for assessment surveys, with this method position accuracy reduces with distance so keep the measurements short. In poor visibility it may be useful to have a second diver swim up and down the tape to ensure that it is not snagged. By setting a right angle in the ratio 3 : 4 : 5

To set out right angles in the field, a measuring tape, two ranging poles, pegs and three persons are required. The first person holds together, between thumb and finger, the zero mark and the 12 metre mark of the tape. The second person holds between thumb and finger the 3 metre mark of the tape and the third person holds the 8 metre mark. When all sides of the tape are stretched, a triangle with lengths of 3 m, 4 m and 5 m is formed (see Fig. 2.22), and the angle near person 1 is a right angle. NOTE: Instead of 3 m, 4 m and 5 m a multiple can be chosen: e.g. 6 m, 8 m and 10 m or e.g. 9 m, 12 m and 15 m.

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Figure 2.22

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By setting a right angle by optical square.

Optical squares are simple sighting instruments used to set out right angles. They can be provided either with mirrors or with one or two prisms. Because of practical difficulties in using squares with mirrors, they have been replaced by squares with prisms: "prismatic squares".

Figure 2.23

The procedure to follow is: Step 1 The operator should stand with the instrument on the base line (connecting A and B). To check this, the assistant, standing behind pole (A) (or B), makes sure that the plumb bob, attached to the instrument, is in line with poles (A) and (B) (see Fig. 2,24a). The operator then rotates the instrument until the image of pole (A) can be seen.

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Figure 2.24a Step 2

The operator then moves the instrument along the base line until he finds a position for which (when looking through the instrument) pole (C) is in line with the image of pole (A) (see Fig. 2.24b). While searching for the right position, the operator must keep the instrument always in line with poles (A) and (B). This is done under the guidance of the assistant standing behind pole (A).

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Figure 2.24b Step 3 When the correct position of the instrument is found, peg (D) is placed right under the plumb bob. The line connecting pole (C) and peg (D) is a line perpendicular to the base line (see Fig. 2,24c).

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Figure 2.24c

Figure 2.25

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2. Oblique offsets The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.

Figure 2.26

Figure 2.27

Note: Offset distances must be kept as short as possible to preserve the accuracy of locating detail, since the right angle to the survey lines is usually estimated by eye. Suggested maximum offset lengths under these conditions are 5 metres on a scale of 1/1000 and 2,5 metres on a scale of 1/500. 

Tie or subsidiary lines:

A tie line joints two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, building etc. 

Ties Method 1. Set up two control points on site, through the middle of the area to be recorded ensuring a clear line of sight between them. 2. Lay a tape measure between the two points, ensure that the tape is at the correct tension. 3. Ensure each point is securely fixed, as it will be pulled on by the tape measure. 4. One diver visits each feature and holds the free end of the tape measure. The other diver records the distance from the feature to two points on the baseline. 5. Note which side of the baseline the feature is located, left or right. The left side of the baseline is on the left when looking down the tape measure from the zero end.

Other points to remember:

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The distance between the points on the baseline should be roughly the same as the distance between the feature and the baseline

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The distance from the control point should be limited to 30m for assessment surveys

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In poor visibility it may be useful to have a second diver swim up and down the tapes to ensure that they are not snagged

Running survey line

By this is meant the routine of chaining the line, and locating the detail adjacent to it, by means of offsets.

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For chain surveying, at least two men are required, but frequently three people are employed. They are:   

The surveyor, who does the reading and booking, The leader, and The follower.

(i) To start the chaining of a line the follower holds the zero end of the chain in contact with the peg at the beginning of the line and presses the handle with his feet and stands firmly over it. (ii) The leader holds another end of the chain and goes along with the arrows and ranging rods on the line. (iii) Nearly at the end of the chain length, he stops and aligns with the help of ranging rod which he keeps vertical and faces the follower, who gives him instructions by his arms. (iv) After alignment, the leader pulls the chain and inserts an arrow in to the ground to mark the end. (v) The lateral measurements or offsets are taken from the chain line to any object that is to be plotted on the plan. (vi) The chain line should be such that these offsets are as short as possible. While pulling the chain, care should be taken. (vii) After taking the offsets, the leader picks up the staff rod and remaining arrows keeping the chain a little away from the line so that the arrow placed is not disturbed, starts moving ahead as before. (viii) As the follower reaches the arrow with the near end of the chain, he should speak loud “chain” or “tape” to give a warning to the leader that he has nearly reached the arrow or a chain length and immediately the leader stops. (ix) The follower holds the handle against the arrow and directs the leader to come in line as before. (x) The leader again stretches the chain and fixes the arrow in the ground at another chain length or make a cross if the ground is firm. (xi) Again, the leader walks in the line in the same manner and the follower now picks up the first arrow, comes to the second arrow and gives instructions for the third chain length. (xii) Thus, the whole process is repeated until the end of the line is reached. (xiii) The number of arrows with the follower is an indication of the number of full chain lengths completed at any time.

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(xiv) After some time the number of arrows should be checked mutually by the follower and the leader so that no chain length is missed and no arrow is lost. Generally, the number of arrows taken is ten and hence after fixing the tenth arrow, the leader speaks out “arrows” which means that this was the tenth chain line. (xv) The follower then goes to the tenth arrow and picks it up after fixing a ranging rod there. The arrows are then handed over to the leader, and a record is made in the field book by the surveyor. (xvi) For the fractional length of the Chain, the leader stretches the chain beyond the end station. While the follower holds the rear handle of the chain against the last arrow. The leader reads the fractional chain length loudly, and the surveyor notes the entire length of the line.

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Figure 2.28

Check line – The line joining between the apex of triangle and some fixed point on survey line or on base line is called as check line. 2.2 THE FIELD BOOK The notebook in which field measurements are noted is known as the ‘field book’. The size of the field book is 20 cm x 12 cm and it opens lengthwise. Field books may be of two types:

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1. Single –line, and 2. Double-line.

1. Single-line field book In this type of field book, a single red line is drawn through the middle of each page. This line represents the chain line, and the chainages are written on it. The offsets are recorded, with sketches, to the left or right of the chain line. The recording of the field book is started from the last page and continued towards the first page. The main stations are marked by “and subsidiary stations or tie stations are by “ 2. Double-line field book In this type of field book, two red lines, 1.5 cm apart, are drawn through the middle of each page. This column represents the chain line, and the chainages are written in it. The offsets are recorded, with sketches, to the left or right of this column. The recording is begun from the last page and continued towards the first. The main stations are marked by “and subsidiary or tie stations by “This type of field book is commonly used. 2.2.1 Recording the measurements in the Field Book (Field Measurement Book) The book in which the chain or tape measurements are entered is called the field book. It is an oblong book of size from 10cm x 15cm to about 13 cm x 23 cm and opens lengthwise. The main requirements of the field book are that it should contain good quality stout opaque paper. it should be well-bound and of a size convenient for the pocket. The chain line may be represented either by a single line or by two lines spaced about 1 to 2 cm apart. ruled down the middle of each page. The double line field book as per below drawing no-1 is most commonly used for ordinary work, the distance along the chain being entered between the two lines of the page.

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Single Line Filed Book No 1 Single line field book as per above drawing no-1 is used for a comparatively large scale and most detailed dimension work. A chain line is started from the bottom of the page and works upwards. All distances along the chain line are entered in the space between the two ruled lines while the offsets are entered either to the left or to the right of the chain line, as the case may be.

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Offsets are entered in the order they appear at the chain line. As the various details within offsetting distances are reached, they are sketched and entered as shown in as per the above single line filed drawing no-1 and as per below drawing no-2 Every chain line must be stained from a fresh page. All the pages must be numbered.

Double line Filed book drawing no-2

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Sample of field book recording In addition to the previous instructions upon booking, the following notes are appended: 

neat figures and legible printing,

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clear sketches,

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clearness in representing the points to which offsets have been taken,

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add explanatory notes where necessary, to avoid ambiguity,

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cross-index wherever possible,

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leave nothing to the memory,

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use a hardish pencil (about 2H) and keep it sharp,

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keep the field book clean.

The field book must include: 1. Title page 2. Index 3. Index diagram 4. Recording of the field work 5. Separate sketches of detail too intricate to show on the chainage pages

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The title page will give: 

An accurate description of the area surveyed,

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A list of the tapes and or other apparatus used,

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Name of Surveyor, chainman and booker,

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Date of survey

Index

The index records: • locality covered • gives book number, and • item number

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Index diagram This consists of a diagrammatic sketch, not to scale, of the survey lines. It shows the following:

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The number of each survey line,

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The name of each survey station,

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The length of each survey line, written against the line and reading in the direction of measurement so that the whole skeleton may be plotted from the diagram,

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A reference to the book page number where detail surveyed from each line is recorded.

2.2.2. Precautions to be Taken While Entering the Field book 1. All measurements should be noted as soon as they are taken. 2. Each chain line should be recorded on a separate page. Normally it should start from the bottom of one page and end on the top of another. No line should be started from any intermediate position. 3. Over –writing should be avoided. 4. Figures and hand-writing should be neat and legible. 5. Index-sketch, object-sketch and notes should be clear. 6. Reference sketches should be given in the field book, so that the station can be located when required. 7. The field book should be entered in pencil and not in ink. 8. If an entry is incorrect or a page damaged, cancel the page and start the entry from a new one. 9. Erasing a sketch, measurement or note should be avoided. 10. The surveyor should face the direction of chaining so that the left-hand and right-hand objects can be recorded without any confusion. 11. The field-book should be carefully preserved. 2.2.3 PROCEDURE OF FIELD WORK Field work of chain survey should be carried out according to the following steps: 

Reconnaissance

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Before starting survey work, the surveyor should walk over the whole area to be surveyed in order to examine the ground and determine the possible arrangement of framework of survey. During this investigation, he should examine the intervisibility of the main survey stations. He should ensure that the whole area is enclosed by main survey lines, and also that it is possible to form well-conditioned triangles. He should observe various objects and boundary lines carefully and select the survey lines in such a manner that the objects can be located by short offsets. The base line should preferably be taken through the centre of the area and on fairly level ground. 

Index sketch

After preliminary inspection of the area, the surveyor should prepare a neat hand sketch showing the arrangement of the framework and approximate position of the objects. He should note the names of the stations on the sketch maintaining some order (clockwise or anticlockwise). The field work should be executed according to this index sketch. The names and sequence of chain lines should be followed as directed in the index sketch. The ‘base line’ should be clearly indicated in the index sketch. 

Marking the stations on the ground

After reconnaissance, the stations are marked on the ground by wooden pegs. These pegs are generally 2.5 cm square and 15 cm long, and have pointed ends. They are driven into the ground firmly, and there should be a height of 2.5 cm above the ground. The station point is marked with a cross so that it can be traced if the wooden peg is removed by somebody. 

Reference sketches

To take precautions against station pegs being removed or missed, a reference sketch should be made for all main stations. It is nothing but a hand sketch of the station showing at least two measurements from some permanent objects. A third measurement may also be taken. 

Taking measurements of survey lines and noting them in the field book

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Ranging and chaining is started from the base line, which should be measured carefully. The magnetic bearings

of the base

line

are measured by

prismatic compass.

These

measurements are noted in the field book showing the offsets to the left or right according to their position. Then the other survey lines are ranged and chained maintaining the sequence of the traverse. The offsets and other field records are noted simultaneously. The check lines and tie lines are also measured and noted at the proper place. The station marks are preserved carefully until field work is completed.

2.2.4 Sketches To the inexperienced surveyor, the making of satisfactory sketches is often troublesome. The tendency is to allow insufficient room for sketches of intricate detail, so that dimensions cannot be entered legibly. N attempt should be made to sketch strictly to scale. All dimensions of buildings and similar detail should be measured. Even if the two ends of a wall is fixed, the length of the wall should still be measured, so that he building can be separately plotted.

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Identification and correction of field errors

The errors formed in chaining can be either cumulative error and compensating error. The errors can be considered either as a negative value or positive value depending on whether the result is a higher value or smaller value.

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Cumulative Error and Compensating Error The error that occurs during the chaining process in the same direction is called as cumulative error. This type of error accumulates with the process of chaining.

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An error that occurs in either direction during the chaining process is called as compensating error. As these errors takes place in either directions, the values compensate.

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Causes of Errors The basic reasons for errors caused in the chaining process in surveying are due to:

1. Erroneous Length of the Chain or the Tape This error is a cumulative error that can either take a negative or a positive value. This is the error due to the wrong length of the chain which is considered as one of the serious error. If the length of the chain is long, then the measured distance is smaller ...