EDM

Electronic distance measuring instrument is a surveying instrument for measuring distance electronically between two points through electromagnetic waves.Electronic distance measurement (EDM) is a method of determining the length between two points, using phase changes, that occur as electromagnetic energy waves travels from one end of the line to the other end. As a background, there are three methods of measuring distance between two points:

DDM or Direct distance measurement – This is mainly done by chaining or taping.

ODM or Optical distance measurement – This measurement is conducted by tacheometry, horizontal subtense method or telemetric method. These are carried out with the help of optical wedge attachments.

EDM or Electromagnetic distance measurement – The method of direct distance measurement cannot be implemented in difficult terrains. When large amount of inconsistency in the terrain or large obstructions exist, this method is avoided.As an alternative to this optical distance measurement method was developed. Still it gained a disadvantage of limited range of measurement. It is limited to 15 to 150m with an accuracy of 1 in 1000 to 1 in 10000. Above all we have EDM with an accuracy of 1 in 105, having a distance range of 100km.

Electronic distance measurement in general is a term used as a method for distance measurement by electronic means. In this method instruments are used to measure distance that rely on propagation, reflection and reception of electromagnetic waves like radio, visible light or infrared waves.

Sun light or artificially generated electromagnetic wave consists of waves of different lengths. The spectrum of an electromagnetic wave is as shown below:

                                             Among these waves microwaves, infrared waves and visible light waves are useful for the distance measurement. In EDM instruments these waves are generated, modulated and then propagated. They are reflected at the point up to which distance is to be measured from the instrument station and again received by the instrument.

The time taken by the wave to travel this 2x distance may be measured and knowing the velocity of wave, the distance may be calculated. However time is too short, measuring the time taken is difficult. The improved techniques use phase difference method in which the number of completed wave and incomplete wave is measured. Knowing the length of wave, distances are calculated.  Built up microprocessors provided in the instrument calculate the distances and display it by liquid crystal display (LCD).

Origin of Electronic Distance Measurement

Electronic distance measurement can be done by instruments like geodimeter, tellurometer or distomat etc. The first EDM instrument called geodimeter was developed in Sweden in the year 1948. Geodimeter is geodetic distance meter developed based on a modulated light beam.

The second instrument for EDM was designed and developed in Africa in the year 1957, named tellurometer. This instrument employs modulated microwaves.

As years passed technology has improved drastically. At present, we have modern EDMs that displays distance in digital form and many gains microcomputers that calculates horizontal and vertical distance i.e. DX and DY. They also show sloped distance (DH).

Electronic distance measurement equipments are incorporated along with theodolites that possess automatic angle readout called as total station (electronic tacheometers) also called as field to finish systems. These record distance and angles simultaneously.

Types of Electronic Distance Measurement Instrument

EDM instruments are classified based on the type of carrier wave as

1. Microwave instruments
2. Infrared wave instruments
3. Light wave instruments.

1. Microwave Instruments

These instruments make use of microwaves. Such instruments were invented as early as 1950 in South Africa by Dr. T.L. Wadley and named them as Tellurometers. The instrument needs only 12 to 24 V batteries. Hence they are light and highly portable. Tellurometers can be used in day as well as in night.

The range of these instruments is up to 100 km. It consists of two identical units. One unit is used as master unit and the other as remote unit. Just by pressing a button, a master unit can be converted into a remote unit and a remote unit into a master unit. It needs two skilled persons to operate. A speech facility is provided to each operator to interact during measurements.

2. Infrared Wave Instruments

In this instrument amplitude modulated infrared waves are used. Prism reflectors are used at the end of line to be measured. These instruments are light and economical and can be mounted on theodolite. With these instruments accuracy achieved is ± 10 mm. The range of these instruments is up to 3 km.

These instruments are useful for most of the civil engineering works. These instruments are available in the trade names DISTOMAT DI 1000 and DISTOMAT DI 55.

3. Visible Light Wave Instruments

These instruments rely on propagation of modulated light waves. This type of instrument was first developed in Sweden and was named as Geodimeter. During night its range is up to 2.5 km while in day its range is up to 3 km. Accuracy of these instruments varies from 0.5 mm to 5 mm/km distance. These instruments are also very useful for civil engineering projects.

Operations of Electronic Distance Measurement Instruments

It is essential to know the fundamental principle behind EDM to work with it. The electromagnetic waves propagate through the atmosphere based on the equation

  Where ‘v’ is the velocity of electromagnetic energy in meters per second(m/sec); f is the modulated frequency in hertz (Hz) and is, the wavelength measured in meters. Mainly the waves that are propagated can be represented like a sine wave as shown in figure below.

Another property of wave called as phase of wave , is a very convenient method of small fraction of wavelength during measurement in EDM. The points A, B, C etc. represents various phase points

 

                                                  Say AB is the survey line to me measured, having a length of D. The EDM equipment is placed at ends A and B. A transmitter is placed at A and a receiver is placed at B. the transmitter lets propagation of electromagnetic waves towards B. A timer is also placed. At the instant of transmission of wave from A the timer at B starts and stops at the instant of reception of incoming wave at B. This enable us to know the transit time for the wave from the point A to B.

                                                 From the transit time and known velocity, the distance can be easily measured. Now to solve the problem arise due to difficulty in starting the timer at B, a reflector can be placed as shown below instead of a receiver at B.

    

Measurement of distance with EDM and a Reflector

As explained let the waves get transmitted from A and reflected from B. If the received signal is out of phase by a measure of , then equivalent distance is

                                                                                   Thus, the distance

                                                                   where n is the integral number of wavelength,  in the double path

Error in Electronic Distance Measurement Instruments

Personal Errors

• Inaccuracy in initial setups of EDMs and the reflectors over the preferred stations
• Instrument and reflector measurements going wrong
• Atmospheric pressures and temperature determination errors

Instrumental Errors

• Calibration errors
• Chances of getting maladjusted time to time generating frequent errors
• Errors shown by the reflectors

Natural Errors

• Atmospheric variations in temperature, pressure as well as humidity. Micro wave EDM instruments are more susceptible to these.
• Multiple refraction of the signals.

The advantage of using EDM instruments is the speed and accuracy in measurement. Several obstacles to chaining are automatically overcome when these instruments are used.

Total station

Capability of a Total Station:

Microprocessor unit in total station processes the data collected to compute:

1. Average of multiple angles measured.
2. Average of multiple distance measured.
3. Horizontal distance.
4. Distance between any two points.
5. Elevation of objects and
6. All the three coordinates of the observed points.
   

Data collected and processed in a Total Station can be downloaded to computers for further processing.

Total station is a compact instrument and weighs 50 to 55 N. A person can easily carry it to the field. Total stations with different accuracy, in angle measurement and different range of measurements are available in the market. Figure below shows one such instrument manufactured by SOKKIA Co. Ltd. Tokyo, Japan.

Brief Description of Important Operations of Total Station:

Distance Measurement:

Electronic distance measuring (EDM) instrument is a major part of total station. Its range varies from 2.8 km to 4.2 km. The accuracy of measurement varies from 5 mm to 10 mm per km measurement. They are used with automatic target recognizer. The distance measured is always sloping distance from instrument to the object. Angle Measurements: The electronic theodolite part of total station is used for measuring vertical and horizontal angle. For measurement of horizontal angles any convenient direction may be taken as reference direction. For vertical angle measurement vertical upward (zenith) direction is taken as reference direction. The accuracy of angle measurement varies from 2 to 6 seconds.

Data Processing :

This instrument is provided with an inbuilt microprocessor. The microprocessor averages multiple observations. With the help of slope distance and vertical and horizontal angles measured, when height of axis of instrument and targets are supplied, the microprocessor computes the horizontal distance and X, Y, Z coordinates. The processor is capable of applying temperature and pressure corrections to the measurements, if atmospheric temperature and pressures are supplied.

Display:

Electronic display unit is capable of displaying various values when respective keys are pressed. The system is capable of displaying horizontal distance, vertical distance, horizontal and vertical angles, difference in elevations of two observed points and all the three coordinates of the observed points.

Electronic Book:

Each point data can be stored in an electronic note book (like compact disc). The capacity of electronic note book varies from 2000 points to 4000 points data. Surveyor can unload the data stored in note book to computer and reuse the note book.

Use of Total Station

The total station instrument is mounted on a tripod and is levelled by operating levelling screws. Within a small range instrument is capable of adjusting itself to the level position. Then vertical and horizontal reference directions are indexed using on board keys. It is possible to set required units for distance, temperature and pressure (FPS or SI). Surveyor can select measurement mode like fine, coarse, single or repeated.

When target is sighted, horizontal and vertical angles as well as sloping distances are measured and by pressing appropriate keys they are recorded along with point number. Heights of instrument and targets can be keyed in after measuring them with tapes. Then processor computes various information about the point and displays on screen.

Use of theodolite

USE of THEODOLITE

  

 Introduction

The theodolite is a versatile instrument and is commonly used for the following tasks.

a)      Measurement of horizontal angles

b)      Measurement of vertical angles

c)      Setting out horizontal angles

d)      Ranging
                                                                                              

e)      Levelling

f)        Optical distance measurement

g)      Controlling verticality

 Measurement of horizontal angles

The reiteration method is a common method of observing horizontal angles.  The procedure is as follows:

a) Accurately center and level the theodolite over a ground mark

b)      Sight the left hand target (face left) with a small reading on the plate using the lower plate clamp and slow motion screw.  Do not touch the lower plate again

during this round of angles.  If several rounds of angles are to be observed, the initial plate setting is changed by about 90 each time.

c)      Sight on to the right hand target(s) using the upper plate clamp and slow motion screw, noting the reading each time.

d)      When the last target has been sighted, change face,  This is done by rotating the telescope vertically through 180 and the upper plate horizontally though

180 to sight back onto the last target.

e)      If face right re-observe all the targets.

f)      It is essential that the plate readings are checked for accuracy on completion of each round of angles.  Check that there is 180 difference between the

readings.  Any variation from the 180 difference is an indication of instrumental error and should be reasonably constant.  This will discover gross errors due to

misreading scales, using wrong slow motion screws, sighting wrong targets, etc. The targets can be re-sighted and the readings corrected before changing the

lower plate.

g)        Horizontal plate readings and reduced angles can be recorded in a standard field book.

Note the different initial plate settings for each round, the use of the remarks column and the summary of angles.

The operation of one second theodolites is practically the same as that outlined above.  The only difference occurs during the initial sighting of the left hand target.

Sight the target first and then set the required plate reading.

 Measurement of vertical angles

Vertical angles are useful in applying slope corrections to distance measurement and for determining reduced levels of inaccessible points.

The observing procedure is practically the same for all theodolites.

a)      Sight the target with the horizontal cross wire.

b)      Level the altitude bubble, unless the instrument has automatic vertical indexing in which case there may be a release button to press

c)      After adjusting the micrometer note the plate reading.

d)      Change face and repeat

The orientation of the vertical circle varies from one instrument to another and several examples are in Figure 4.6.  Study your theodolite carefully as it is

necessary to reduce vertical angles.
     Levelling

The theodolite could be used for leveling provided a number of precautions are taken.

a) The altitude bubble should be centred and the telescope locked with a vertical angle of exactly 00-00-00,

b) Read the staff.

a) Change face and repeat the above steps

b) The mean of the two staff readings will give a reasonable result over short distances.

Levelling by theodolite must never be regarded as an acceptable alternative to the surveyor’s level where accuracy is needed.

        Optical distance measurement



Horizontal distances can be measured using theodolite and leveling staff.  These distances can be accurate to 0.1 m and cannot be used where accuracy is

required.

4              Sight a vertically held leveling staff and read the staff where it is cut by the horizontal crosswire and the two stadia hairs.

5              Check the staff readings.  The difference between center and top readings should equal difference between centre and bottom readings.  Read the staff again if there is a disagreement.

6              Note the vertical angle after levelling the altitude bubble.

7              Compute the horizontal distance from

100 xsxcos2  vertical angle

where s= difference between top and bottom stadia readings
 Sources of error

(a) Instrumental errors

      The geometry of a theodolite comprises three axes directly related to ech other called vertical, trunnion and optical axes.

When the theodolite is in adjustment:

i)The vertical axis is vertical

ii)The trunnion axis is at right angles to the vertical axis

iii)The optical axis is at right angles to the trunnion axis.

iv)The zero line of the vertical circle is horizontal when the altitude bubble is level.

v)The crosshairs are vertical

vi)The optical plummet provides a vertical line of sight once the instruments is level

These sources of error can be detected by undertaking a series of standard tests (see paragraph 3.7).

Changing face will eliminate errors due to (ii), (iii), (iv)

Note: Changing face will not eliminate errors crept in while levelling  the instrument.

 
b)Manipulation errors

i)Parallax not properly eliminated

ii)Sighting wrong target, or part of target

ii)Using stadia hair in vertical angle measurement

iii)Incorrect use of upper and lower plate clamps and slow motion screws

iv)Poor centering of instrument over a ground mark

v)Targets not properly centred over ground marks

vi)Incorrectly set up tripod

c)Reading and booking errors

i)Incorrect reading of scales

ii)Incorrect booking of plate readings

iii)Arithmetic errors – there is no convenient arithmetic check.

iv)Check left and right face readings – is there the correct difference between them?

v)If several rounds of angles have been observed, look for the odd one out.

 Theodolite checks



There are six standard checks on a conventional theodolite and these should be done on a regular basis.

a)Plate bubble

    Simply leveling the theodolite and “freezing” the bubble will check for plate bubble error.  If the bubble “freezes” away from the central position, the theodolite is

still leveled.  Centralising the bubble by adjusting the bubble tube is not a necessary adjustment.

b) Verticality of crosshairs

With the instrument correctly leveled the vertical crosshair should be truly vertical.  To check this sight on to a well defined distant object and move the telescope

up and down.  The object should remain on the vertical crosshair throughout its entire length.

c) Horizontal collimatioin

This check determines whether the optical axis of the telescope is at right angles to the trunnion axis.  With the theodolite set up and leveled sight a well defined

mark, say 50 m away with the lower plate clamped.  Record the horizontal plate reading.  Change face and sight on to the mark again.  Record this plate reading.

The difference between the two readings should be 180 and any variation from 180 is twice the collimation error.  Collimatioin errors of 40 seconds or less are

acceptable for most site applications.  This error will be eliminated by observing on both faces of the instrument.

a)      Trunnion axis error

This check determines whether the trunnion (or horizontal) axis is at right angles to the vertical axis.  With the instrument set up and leveled, sight a high target

using the vertical crosshair (vertical angle approximately 40 – 50).  Depress the telescope and read a staff or other graduated scale laid horizontal under the target

and facing the theodolite.  Change face and repeat this operation.  If the two staffreadings re within 5 mm the trunnion axis error is within 5 mm the trunnion axis

error is within acceptable limits for mot site applications.

By observing on both faces of the instrument, this error will be eliminated. However, this is quite impractical if the theodolite is used for plumbing purposes.

e)Vertical collimation error

The zero line of the vertical circle should be horizontal when the altitude bubble is levelled or the automatic compensator is in operation.  The check follows the

procedure for measuring vertical angles (see paragraph 3.3).  The difference between the two reduced vertical angles is twice the vertical collimation error.

Collimation errors of 40 seconds or less are acceptable for mot site applications.  By observing on both faces o the instrument, this error will be eliminated.

(f) Optical plummet

Set up and level the theodolite.  Place on the ground a white card with a cross, drawn on it.  Position this card so that that the cross is exactly in the center of the

plummet graticule..  Tum the instrument through 180 and observe whether the cross remains in the center of the circle.
 Maintenance of equipment 



See paragraph 2.6 – Maintenance of levelling equipment.  

Tacheometry

Tacheometry (Surveying)                                                                                                   

Tacheometry is the branch of Surveying in which we determine the horizontal and vertical distances with the angular measurements with an instrument , Tachemometer. It is not so accurate method of finding the horizontal distances as the Chaining is, but it is most suitable for carrying out the surveys to find the distances in the hilly area where other methods are quite difficult being carried out. It is generally used to locate contours, hydrographic surveys and laying out routes of highways, railways etc.

The instruments required for carrying out the Tacheometric survey are:

(1) A Tacheometer (2) A Stadia Rod.

• Tacheometer: Tacheometer is more or less a Theodolite installed with  a stadia diaphragm. Stadia diaphragm is equiped with three horizontal hairs and one vertical hair. So we can take three vertical staff reading at the same instruments setting, lower most hair reading, central hair reading and the top hair reading. The difference between the lower hair reading and the upper hair reading gives the staff intercept(s).

The Tacheometer with the analactic lens are famous because their additive constant is 0. There is one concave lens introduced between the eye piece and the object piece to eliminate the additive constant of the instrument. It simplifies the calculations.

Methods of Tacheometric Survey:

(A) Stadia Hair Method

1. Fixed Hair Method
2. Movable Hair Method

(B) Tangential Method

(A)Stadia Hair Method: 

• As the name suggests in this method theodolite with the stadia diaphragm is used to find out the staff intercept between the lower and upper hairs and also the central hair reading is noted.

Principle of Stadia hair method is that the ratio of the length of perpendicular to the base is constant in case of similar triangles.

1. Fixed Hair Method: In the fixed hair method the cross hairs of the diaphragm are kept at a constant distance apart and the staff intercept varies with the horizontal and vertical position of the staff with respect to the Theodolite.
2. Movable Hair Method: In this method the staff intercept between the lower hair and the upper hair is kept constant by moving the horizontal cross hairs in the vertical plane.

Formula to carry out calculation works:

Case:  

(a) Staff held vertical:

             D = (f/i).s+ (f+d)

   where, f/i = multiplying constant

               s = staff intercept between the bottom and top hair

               f+d = Additive constant

              D = Horizontal distance between the staff station and the observer's position

(b) Inclined sights staff held vertical:

            D = (f/i).s. cos^2A + (f+d) cosA

            V = {(f/i).s.} .[{sin(2A)}/2] + (f+d) sinA

                Where A is the angle of elevation or angle of depression.

(c) Inclined sights upwards, staff held normal:

    D = [(f/i).s+ (f+d)]cosA - h.sinA  ;    V= [(f/i).s+ (f+d)].sinA

     h= central hair reading.

R.L. of staff = H.I. + [(f/i).s+ (f+d)].sinA - hcosA

(d) Inclined sights downwards, staff held normal:

  D = [(f/i).s+ (f+d)]cosA - h.sinA  ;  V= [(f/i).s+ (f+d)].sinA

    R.L. of staff = H.I. + [(f/i).s+ (f+d)].sinA - hcosA

(B)Tangential Method:

In Tangential method only central hair reading is noted down and generally two angular observations are taken to calculate the horizontal and vertical distances.

prismatic compass

PRISMATIC COMPASS

Comapass surveying is a branch of surveying in which directions of surveying lines are determined with a compass and the length of lines are measured with a tape or chain. In practice the compass si generally used to run a traverse.

In surveying,"Traverse" consists of a series of straight lines connected together to form a open or a closed polygon.

Methods of traversing: Depending on the type of instrument used for the measurement of angles the method of Traversing can be classified as under;

1.Chain Traverse

2.Compass Traverse.

3.Plane Table Traverse.

4.Stadia Traverse.

5.Theodolite Traverse.


In Compass Traverse the direction of the traverse lines are determined with a magnetic compass. 

The magnetic Compass may be "SURVEYOR COMPASS" OR "PRISMATIC COMPASS"

PRISMATIC COMPASS:

The prismatic compass is one of the magnetic compass in which there is a prism for taking observations. The prismatic compass is generally smaller in size than a surveyor compass.The prismatic compass is used to determine the whole circle bearing of the lines.It consists of 

1.circular box, about 85 to 100 mm in diameter. The box is made up of brass or a non metallic material.At the centre of the box there is a hard steel Pivot which supports the magnetic needle. The needle used in a prismatic compass is board of form. The box is fitted with a glass disc at its top. When the compass is not in use,the box is covered with the brass disc.

The prism when carried on a mounting frame which can be raised or lowered for focussing of the prism. The image of the graduations is viewed through a small circular aperature in the prism mounting. Just above the aperature ther is an narrow slit or a small V-cut used as an eye vane. The object vane consists of a metal frame hinged to the box. It has a vertical hair. The object vane is usually provided with a hinged mirror so that the object which are either too low or high can be sighted by inclining the mirror. Dark coloured glasses are provided near the eye vane which can be interposed between the eye and the prism when sighting illuminous objects or the sun.

when the instrument is not in use, the object vane is folded on the glass cover. In this process, the lifting pin is pressed which lifts the needle off the pivot and holds it against the glass cover. Thus undue wear of the pivot is prevented. TOo dampen the oscillations of the needle and to bring it to rest quickly, a light spring brake is fitted inside the box. A brake pin provide belwo the base of the object vane when pressed comes into contact with the edge of the alluminium ring and stops its oscillations.The prismatic compass is mounted on the tripod while taking readings.

PartsEdit

The essential parts of the prismatic compass are listed below:-

• Magnetic needle
• Eye vane
• Eye slit
• Eye hole
• Mirror (adjustable)
• Glass cover
• Graduated ring
• Lifting lever
• Lifting pin
• Metal box (85–100 mm dia)
• Focusing stud
• Object vane
• Sun glasses
• Sliding arrangement for mirror
• Horse hair

Chain survey

Chain Survey

Chain survey/surveying is an very old method of Surveying. This article includes definition of chain survey along with all detailed information with necessary images about various aspects of chain surveying. 

Chain survey is the simplest method of surveying. In chain survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. Here only linear measurements are made i.e. no angular measurements are made.This is most suitable adapted to small plane areas with very few details. If carefully done, it gives quite accurate results.

The necessary requirements for field work are

• Chain
• Tape 
• Ranging Rod 
• Arrows 
• Cross staff

Suitability of Chain Survey

Chain survey is suitable in the following cases: 

1. Area to be surveyed is comparatively small
2. Ground is fairly level 
3. Area is open and
4. Details to be filled up are simple and less.

Survey Station

Survey stations are of two kinds 

1. Main Stations
2. Subsidiary or tie

Main Stations

Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations re called the main survey line or the chain lines. 

Subsidiary or the tie stations

Subsidiary or the tie stations are the point selected on the main survey lines, where it is necessary to locate the interior detail such as fences, hedges, building etc. 

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. 

Base Lines

It is main and longest line, which passes approximately through the center of the field. All the other measurements to show the details of the work are taken with respect of this line. 

Check Line

A check line also termed as a proof line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check line is measured to check the accuracy of the framework. The length of a check line, as measured on the ground should agree with its length on the plan. 

Offsets

Offsets 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
2. Oblique offsets.

The measurements are taken at right angle to the survey line called perpendicular or right angled offsets. For setting perpendicular offsets any one of the following methods are used: 

• Swinging
• Using cross staffs
• Using optical or prism square

Perpendicular Offset by Swinging:

Chain is stretched along the survey line. An assistant holds the end of tape on the object. Surveyor swings the tape on chain line and selects the point on chain where offset distance is the least (Fig. 12.13) and notes chain reading as well as offset reading in a field book on a neat sketch of the object. 

Perpendicular Offsets Using Cross Staffs

Figure 12.14 shows three different types of cross staffs used for setting perpendicular offsets. All cross staffs are having two perpendicular lines of sights. The cross staffs are mounted on stand. First line of sight is set along the chain line and without disturbing setting right angle line of sight is checked to locate the object. With open cross staff (Fig. 12.14 (a)) it is possible to set perpendicular only, while with french cross staff (Fig. 12.14 (b)), even 45º angle can be set. Adjustable cross staff can be used to set any angle also, since there are graduations and upper drum can be rotated over lower drum.
  

FIELD BOOK

All observations and measurements taken during chain surveying are to be recorded in a standard field book. It is a oblong book of size 200 mm × 120 mm, which can be carried in the pocket. There are two forms of the book (i) single line and (ii) double line. The pages of a single book are having a red line along the length of the paper in the middle of the width. It indicates the chain line. All chain-ages are written across it. The space on either side of the line is used for sketching the object and for noting offset distances. In double line book there are two blue lines with a space of 15 to 20 mm is the middle of each book. The space between the two lines is utilised for noting the chain-ages. Figure 12.17 shows typical pages of a field books. 

Procedure in chain survey

1. Reconnaissance: The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor inspects the area to be surveyed, survey or prepares index sketch or key plan.
2. Marking Station: Surveyor fixes up the required no stations at places from where maximum possible stations are possible.
Some of the methods used for marking are:
• Fixing ranging poles
• Driving pegs
• Marking a cross if ground is hard
• Digging and fixing a stone.
3. Then he selects the way for passing the main line, which should be horizontal and clean as possible and should pass approximately through the center of work.
4. Then ranging roads are fixed on the stations.
5. After fixing the stations, chaining could be started.
6. Make ranging wherever necessary.
7. Measure the change and offset.
8. Enter in the field the book.