GEOGRAPHY – Form Five – SIMPLE SURVEY AND MAP MAKING

SIMPLE SURVEY AND MAP MAKING

INTRODUCTION TO SURVEYING

Surveying is an art or a science of measuring and recording distances, shape and heights in the field on the earth surface from which maps or plans can be developed.

Or is an art or a science of making measurements of natural and man made features on the earth surface made on the earth surface in the field areas help the features. The collected information from field areas about the natural and artificial features are finally represented in a form of a map or a pan of which drawn at a specified scale

  5.1 VARIABLES USED IN PROCESS OF SURVEY

The conducted surveys are of wide range and thus fall into different types as reasonably classified. The wide range of survey classified according o the size of areas whose measurements determined and according to specific objectives.

By considering the size and nature of the surface areas that has been surveyed or would be surveyed, the art of survey is divided into two variables

•         (i)Geodetic survey
•         (ii)Plane survey

Geodetic survey

It a type of survey which covers such larger areas like the whole of the earth surface, a continent or a country and takes into account of the curvature of the surface area. Geodetic survey is mostly used to establish Saturn points (bench marks )

Plane survey

It is types of survey used for the smaller areas of which the surface is considered flat and the earth curvature is not taken into consideration. Areas surveyed are not more than 10km X 10km size

Survey according to the specific objective include the following significant types

  -Topographical survey

•         -Engineering survey
•         -Geological (mining) survey
•         -Hydrological survey
•         -Photogrammetric survey

Topographical survey

It is a survey by which the topographical features of the earths surface such as road, revivers, mountains plateaus ocean building lakes, vegetation and others are measured and whose measurements used to develop maps or plans which show their relative positions both horizontally and vertical. Maps developed after the conduct of topographical survey are known a topographical maps

Cadastral survey

Cadastral survey is also known as boundary survey. It is a kind of survey conducted with an aim of preparing legal documents on boundaries of towns, cities, industries for owner ship purposes. i.e A sort of survey carried out to produce plans which shown properties boundaries. It is mostly conducted to develop administrative areas.

Engineering survey

It is also as built up survey. It is a kind of survey which provides details (special information) for construction purpose. The construction can be of buildings, reads, railways, bridges, water dam etc. They are commonly carried of during and after planning the construction of engineering projects.

Geological (mining) survey

It is a kind of survey conducted by the geologists with the aim of realizing the distribution of rocks and their content. Or all surveys for mining works on the earth or under ground.

Hydrographical survey

It is a sort of survey for water areas particularly seas and lake, They are carried out to determine the depth of water bodies at different points and nature of the bottom of the sea. These are commonly carried out for harbor construction, oil drilling and natural gas exploration as well as exploitation at sea area

Note

With all types of survey, topographical survey is of the major concern in our level. This is one of the major concerning in our level. This is conducted by different ways which include the following

•         Chain survey
•         Plane table survey
•         Prismatic survey
•         Leveling

IMPORTANCE OF CONDUCTING SURVEY

•         (i)To determine altitude size and distance of different areas
•         (ii)To establish position location of both natural and artificial features
•         (iii)They are potential for maps construction. Details on heights bearing, distance and size of features for areas which appear on maps derived from survey works.
•         (iv)To plan and control various kinds of engineer projects such as roads, railways, buildings canals and dams contraction
•         (v)To develop data bank of land and natural resources which aid in developing and management of environment

 

SURVEYING PROCESS

The conduct of surveying particularly topographical survey in the field area, should involves the following three significant steps

•         Reconnaissance survey
•         Observation and measurement
•         Presentation or plotting of the data

 

1.     I  Reconnaissance survey

The site whose details to be taken by conducting survey should be pre visited. This is commonly known as reconnaissance survey and it is defined as preliminary survey by which surveyor takes a general picture about the areas into his or her mind by working all over the site. During the pre visit; the surveyor should do the following beside walking all over the site

•         • Making a sketch plan of the site in the field to be surveyed
•         • Choose the surveying stations
•         • Marking of the surveying stations by establishing station pegs.

II Observation and measurement

It is considerably to be the conducted to actual surveying. During this state, the surveyor determines the relative positions and size of the natural and artificial features on the lad surface. There are three forms of measurement required

(a) Liner measurement

(b) Angular measurement

(c) Heights measurement

 

Liner measurement involve the determination of distance between to one point and another on the surface on the earth. Measurements are done with the use of chain or tapes.

Angular measurements involve the taking of the bearings between stations or from one station to another. Angular measurements are commonly taken with the use of prismatic compass.

Heights (altitudes) are determined with the use of leveling staff and leveling instrument and this particularly determination is made in the conduct of leveling.

 

III. Presentation (plotting)

It is all about the presentation of the data collected from the field during surveying in the form which allows the information to be clearly interpreted and understood by others. It is done by entering the measurements taken at the field in the field notes. The process is known as booking of field data. Thus booking is the process where by the measurements which are done in a field areas entered in a field note book for recording

Commonly booking is done by surveyor and not by assistant for the sake of avoiding unnecessary errors.

In the process of booking all measurements entered must be in raw units from the bottom up wards. All the linear measurements should appear at the center of the column. Features at the right hand side must be recorded on the right hand side, while those in the left side must be recorded on the left hand side of the column. All offsets must be shown on the relevant side as they appear along the survey line. The forward and backwards bearings must be known and should be circled.

Booking can be by either single line or double line bookings

  

5.2 CHAIN AND TAPE MEASURE SURVEY

Chain and tape survey is a science of measuring series of straight lines on the ground with the use of a chain or tape measure. It is the simplest and commonest method of making linear or horizontal measurements on land particularly for the smaller distances.

Significant uses of chain survey

•       • Chain survey is conducted to develop details which can be used for mapping areas.
•         •Chain survey is conducted to develop details for construction purposes.
•         •Chain survey is conducted to collect data to be added to the existing data.
•     •Chain survey is conducted to supplement other topographical survey techniques like plane table survey and prismatic compass survey.

Tools used in the conducting chain survey

The main tools used into the conducting chain survey include the following:-

1. i. The chain:-

It is a tempered steel wire with length of about 20m or 30 m. It has links connected by small rings in such a way it can be easily folded and carried in the field. The length from ring centre to another is of about 200 mm. It is used to measure distance on the ground along the line of tracery.

2.     ii. Tapes

(a) Those which made up of steel and available in varied length up to 100m

(b)   Those make up of fiberglass and they are available in lengths of 10m, 15m, 20m, and 30m Tapes like chain, are also used for liner measurements particularly of short distances.

3.   iii. Cross staff

They are made of metal or wood with eye slits at right angles. They are used to measure right angles of the off sets from the line of traverse.

4. iv. Ranging rods /poles

They are poles of woods or light metal 2m, 2.5 or 3m long. They are painted alternatively in red and white bans each 500 mm long so that they can be easily seen from a distance. They have pointed metal ends for driving into ground. They are used to mark points required to be seen in linear measurement

 

 

5.     v. Arrows

These are steel skewers of about 0.4m long. They are used for marking points an ground when chaining a long (line) distance, They are potentially used when the chain has to be laid down several times and the positions of the end of chain are marked with the arrows.

1. 
   
   
   pegs
2.     They are wooden station marks with 40mm square and 50 cm long. They are driven into the ground to mark positions permanently during survey.
3.     vii. Optical square

It is optical instrument of camera like for setting out angles. It is commonly used for marking off sets at right angles from the line of traverse.

8. viii.   Prismatic compass

It is a compass box with needle in the magnetic liquid to enable the needle to swing freely at the centre. There is a card which is graduated in the degrees to enable the reader or the viewer to read the given bearing or observed bearing

9.   ix. Field sheet/Note book

They are writing sheets They are used for recording or booking all necessary field work information

FIELD WORK OF A CHAIN SURVEY

(METHOD OF CHAIN SURVEYING)

The conduct of chain survey in the field area

             1. Preliminary inspection of the field area
       The site whose details to be taken by conducting chain survey should be pre visited. This is commonly known as reconnaissance survey. During   the pre visit, the surveyor work all over the site to obtain a picture in his or her mind of the whole area. More over, the surveyor should do the following beside walking all over the site.

•         Making a sketch plan of the site in the field to be surveyed.
•         Choose the surveying stations.
•         Marking of the surveying stations with leader.

 

2. Organization of chain survey team

It should have three people including the leader, follower and booker

•         The leader has a role of reading measurements taken in the field with the use of chain. He also pulls the chain forwards and insert the arrows at every point or station
•         The booker has a work of recording data as given from eh leader
•         The follower has a role of organizing the instruments during the field work. He also directs the leader where to insert the arrow and collect them.

3. The main chain survey work has to start as follows

(a) The chain is thrown and extended from one position to another and disentangle knots

(b)   Leader takes arrows and ranging road, and follower takes ranging rod

(c) Follower erects road at first base point places brass handle of the chain against rod. In this leader ex tends the chain follower by the follower

(d)   Leader straightens the chain and inserts arrow at end of brass handle. Offsets, liner distances and the point can be taken

(e) The leader drags chain so that follower is on a leader is arrow. Then follower moves to the leader arrow and places pole behind arrow.

Note:

The process of dragging the chain, signing and aligning is repeated until the line is completed the same process is also repeated with another lines until the whole area is surveyed.

  correct signals as seen by surveyor leader. (a) Move chain to left (b) Move chain to left (c) Chain correct



     Point fixation in chain survey.
  Fixation of points as chain survey conducted is by following methods.
  1.Trilaterization method.
it is by two measured distances. Where the equal measure distances from the baseline meet, the point is fixed.

 
i.e The AP=BP
 
2 Radiation method
it is also known as bearing and distance method or polar method. it is by taking both bearing and distance of position point from the line traverse.
  The method is illustrated as follows:
 
3 intersection or triangulation method.
  The position of point is given by the intersection of two rays of bearing and distance.’


4 Off set from baseline.
  It is by measuring right angle and distance of the off-set from the line of traverse.

 

NOTE

1 Chain survey can be accurately conducted if the following are adhered to:

•         -The use of few lines as possible.
•         -Avoid steep slopes and major obstacles.
•         -Select one major line.
•         -Keep chain lines short and measure them accurately.

2.Chain errors can be avoided by doing the following

•         -It should be checked that all chain lines are included.
•       – Read markers on chain carefully.
•         -Call measurements to booker clearly and ask him to repeat.
•         -Ensure correct position of arrows as they must touch edge of  handle or measuring tape and perfectly upright.

OBTACLES IN CHAIN SURVEY

Those are the features (object) lying with in the area to be surveyed along the survey line which may hinder or prevent chaining or ranging.

Types of obstruction

There are three types of obstruction in chain survey

•         (a) Those which do not allow crossing from side to side, river, a lake, pond preventing chaining.
•         (b) Those which will prevent vision but not chaining e.g a hill
•         (c) Those which prevent both chaining and vision e.g building.

TECHNIQUES (WAYS) TO OVERCOME THE OBSTACLES

There are seven ways on how to overcome the obstacles. The methods depend on the nature of obstacles as follow

1.     1. HILL

Hill is a rounded upland it is a considerable obstacle if lies on the path of the chain line. The obstacle of hill can be avoided by the following consideration techniques.

(a   (a) Simple system of offsets

This technique, a surveyor sets out equal perpendicular far long enough distances to clear the obstacle as follow

In the above application BD distance= AC distance This it obvious, as BD= AC the measured DC distance is equal to BA distance


(b)The method of similar triangle

(b   It is applied as follows

(c   AOC = BOD

Them

As Aoc=Bod The measured DC distance is equal to AB distance

 

(c)The method of equal triangle

 

With the technique of equal triangle a applied above C is the mid point of AO and D is the mid point of BO it thus AB=2Xcd

Trigonometric method

(d   From point A measure the length of random line chosen to avoid the hill to C. Then chain a line at right angle from AB to C Calculate the length of AB by Pythagoras theorem

 

2. POND

The obstacle of pond can be avoided by the following application.

Measure the perpendicular the same length from AB (AX and BY) Measure the length of XY. Then AB = AY

 

3. RIVER

Make chain line from one side to the opposite side i.e from B to D

Set up ranging pole at A and develop chain line along the river to C and it should be assured that AO=Oc from C develop chain line of right angle to D

Then
AO = OC

BO=OD

CD=AB

 

4. BUILDING

•         Set up ranging pole at A and B along the line traverse
•         Measure an equal distance along the line of traverse AQ and BR check corner of the rectangles so farmed are right angles
•         Extend the line of traverse through C and D
   

BOOKING AND PLOTTING CHAIN SURVEY

(A)BOOKING

Booking is a process where by the measurements done in a field are entered in a field note book for recording. Or is the process of entering the measurements take on the ground into the field note book. This is normally done by surveyor and not by assistant so as to avoid unnecessary errors

In booking the details the following should be taken into consideration

•         All linear measurement should appear at the centre of the column and it has to start from the botton upwards
•         Feature at the right hand side must be recorded on the right hand side while those at left hand side must be recorded on the left hand site of the column.
•         All offsets and ties must be shown on the relevant side as appear along the survey line.
•         The forward and back ward bearings must be known and they should be circled.

Methods of booking

There are two main methods of booking details in a field note book after they have been taken from the field by the conduct of chain survey

(i)                 Single line booking
By the single line booking the details on linear measurements are entered along the single line at the centre as illustrated below.

 

(ii)   Double line booking
By the single line booking the details on linear measurements are entered inside along the double lines at the centre as illustrated below.

 

In book of the chain survey detail involves the following significant steps:

•       – Plot your survey booking north so that the top, bottom, left and right respectively represent top bottom left and right respectively represent north, south west and east
•       – Always keep you paper perfectly flat, do not remove from the drawing table
•       –  Measure every line to away features from the survey line on right and left hand sides
•       -Mark out around with small circle and dot inside to represent station
  Example 1.

 

 

According to the booking into field note book develops the following interpretation

•         The claim survey covers two stations A to B
•         The survey line has a total distance of 6.5 m
•         At a distance of 1.5m observer seen the edge of the bush at off set distance of 2.4m
•     At distance of 6.5 observer seen also the edge of the bush at a distance of 2.3
  
  
  
  The figure to the last immediate page shown as follows

1.     1.The chain survey line covers four stations A to D with total distance of about 225m
2.     2. At point A (Station A) the survey line runs through the bearing of 326 to B
3.     3. At station A one observe assembly hall at bearing 43 of the right side of the survey line
4.     4. Along the survey line at a distance of 60m the surveyor observed assembly hall at 51 right side of the survey line
5.     5. At station B, 100m from A, surveyor observed pavilion at the bearing of 331 at left of the survey line.

6.The surveyor also observed an assembly hall at 40 to the right hand side of the surveyor line

6.     7. From point B the surveyor changed direction and surveys through direction 7
7.     8. At a distance o f160m away from B towards C the surveyor observed line) and also at right angle at the same distance point observed a swimming pool (right hand side of the survey line
8.     9. At point C 315 from B surveyor observed BB of 187º from C to B
9.     10. At station C the surveyor observed a science block at a bearing 50º left side of the surrey line to the left side of the survey line a bearing of 275º be observed assort pavilion.

11  11. From point C surveyor changed direction and took 96º

12  12.The last station D was at distance of 225m. He observed the BB of 27º to C. He observed science book (left side of survey line).

 

(b)   PLOTTING
Plotting is a process of putting or transferring the information or details obtained in the field into a piece of paper to develop conventional visual appearance of the surveyed area. The plotting date mostly from what have been bake in the field at book.

In plotting the surveyor should have the following equipment.

•       •  A Scale ruler
•         • A pair of compass
•         • Drawing paper
•         • Protector
•         • Square ruler
•         • Pencil and rubber
•         • T square
•         • Drawing table
•         • Drawing pins

Plotting procedure

•         1. Choose suitable scale like that of 1cm to represent 10m
•         2. Select the starting point on the nature of data booked in the field note book
•       3. Establish the north direction

4. By considering the north direction the elected starting point as well as the booked data established the survey lines, off sets and ties, and respective features

 A FIELD NOTE BOOK

 

 

Errors in Chaining

The operation of the chain survey may get subjected into errors. The errors result due to the following:

•         -Incorrect ranging length of the chain
•         -Chain not being too tight and not properly aligned
•       – Sag in the chain
•       – Expansion and contrition due to differential temperature conditions.
•         -Incorrect booking made during data collection or actual surveying.
•         -Arrows being not properly insured.
•       – Presence of sleeps on the way.

Advantages and disadvantages of chain survey

Advantages

•         It is useful method for surveying small areas.
•         It is useful method of taking linear measurement over smaller distances.
•         Chain survey is much applied in other surveying techniques like prismatic compass survey.

Disadvantage

•         Sometimes it becomes time consuming method as it is so much slow in the conduct of it
•         It is heavy and tiresome to work.
•         It is subjected to the problem of errors accumulation.
•         It is required to the problem of errors accumulation.
•         It requires more than two people to run survey effectively.
•         It requires extra time for office work particularly in making of plotting of the data.

 

5.3 PRISMATIC COMPASS SURVEY

Prismatic compass survey is a branch of topographical survey in which directions of survey lines are determined with prismatic compass and the length of  lines are measured with a tape or chain to develop a controlled network of connected positions (point) known as traverse. Or it is surveying technique with the use of prismatic compass entailing the fixing of object position in the field by measuring the angle of bearing between the line of magnetic north and the line of sight to the object. In practice the prismatic compass survey is generally used to run traverse. In surveying connected together to form an open or closed polygon.

Significance of conducting prismatic compass survey.

•         -Prismatic compass is mostly used to have details for rapid mapping of areas in relation to details collected by other topographical surveying techniques.
•       –  It is used to take angular measurement of objects relatively to one another in terms of bearing.
•         -Used to develop transverse of the survey lines by confectioning several stations.
•       – It is used to locate position of objects in the field by either intersection or resection method.
•         -Used for setting the magnetic north of the areas to be surveyed as well as to be mapped.

Equipments used in the conduct of prismatic compass survey

1.   1. Prismatic compass

It is the circular compass box with a small magnetic needle swinging horizontally and circular card inside it. The circular card marked clockwise in degree number values. The magnetic needle shows the north direction and the degree number along the sightline is the bearing of an object from the position of observation. The prismatic compass is used to determine the bearing of an object

2. 
   Ranging pole

It is used for making stations of the survey lines whose distance are to be measured.

3. Arrows

They are used to mark the end of chain along the survey line.

4. Tape measure or chain

Either one of the two used to take linear measurements along the survey lines from are position to another.



     

 

5.Field note book and pencil

Used to record the details obtained in the conduct of prismatic compass.

PROCEDURE IN PRISMATIC SURVEY

1.Preliminary surveying should be firstly carried out. During the preliminary survey the following should be done

•                 -The main stations should be chosen and they are to be marked by ranging poles or any natural feature like a tree
•                 -The survey lines should be clearly laid out. In laying the survey lines should be as long as possible

2.Mark the stations e.g A,B,C etc

3.Angular measurement are made to the next station e.g A to B,B, to CC to DD etc. In determination of the angular measurements the FB and the BB are taken and entered in the field note book.

4.Linear measurement are made using the chain or tape measure and also entered in the field note book.

5.Simultaneously, the offsets and ties to most important object are taken in the course of traversing.

6.The booking of the date in the entry book may appear as following the next page.

Reading of bearing in prismatic compass survey.

Bearing means an angular distance along the sight line from one position to another position detected clockwise from magnetic north of the used prismatic compass.

A bearing of the observer to a distant object is called forward bearing. A bearing from the distance object to an observer is called backward bearing

Principle involved in taking the bearings

•         The forward and backward bearings differ exactly by 180.
•         If the forward bearing is greater then 180 substract 180 to get the backward bearing

 

Bb=fb-180

•         If the forward bearing is less than 180 add 180 to get the backward bearing

 

BB=FB+180

Precaution of Using prismatic compass

Before using the prismatic compass the following should be observe

•         The prismatic compass should be checked if it is in good order or not in order to avoid errors inherited from such instrument
•         Walk over the area of the survey checking whether there are materials made of iron and remove them when is possible.
•         Check whether you have any instrument made of metal such as watch before using magnetic compass. Such materials make attraction during actual survey.
•         In taking the bearings check the difference between FB and BB. The difference should be exactly 180

Lack attractions in magnetic compass survey

It is made by the presence o metals or any thing in the field area that may disturb the earth magnetic field at the point of observation and may affect the correct reading of the compass needle the magnetic needle of the magnetic compass is disturbed not to point exactly to the magnetic north, in stead it is elected seriously from its normal position. Example of elements which make attraction in clued keys watches rock material metallic properties in the ground, electric cables, spectacles ring telephone and others which make the same.

Detection of local attraction

Local attraction detected by observation the difference between FB and B

•         If the difference is 180 there is no local attraction
•         If the difference is greater or less than 180 there is local attraction between stations

Correction of local attraction

Local attraction are corrected with the use of means error of which computed as follows

Mean error = Amount of error                                 2

 

Amount of error = The obtained diff – stand diff

 

With the mean error correction is made under the following principle considerations.
  – If BB>FB: Then; FB + Mean error & BB- mean error.
– If BB< FB: Then FB – Mean error & BB_ mean error.

PLOTTING IN PRISMATIC COMPASS SURVEY

Plotting in prismatic compass survey is a process of putting or transferring the information or details obtained into field onto a piece of paper to eyed area in a form of traverse.E

Procedure

1.     1.Choose suitable scale according to the size of paper you have.
2.     2.Choose suitable starting point.
3.     Set the north direction (MN).
4.     By using protector, measure the angle for direction and mark the distance according to the scale. In close traverse, the finishing point should coincide with starting point if the traverse do not coincide, adjust the traverse using graphical method

The process of establishing a traverse as a net work of connecting series of lines is known as traversing.

COMPASS TRAVERSING
As it has already been given in the introductory part, the position on the ground is established if its bearing and distance from another known point are measured. Te process of establishing the positions of successive points is known as traversing

A traverse is therefore defined as a plot of a series of survey lines connecting established points. There are two types of compass traverse including

(a   An open traverse

(b   A closed traverse

The open traverse

Closed traverse is also known as free traverse. It is the one of traverse of which proceeds from known point to another, the finishing point coincide with the starting point. The open traverse is established following the conduct of open traverse survey.

The open traverse survey from which the open traverse polygon established is conducted to have a good plan of route for road or railway construction or for pipe line establishment

 

 

Closed traverse is a series of linked traverse lines where the terminal point closes at the starting point. Or the one which proceeds from known point to another and the finishing point coincides with the starting point

Closed traverse is useful in making the boundaries of wood or lake. Civil engineers utilize this practice for preliminary surveys of proposed projects in a particular designed area.

 

 

Adjustment of closing error

In plotting a scales closed traverse, the terminal point may not coincide with the starting point. If this is revealed, a closing error detected and it has to be corrected (adjusted).

Closing error is adjusted by the graphical method as follows:

•         • Measure the distance of the disclosure (closing error) with our example the distance should be measured from a A
•         • In all direction of the closing error, draw parallel lines at all station to the disclosure.
•         • Find the total distance of the traverse
•         • With suitable scale, draw a horizontal line representing the discourse A to AI
  • Mark the distances between each station on the horizontal line between each station
•         • At station A draw a perpendicular line representing the closing error
•         • Draw the hypotenuse line joining A and A
•         • Join other perpendicular lines from each station on the horizontal line to the hypotenuse line
•         • Measure each perpendicular line
•         • Lastly use measures to adjust the disclosure.

 

 

Intersection and resection

In prismatic compass traverse

These are the methods used in recognizing position of objects in areas on the earth surface by doing prismatic compass survey and also applied on maps.

Intersection

It is a method of identifying an object by taking bearings towards it from two or more fixed points radio tower is found at bearing of 22.30 and be recognized by taking the bearing from the two points of observation to it. The application is as follow.

Resection

It is a method of taking bearings to two objects in the field which can be located on the map. The reading are converted into backward bearing, then converted to true bearing and angle lines drawn from the points to meet at observers position. The methods is used to recognize the point of observer by taking bearings from the two observed object.

E.g The compass bearings of Y and Z as taken from A were 55 and 88 respectively. The point of observers recognized by resection method a follow

 

 

Chaining errors in the prismatic compass survey

Error is a state of being incorrect in the field during data collection

Errors are numerous and fall into the following groups

1.     (1)Mistake
2.     (2)Systemizing or cumulative errors
3.     (3)Accidental or compensation errors

Mistakes

These result due to experience or due to carelessness on the part of the surveyor. The typical mistake in clued.

•         • Misreading of the chain
•         • Miscounting./loss errors
•         • Reading from the wrong end of the chain
•         • Wrong booking
•         • Miscounting chain length.

  Systematic or cumulative errors

The error arise from eh chain itself by being not accurate e.g the chain may not have 100 links

Accidental or compensation errors

The error arise from a chain itself by being not accurate e.g the chain may not have 100 links

5.4 PLANE TABLE SURVEY

Plane tale survey is a surveying technique used to fix the position of distant object by intersection radiation or resection by employing a drawing board (plane table) placed on tripod stand. It represents the most accurate and rapid way of fixing position of objects for map making.

In plane table survey, the table is covered with drawing paper on which the details are entered during the survey

 

 

 

 

EQUIPMENT USED IN PALANE TABLE SURVEY

1.     i.Plane table

It is device used in surveying to provide solid and level surface on which field drawings, charts and maps established. It is of any size and the most suitable are those of 45 x45 cm, 45 x 60 cm nd 60x75cm Commonly, inside the table there is a socket which allots it to the mounted on a tripod stand when in use it is set over apoint of precise horizontal level then a drawing sheet is attached over its surface.

2. ii.The tripod stand

It is wooden or metallic stand with three sides which can be easily moved and adjusted. A tripod stand needs to have some privet to allow the table to be rotated in a horizontal plane and clamping pin and screw which hold the table steadily or firmly the table.

3. iii.The alidade

It is a device that allows one to sight a distant object and use the line of sight to perform a task. It is fundamentally used to sight and measure distant objects from the position of the table. With alidade the distance and direction angle of the object can be measured and established on the table by drawing over the paper sheet. Alidade is so used to cheek the table to the exactly level if it has water level

4. iv.Trough compass

It is also known as plane table compass. It is a rectangular wooden box with magnetic needle floating freely at the center. It is used to find the magnetic meridian in a field area to be plotted on the drawing paper sheet over the table.

5. v.The plumb bob (plumbing fork)

It is attached to the edge of the board and the metal weight directly over the ground station. It is used to fix the table exactly.

6. vi.Sprit level or one level

It is used for leveling the table if alidade has no water lever.

7. vii.Chain or tape measure

Used to measure ground of the sightlines to distant object if the alidade is not capable.

8. viii.The cover

A plastic cover is used to prevent the table during rain while surveying.

9. ix.Good drawing paper

The paper should be mounted on the table either by pins or clamps.

1    x.A hard pencil

It is for drawing in the field. Also a rubber should be organized for rubbing incase mistake entered.

1   xi. An umbrella

It is used to avoid hot sun or rainfall.

 

METHODS OF PLANE TABEL SURVEY

Fixing of data on a table sheet is done differently by the plane table surveyors depending on whtat is intended to be established about the position of the distant object. Considerably to this plane table survey appears to be of four methods including the following:

•         -Radiation method
•         -Intersection method
•         -Traversing method
•         -Resection method

Radiation method

By radiation method, it is a kind of plane table survey which is made up of rays radiating from the centre of the surveyed area. It is employed with an emphasize of establishing the position of distant objects from the centre at which the pane table is established.

In this method, the direction of objects (points) to be located are obtained by drawing radial lines along the sight line of alidade. The horizontal sightline then measured with tape or chain and scaled off by marking their position on the drawing.

Procedure

1.     1.Surveyor should walk around the survey area in which the objects lie and select a position for the plane table from all objects . ie Has to walk around then commanding position for plane table should be chosen.
2.     2.Surveyor should set up the plane table at the commanding position. By regarding our visual example the commanding point is at station A and clamp it at station A and clamp it then insert MN line on the paper
3.     3.Stick a pin upright in the centre of the board to represent the position.
4.     4. Layout the edge of the alidade against the pin and sight on each of the objects V, W, Y, Z
5.     5.With tape or chain, measure ground distance from point A to each of the objects in the field. The distance should be converted into smaller unit values in response to scale.
6.     6.The details can be plotted clearly on the drawing paper over the table.

 

 

Intersection method

It is a technique of plane table survey in which ruling lines (rays) for distance object (s) are obtained by intersection from two different commanding position. The two different commanding positions are set along the same ground base line

Procedure

1.     1. Choose a ground baseline which should be on a rather flat areas to be surveyed. The two end points of the base line should be conditionally intervisible and long enough to mark the end points with either permanent features or ranging poles.

 

2.     2.Measure the ground base line with either tape or chain.

 

3.     3. Find the suitable scale by considering the length of the ground base line as well as the size of plane table sheet.

 

4.     4.Draw the base line on the your drawing paper

 

5.     5.Points can be then established by intersection from the base line i.e Establish the sight line to the points from one position of ground base line end. Then establish the sight line to the same point from another position of ground base line end. Where the two sight line intersect the position can be established

   

 

Traversing method

It is a plane table surveying technique employed to develop a traverse compound of interested feature like school, college industry and others,

Procedures

•         -Make pre-visit of the area to be surveyed and identify the important corners of the compound area.
•       – Set up the table in one corner of the field so that one side is roughly parallel to the side of the area to be surveyed.
•         -Select some objects in the next corner of the area to be surveyed or mark the stations with ranging poles.
•         -Choose the suitable scale to the area.
•         -Set up the table at A and a drawing towards by to the scale
•       – Measure the distance AB and mark B to scale
•       – Leaving some thing to mark the station, pick up the table to B and set it up.
•       – From B measure the ground distance from B to C.
•         -Draw in the ray to C on the paper and mark it C.
•       – Repeat the process at CDE etc, until you are through to all stations.
•         -Plot the surrounding details by radiation and intersection.

  

Resection

By resection the position of two or more points are located from the same position of pane table and from one commanding position of the plane table other features are located by considering sight lines on the plane table established. It is meant that; ray lines on the plane table established from the same position to different points of interest.

 

Advantage of plane table

•       – Is a rapid method of survey no booking is required
•         -It permits rapid sketching of details on the map
•         -The technique used is an excellent discipline in observation and cartography

 

Disadvantage of the plane table survey

•         -The method can not be used during damp or rain season.
•       – A simple error in the ground base line measurement could throw out all angle measurement
•         -It needs much higher skills to be conducted

 

5.5 LEVELLING
Leveling is the process of determining the difference in elevation of two point

Or

the method of surveying where by the relative heights of a number of points on the earths surface are determined by sighting through a leveling instrument and reading the heights on the leveling staff.

Or

The measurements of height difference between points on the ground

The heights difference between points on the ground

The heights difference to be measured has to be deferred to same reference level. Stable reference points for heights are called benchmarks (BM) which have heights fixed with reference of mean sea level.

Leveling in cartographic concerns used to establish altitude on maps by contours, spot height, trigonometric points as well as bench marks. All altitudes seen on maps by contours, spot heights trigonometric points and bench marks have been determined by surveying process known as leveling

 

Instruments used in leveling

1.     1. Leveling instrument

They are level sighting instrument toward the station with leveling staff. There many types of leveling instruments used in leveling including. Dumpy level tilting level, the delete, abbey staff, clinometers alidade and other

       

 

2.     2  Leveling staff:

It is a long big scaled rod graduated in tenth meters, hundreds meter. It is used together with a leveling instruments to measure the vertical distance above a reference plane.

 

3.     3 Chain or tape

They are used for measuring horizontal distances in the field area.

 

 

4.     4Pegs

They are popularly known as station pegs and used for marking different station points in the field area

5.     5Field sheets and pencil

They are used recording or booking all necessary field work information

6.     6Arrows

They are used to mark the end of chain along the survey

METHODS OF MEASURING HEIGHTS
  There are two methods of measuring heights;
  including

-rise and fall method
-Height of collimation method

      The rise method of leveling

It is done by considering the reference height to another point. Back sight and fore sight taken and subtracted to get the difference. If the difference is positive it means the land rises hence the height is determined by taking the amount of difference.

Between BS and Fs added to the amount of reference point
Example

  

•         The travelling instrument is set up on its tripod at X
•         He telescopic staff is erected at A and a reading is o bout 3.561m, Such amount of reading of the staring point to thelevelling instrument is known as back sight
•         The level is then swung rounding a horizontal plane. The telescopic staff is set as B and a second reading taken is (1.352m)
•         The difference is level between A and B is rise of (3.561m) (1.352)= 2.209m
•         So long the height of A known the height of B can be calculated as (635.562m +(2.209m)=637.777

The fall method of leveling

It is also by considering the reference height to another point. Back sight and fore sight taken and subtracted to get the difference. As the difference is negative it means the land falls hence the height is determined by taking the amount of difference between BS and FS (Fall) subtracted from the amount of reference point

  

Height of collimation method

The height of collimation is found by adding the staff reading of a point of known level (back sight) to that point of known height (bench mark) to get height of instrument (HI) Then the HI subtract the fore sight reading to get the height of the recommend ended position.
  Example

         

Procedure

By considering the longitudinal section above the height of print by the height of collimation methods done as follows

•         The height of D is known and it is of about 635.782m
•         The levelling instrument is set up at Y and directed to D to read BS According to above the Bs is of about 2..454
•         The known height 635.782 should be added to the amount of Bs 635 + 2. 454 = 6338 (HI)
•         The amount on height of instrument should subtract the amount of 633.328.0.973

TYPES OF LEVELING

Leveling is of two types up on what measured relatively to heights. The main types include the following

1.     Differential leveling
2.     Profile leveling

 

Differential leveling

The technique involves the determination of the elevation difference between one point apart.

Procedure

It is in an assumption that, we need to know that difference in elevation of point A and B of a farm. Point A is a bench mark whose evection may be known or assume . This can be illustrated as follows

1                    Leveling staff is held at A an instrument level is set at the point P the first reading taken at section A is back sight. The readings added to the reading of station A to get the height of the instrument.

2                    The staff man moves past position 1 station 1 (51) the instrument is turned so that reading is taken at of station. This is the foresight (FS) Say 0.9m) The elevation of this station is determined by subtraction The FS from the Hi

3                    The level man moves to station P2 and sets the instrument to read the staff at station SI (SAY 1.2M) This reading is added to the elevation of station SI to get the new height of instrument i.e Hi 400 + 1.2

4                    The staff man moves to station 52 and a foresight reading is taken (say =4) This reading is subtracted form the HI to get the elevation of s2 ie 401.8 – 1.4 = 400

5                    The level man moves to position P3 from which reads the back sight from staff at position s5 (say position s2 to get new height of instrument. E. Hi= 400.1.4+1.8=402m

DIFFERENTIAL LEVELING
Bench mark A-B

 

Profile leveling

Profile leveling is the process of determining the elevation of a series of points at measured interval along a survey line e.g along route way.

•         A bench mark is chosen where the staff level will be stationed to take the reading as back site
•         An instrument is set at position X (PX) and reading is taken as back sight (BS) from the beach mark say (0.6m)
•         The staff man moves to station A and a reading is take (IS) The elevation of point A is determined by subtracting this reading from the height of instrument ie 600.6 = 599.7m.
•         Staff man moves to station s1 and reading (IS) is take (say 1.2m) This reading is subtracted form HI to get the elevation of SI 600.6 – 1.2.
•         The staff man moves to station s2 and reading (IS) is taken (say 2.1) This reading is subtracted from Hi to get the elevation o 52 ie 600 6-2= 598.5m.
•         The staff man moves to station 53 and a reading (FS) is taken (say 2.9m) This reading is subtracted form HI to get the elevation of point s3 i. 600.6 2.9=597m.
•         The level man moves to position PY and sets the level. A reading is taken at s3 as a back sight (BS say 0.5m) piint s3 as a back sight (BS say 0.5) point s3 is called turning point. The reading is added to the elevation of points s3 to get new height of the instrument (HI) ie 597+ 0.9=597.3
•         The staff man moves to point s5 and riding is take (IS say 1.4) This is subtracted from the HI to get the elevation of point s5 i.e 598.2-2=596m
•         The staff man moves to poking B and a final reading is taken which is a foresight (FS say 2.3m) The reading is subtracted from HI to get the elevation of point B. E.E 598.2-2.3=595m
  
  
  
  

Usefulness of leveling

•         Leveling helps to determine the relative heights on land that can be used in contour mapping.
•         Housing foundations, the location of industrial sites the routes communication and sitting of codlings can be located and determined with the help of leveling.
•         Leveling can be used to determine the heights of land surface such a hills, valleys and plains.
•         Leveling is used to develop the longitudinal section of roads, drainage ditches, railways and others