LAND SURVEYING

I. INTRODUCTION

Definition of Land Surveying

One Typical definition: ‘Land surveying’ is a discipline dealing with the determination of the shape, size and the relative position of features (points) on or near the earth’s surface.

Some Historical Evidence of Surveying Skills in Application

-Early land measurement can be traced in Babylonia and Egypt in the form of field measurements. Example, Egyptians made exploration of the Nile and constructed the great pyramids by applying surveying principles.

-Masterpieces of the Greek Architecture demonstrate surveying skills in application.

-The Great Sewer of Rome, the Roman Aqueduct at Tarragona required great skills and knowledge of surveying in their creation.

Main Uses of Land Surveying

 Mapping: -Measurement of land and its physical features accurately and to record the same in form of maps or plans.  Measurement of Areas and Volumes. -Eg. Acreage, area of catchment, -Volume of reservoirs, e.t.  Setting out. -Operations necessary for the correct positioning of proposed works on the ground and dimensional control during the construction process.

Branches of Land Surveying

The two widest principle classifications are:

A. Plane surveying -Relatively small areas are involved <300km 2 - Earth’s surface is taken to be flat (plane) i. the curvature of the earth is ignored - Relatively simple equipments, techniques and methods employed B. Geodetic surveying - Relatively large areas of the earth’s surface are involved -The curvature of the earth is taken into account -Relatively superior equipments, techniques and methods employed to achieve the Higher accuracy requirements - Used to provide horizontal control and study of the earth’s magnetic field and Detection of continental drift

Main Types of Plane Surveying

 Topographical surveys -Deals with establishment of position and shape of man-made and natural features over a given area (Topographical maps generation) for purposes of recreational, geographical, navigational, geological or engineering applications  Cadastral surveys -Deals with establishment of boundaries and ownership of land and property for registration, valuation and legal purposes.  Engineering surveys -Embrace all survey work required before, during and after any engineering works such as roads, railways pipelines, dams, airports e.t.  Hydrographic surveys -Carried out on water bodies such as seas, oceans, rivers, and dams to determine the depth of water and investigate the nature of the sea bed. To map coastlines in order to produce navigational charts and offshore oil exploration. For construction and maintenance of harbours and inland water routes, control of pollution and the scientific study of the ocean (Oceanography)  Photogrammetry -Mapping from the Aerial stereo photographs by use of photogrammetric stereo-plotters especially for military and engineering purposes.

METHODS OF SURVEYING

Chain surveying: The mapping of a point feature(C) is achieved using simple techniques (mainly offsets and ties from a baseline)

Traversing: The mapping of a point feature(C) is achieved using bearing and distances only of a given line from a known position

Trilateration The mapping of a point feature(C) is achieved using intersecting distances only from the end points of a given baseline

Triangulation The mapping of a point feature(C) is achieved using intersecting angles from the end points of a given baseline

Baseline

C

A B

Offset

B Baseline B

C

A

T1 T

Baseline

C

A B

D 

Baseline

C

A

D1 D

 θ

Baseline

C

A B

Degrees of Reliability in Surveying

 Accuracy -The closeness with which a set of measurement of the same subject are to the true value.  Precision - The closeness with which a set of measurement of the same subject are to each other.

Some Important Conversions in Surveying:

-Angular Measure 2π = 360º= 400g -Linear Measure 1 m = 3 ft; 1 ft=0 -Area Measure 1 ha = 2 acres = 10,000 m 2 ; 1 acre = 4047 m 2 -Volume Measure 1 m 3 = 1000 litres

Important Factors to Consider when Starting a Survey

1. The purpose of the survey
2. The degree of precision and/or accuracy for that purpose and for each measurement
3. The possible sources of errors and how to minimize or eliminate them
4. How to verify the correctness of survey
5. The organization of the survey for cost effectiveness

Important Points to Note when Executing a Survey

1. After recce, make a sketch of site
2. Make enough, including, redundant measurements
3. Taking a bearing of at least one of the lines (especially the baseline)

Some Necessary Precautions in Surveying in Order to Avoid Mistakes

 Record all field data in the field book the moment it is determined  Adopt a system to check all data at the time it is recorded  Arrange the survey so that the results can be checked by office computation.  Reasonable care against mutilation or spoiling or watering of field book  No erasing of mistakes in the field book instead cancel and countersign  Field book should never be lost

Typical Scale Ranges

-For detail plans 1:20 ............ 1: -For site plans 1:500 ............ 1: -For large scale maps 1:10000.............. 1: -For small scale maps 1:1000000 .............. 1:

ENGINEERING SURVEYING PROCEDURES

1. Undertake a thorough reconnaissance
2. Construct the control stations-either semi-permanently or permanently as the situation dictate
3. Take all the necessary (and even redundant measurements) of angles, distances, and heights and record appropriately
4. Compile planimetric (x,y) and elevation(z) positions of control points
5. Execute detailed surveying if a plan is required NB: The detailed plan is the basis of engineering design.
6. If setting out is to be done, calculate the angles and distances from/to control points to/from individual engineering feature.
7. Produce additional drawings for x -section and profile and execute further computations for earthworks as necessary.

II. LINEAR MEASUREMENT

Direct Linear Measurement:

Distance as a parameter is obtained directly.

Technique: Chain Surveying Technique

Methods: Ground chaining; Step chaining; and Catenary system

Indirect Linear Measurement:

Distance as a parameter is derived from other measured parameters

Techniques include:

1. Optical Distance Measurement. Example: - Tacheometry where the observed parameters that are used to derive distances are vertical angles and stadia readings. Tachy equations are then used not only to derive horizontal distances between the tacheometer and the leveling staff but also the reduced level (elevation) of the staff position.

2. Electromagnetic Distance Measurement. Example by use of an Electromagnetic Distance Meter (EDM) and a reflector. The EDM utilizes electromagnetic energy that propagates to and reflected by the reflector back to the EDM for computation and display of the distance after atmospheric errors corrections.

3. Satellite Based Techniques. Example is by use of Global Positioning System (GPS) which apply the basic principle of resection to fix the ground position of the GPS receiver given the known positions of at least three satellites poisoned in the outer space. A very cost-effective method for mapping large areas of difficulty terrain and poor inter- visibility.

CHAIN SURVEYING

-Simplest and the oldest form of land surveying. The principal equipment used for linear measurement being (then) the surveyor’s chain.

-It is based upon the fact that if a triangle is set out upon the ground, and lengths measured it may later be plotted at any desired scale.

-It is of lower order accuracy

Main uses:

 Preparation of site plans  Area computation  Setting out simple works

Equipment used:

a) For linear measurement: I. The surveyor’s chain.

-Usually made of steel links measuring 0 each.

-Brass or plastic tags attached every 2m for some designs and red tags attached every 5m and yellow tags every 1m for some other designs.

-Found in 20m, 30m or 50m in length.

Advantages:

-Robust, easily read, easily repaired if broken in the field and can be read from both sides.

Direct Distance Measurement

Ground chaining: The Chain/ tape/ band lied on ground surface.

AB = L1 + L

Step chaining: Short horizontal steps are used at every change of slope to determine the total distance.

Catenary system: The tape is suspended in catenary where it is challenging to place it on the ground

Factors to Consider when Locating Survey Stations

General topography; -Avoid step and uneven slopes. Obstructions; -Avoid obstructions as is practicable.

L1 L

B A

H H2 H

D= H1+H2+H

L1 L

S1 S

Baseline; -The baseline to be as long and as central as possible. Survey lines; -To be as few and as close and as parallel to detail as possible. Check lines; -To be provided for all independent figures. Triangles; -To be as well-conditioned as is practically possible

Field Procedures

Reconnaissance (Recce);

• taking a general view of the site to obtain a general picture of what the situation is before the actual survey. -It involves: -Decision on the purpose of the survey. -Determination on the degree of accuracy/precision of measurement. -Establishment of the method or technique of measurement. Ranging and measuring. -The chainman holds the front of the chain while the surveyor holds the other end. -Ranging rods {poles} are placed at the two ends A and B of the chain line AB. -By sighting the end B rod, the surveyor directs the assistant to be on line AB and to pull the chain to be straight and tight to a full chain length or to be at B. -The assistant places a chaining arrow to mark the end if one full chain. -The chairman then pulls the chain again towards B until when the surveyor is at the chaining arrow. -The surveyor removes the arrow and replaces it with a ranging rod. -The above steps are repeated until the end B is reached. -NB. All the required off sets and subsidiary measurements are taken before the chain is dragged ahead and booked accordingly. -TOTAL LENGTH: AB= (No. of Red Tags *5 + No. of Yellow Tags 1+ No. of Links 0) m

Land Surveying Notes by Magondu G

Field Offsetting: Some of the equipment/ tools that could be used to do the offsets include: Cross staff, Optical square, Optical prism, and box sextant. Some Geometrical Methods of Erecting Offsets

1. Using Pythagoras Theorem (3-4-5)

2. from equal arcs.

B

A C

50

P

40

30

P

P1 P A B

-The baseline P1-P2 is marked by the surveyor’s chain or any other practical method -Point A where the offset is desired is marked with a chaining arrow -A tape is used with zero unit and 12th (3+4+5) unit marks held at a chaining arrow placed at point C, 30 units from point A.

• While holding the 7th (3+4) mark, the tape is pulled tight and horizontally, away from the baseline and a chaining arrow is placed at point B. -Line B-A is perpendicular to the baseline P1-P2 at A

• From point M where the offset is to be done, and using a convenient radius, mark points A & B along the baseline P1-P2 with chaining arrows -Again using a convenient and equal radius from points A and B, mark points C and D. Then line C-D is perpendicular to P1-P2 at M -Alternately, open a long convenient tape length between A and B. Hold the mid-point of this radius and pull it tight and horizontally to points C and D on either side of the baseline. Mark these points with arrows

M

C

D

3. By Creating Isosceles Triangle or by swinging the tape

4. By Creating Two Isosceles Triangles (Students to draw and write notes to explain the construction)

Principle factors/features involved in booking a chain survey

-The field notes should be clear and non-ambiguous.

-The field book is used from the back to the front.

-The two parallel lines (12mm) printed down the centre represent the chain-line and are

used for dimensions only.

-Begin each line at the bottom of a fresh page.

-Take plenty of room and make no attempts to scale the bookings.

-Exaggerate any small irregularities which are plottable.

-Make small sketches of all details inserting explanatory remarks where necessary.

P

T

T

P

M T

-Whereas the construction of isosceles triangle can be done either from the baseline to the feature or vice versa, the swinging the tape method can only be done from the feature to the baseline

-From point M where the offset from the baseline to the feature is required, measure equal distances to points A and B

-From points A and B, measure equal distances to point T to complete the triangle

-Then M-T is the required offset.

-If the offset is required from the feature T to the baseline, then the isosceles triangle can be constructed from T as the apex and A-B as the base. Bisect A-B at M

-Alternately, using a convenient radius, swing the tape in horizontal plane from T to cut the baseline at points A and B respectively. Mark and bisect A-B at M. Then T-M is the required offset.

A

B

-Determine the scale of the final drawing. -Select the equipments and check the funtionability and accuracy. -Carry out the field work keeping in mind the possible sources of errors. -Plot the preliminary and then the fair drawing adding the necessary information.

III. LEVELLING

Introduction

Leveling is the process of height determination. It finds application in all stages of engineering survey,

from topographical mapping to setting out. Through leveling, the heights of points above the mean sea

level (elevations) are determined and hence parameters such as gradient or slope could be computed.

In order to execute leveling, an optical or digital level is required to be used in conjunction with a tripod

and a leveling staff.

Some Basic Terminologies in Leveling

 Ordinance (M.S) datum: Makes international height comparison possible  Arbitrary Datum: Used in a localized site where a Temporary Bench Mark (T.B) is used instead.  T.B: Reference point whose height is assigned arbitrary.  Bench Mark (BM): A permanent reference point whose height is accurately known.  Reduced level (R) is height of a point based on the chosen datum( mainly M.S)  Level line/ level surface: a line/ surface on which all points are perpendicular (normal) to the direction of gravity such a line/ surface is parallel to M.S and hence curved  Horizontal line/horizontal surface is tangential to level line/level surface at a particular point.  Back sight (B): First staff reading/position at any instrument position  Foresight (F) : Last staff reading/position at any instrument position  Inter-sight or intermediate sight (I) : Staff reading/position between a B and F  Change Point (C): A survey point where both F and B readings are observed  Line of Collimation: An imaginary line passing through the centre of the diaphragm and the optical centre of the objective lens  Height of Collimation (H): The reduced level of line of collimation

Fig: Diagrammatic illustration of some leveling terms and the principle of leveling

Principle of Levelling

If a level is in proper adjustment (i. not malfunctioning) and is properly leveled (set up), it establishes a horizontal line of sight which is also parallel to the M.S over short distance. Hence a mathematical relationship between the M.S and the horizontal line of sight can be established

From the figure above, it can be established that

(R+RA) = (R+ RB) = 1st Ht. of collimation

Earth Curvature

Horizontal Line

Level Line

RA

A B C I 1

I 2

RB

RB’ RC

Datum

R R R H 1

H 2

General Levelling Procedure

o Set up the level equipment firmly and securely on a tripod at a convenient point o Use tripod stand legs to bring the circular/pond/ rough bubble approximately to the centre o Sight target and bisect the target staff o Level the main/sensitive bubble (if any) o Remove parallax by first focusing the crosshairs and then the target o Observe the staff reading and book then appropriately Categorization of levels

Category based on precision a) Precise levels: for precise leveling task b) Ordinary conventional levels: for ordinary day to day leveling operations Category based on technology a) Optical level b) Digital levels Category based on design/functionality a) Dumpy level b) Tilting level c) Automatic level

Some Characteristics of Levels Based on Design

 DUMPYLEVEL

-Gives inverted image

-Once leveled remains leveled in all directions

-The telescope and the vertical spindle are cast as one unit

-It has both the circular and the main bubble

NB: It is structurally stable for most engineering works. However errors of reading could occur easily due

to inverted image. It also has a more elaborate leveling procedure.

 TILTING LEVEL

-This level can tilt slighting in the vertical plane

-Gives erect image

-The main bubble must be leveled in every direction of pointing

NB: A tilting level is versatile to use especially in areas of broken terrain.

 AUTOMATIC LEVEL

-Has only the circular bubble

-Has a compensator system [a system of suspended mirrors and prisms] that automatically compensates

for slight mis-levelment

-Has a powerful telescope

-Gives erect/upright image

-Easiest and fastest to level in the field

NB: Automatic level though more robust in construction than the tilting level suffer vibrational

instability [due to suspending compensator sys] and therefore may not be appropriate on construction

sites with heavy earthworks machinery.

Permanent adjustment of levels

The objective is to ensure that the line of collimation would be exactly horizontal once temporary

adjustment has been performed.