lect3survdec204

8/6/2019 Lect3SurvDEC204

1/8

Lecture 3 & 4 - DEC 204( Surveying)

Measurement of Heights :

- Definition of Heights- Use of Heights

- Survey for Heights

Processing for Heights:

Equipment to obtain Heights

- Basic Equipment

- Levelling Instruments

- Levelling Staff

Common Terms in Levelling

- Datum, Level Line, Horizontal Line- Bench Marks, Reduced Level

- Back Sight, Fore Sight, Intermediate Sight

- Change Points

Procedures in Levelling

- The Levelling process to obtain heights

- Differential Levelling, Spirit Levelling

- Fly Levelling

Methods of Booking in Levelling

- Rise and Fall method

- Height of Collimation Method

- Inverted LevelsAccuracy and Errors in Levelling

- Classes of Levelling

- Standards of Levelling, Miscloses- Miscloses and Rejection Criteria

Definition of Height

Heights may be defined as the linear distance along a vertical from a point of concern to a

horizontal reference surface ordatum.

* Along Vertical - Plumb line ie showed by freely suspended weight (plumb bob line)

Datum - Horizontal Surface (MSL) determined by JUPEM over 18.6 YrsDatum station in Port Klang is the reference stn/surface for Heights

Height referred to this surface is transferred to JUPEM in Kuala Lumpur/Jalan

Semarak .

It is called BENCH MARK (Reference Bench Mark ) . From this reference BenchMark other Bench Marks are constructed all over the country

DEC 204 Surveying 1


2/8

* BENCH MARKS (BM)Networks of Bench Mark(BM) throughout the country are available for use by users .

The heights are determined by the Department of Surveying and Mapping, Malaysia

(JUPEM) . It is rigidly built and easily found along Highways .

Temporary Bench Marks (TBM)Temporary Bench Marks (TBM) are built individually for local work . Their heightsare derived (determined)from Bench Marks

* ORTHOMETRIC HEIGHT

This is the commonly used height based on MSL as defined for heights (above) andobtained using spirit levelling. The commonly used unit is meters

Orthometric Heights are for the everyday use and normally used by Engineers,

Surveyors, Users etc.

* DYNAMIC HEIGHT

This is a kind of height with Reference to the gravitational force (gravity values). It isbased on Geopotential Surface with Unit in potential numbers( of g value).

It is not for everyday use.

The height of the mean sea level is of one potential number . Another height may havea different gravitational force and therefore having different potential number. The

difference between the two surface gives a difference of potential number . This

difference in height is the dynamic height difference

Using a specific formula the Dynamic height can be converted to Orthometric Height .

vertical

HorizontalSurface

(msl)

USE OF HEGHT

Basically heights are used for :

i) Scientific investigation purposes eg study of earth shape and earth surfaceii) Determining height difference for points based on MSL

iii) Intervisibility determination

iv) Road construction , Railway Construction and routes etcv) Drainage and Irrigation works for the flow of water as planned

vi) Setting-out works, Cross Sections, Longitudinal Sections, Volumes of Earthworks

vii) Topographic works eg in Contour determination

viii) Spirit Levelling,Height Datum determination and thus BM determination for usersix) Tacheometry

DEC 204 Surveying 2


3/8

SURVEY FOR HEIGHTS

Briefly various methods could be used to obtain heights as listed below.

a) Spirit Levelling

b) Hidrostatic Levellingc) Tacheometric Surveyingd) Trigonometric Levelling

e) Height Traversing

f) Altimetry

g) Contouring

Spirit Levelling/ Differential Levelling

- uses tube bubbles filled with spirit for determining level of instruments

- Process of finding differences between two horizontal surface determined by height orline of collimation of the instrument. To measure this height a staff is used called

leveling staff. Height of collimation is determined by level of cross-hair diaphragm in

the levelling instrument . A line above and below this cross-hair is called the stadia

hair orstadia. Line of collimation is obtained when the instrument is in correct set-up

- Spirit Levelling also called differential levelling determines the differences of height

between points. If the height of one point is known , other points can be determined.The survey therefore starts from one known point ie its value of MSL is known. The

difference of height between two points at one set-up of the instrument gives the height

difference. Height values can be transferred from point to point. It is a simple process.

vertical heightline of collimation

A (RL)

PROCESSING FOR HEIGHT

Equipment for Height Determination

Basic Equipment

DEC 204 Surveying 3


4/8

Levelling is the process of measuring the difference of height between two or more points.

This reflects therefore the need to have equipment that can be used to measure height . Twoinstruments are used :

- Levelling instrument to observe the horizontal line (normal to vertical line)

- Levelling staff to represent the vertical line (plumb line)In lecture 4 explaination about the horizontal surface and the vertical is given . They are the

two basic components in height measurement. The two instruments stated above is used torepresent the basic components in levelling.

Levelling Instruments

This instrument is commonly called the surveyors level or the optical level. It is used to

establish the horizontal line which is tangent to the horizontal surface. The horizontal surface isrepresented by the mean sea level

Basically the levelling instrument consist of a telescope to sight the levelling staff and a spirit

bubble or tube to establish the horizontal line. The telescope is the internal focusing type with adiaphragm having various pattern of cross hairs and stadia hairs. When in focus, the line

passing through the cross hair and the optical centre of the lens of the telescope represents the

line of collimation or the line of sight. All readings of the instrument are taken using this line.Four basic levelling instruments are used :

1. Dumpy Level - fixed or rigid telescope type

2. Tilting Level - telescope can be tilted before readings

3. Automatic Level instrument with compensator for levelling

4. Precise Level - high accuracy instrument with high magnification

5. Digital Level - a new introduction with special features using bar code

Levelling Staff

This instrument is used to measure the distance vertically above the horizontal line. When held

vertical it represents the plumb line or the vertical and the reading represents the height above

the mean-sea level.Levelling staff are designed to be sectional and assembled either telescopic, hinged or by

slotting onto one another vertically in three sections. Most modern staff are made ofalluminium with lengths of 3, 4, 4.25 or 5 meters. The staff markings can be of many form but

the E type is most common. The graduations on the staff is mostly at 0.01m Some staff are

fitted with handles and a leveling facility usually in the form of bubbles to assist verticality .

Common Terms in Levelling

Level Line - Surface where a plumb bob would be normal to it. Actually a curveSurface parallel to the Mean Sea Level.

Horizontal Line - A line normal to the direction of gravity at any point.It is tangentialto the level line at any point.

Datum - A level line of reference. Usually this is the Mean Sea Level

Reduced Level- The height of a point above the chosen Datum.

Bench Marks (BM) - A permanent reference mark with reduced levels accurately determined

Temporary Bench Marks (TBM) - A mark with reduced levels given but transferred from aclose-by Bench Marks. Usually done for temporary purposes

DEC 204 Surveying 4


5/8

Line of Collimation - A line passing through the cross-hair and the optical centre of the

instrument when the instrument is levelled and in adjustment

Height of Collimation - The height of the line of collimation above any Reduced Level

Back Sight - The first reading after the instrument have been set up

Foresight - The last reading before the instrument is shifted

Intermediate Sight - The readings taken after the back sight and before the foresight

Change Points - The position of the staff having foresight and back sight readings.At this point the levelling instrument is shifted to the next station

Procedures in Levelling

Setting up of Instruments

Observation and Reading of Backsight (BS) say Point A (BM)

Observation and Reading of Intermediate Sights (IS) if necessary

Observation and Reading of Foresight (FS) say Point B

Shifting of Instruments to another station

Observation and Reading of Backsight at Point B

Station B is the Change Point

Observation and Reading of IS if necessary or Point C

Repeat of Procedures

Closing of Levelling work at a known BM back to the same point A

Booking of levelling must be done instantaneously with detail andcare

Record of booking must be done in a leveling book

Calculate the Rise and the Fall

Sum up ( ) the Back Sight and the Foresight (14)

Sum up ( ) the Rise and the Fall (15)

Compare the Reduced Level of the first station and last station (16)

Adjust the levelling

Check that the difference between 14 and 15 and 16 are the same

Check accuracy of your levelling work .

Repeat your work if accuracy not within the allowable figure

Detect the sources of your error

Methods of Booking in Levelling

Two methods of booking are practiced in leveling

a) The Rise and Fall Method

b) The Height of Collimation Method

DEC 204 Surveying 5


6/8

The Rise and Fall method

This is the most common form of booking especially when the task is to transfer a height from

one point to another point or to establish a Temporary Bench Mark (TBM) from a Bench MarkFor example you are to establish a TBM at point B and you start from a Bench Mark A

The table below shows the booking in obtaining a height at point B from a known point Awhich is a Bench mark (BM)

DEC 204 Surveying 6


7/8

ReRe

DEC 204 Surveying 7


8/8

ReRe

ReRe

DEC 204 Surveying 8

lect3survdec204

Documents