TOPIC 2 (LEVELLING)
COURSE LEARNING OUTCOME
- Calculate correctly the differences in height, coordinates for surveying stations and area of a traverse.
1.0 INTRODUCTION
What is leveling……….
- Is a method in surveying to determine height of point on earth surface from a datum.
- The difference in height between points is obtained from the differences of the levelling staff reading.
- The land surveyor Department of Malaysia publishes maps that show the height of various permanent stations above sea level.
- These permanent stations are known as Bench Marks (BM).
- When no BM exists close to a construction site, a temporary bench mark (TBM) must be established.
- Levelling must be carried forward from the nearest BM to establish TBM value
3.0 TYPES OF TRAVERSE
3.1 CLOSED TRAVERSE
- Closed traverse begins from one point with known coordinate and ends at the point with known coordinate.
- There are two types of closed traverse.
3.2 OPEN TRAVERSE
- This traverse begins from one point with known coordinate and ends at a point which is unknown; or both points with unknown coordinates.
- Traverse point obtained using this method may not be accurate because checking could not be made.
4.0 Levelling instruments
The basis equipment required in levelling are:
- a device that gives a truly horizontal line i.e. level
- a suitably graduated staff/ruler for reading vertical heights, i.e. the levelling staff
5.0 Definitions of few levelling terms
- BENCH MARK – A relatively permanent and fixed reference point of known elevation above the assumed datum.
- STATION – The point where levelling staff is held.
- REDUCED LEVEL – The height or depth of a point above or below the assumed datum.
- BACK SIGHT – The first reading taken after setting up the level.
- FORESIGHT – The last reading taken before moving the level to another location or instrument station.
- INTERMEDIATE SIGHT – any reading or sight taken between BS and FS
- CHANGE POINT – The point on which both the fore sight and back sight are taken during the operation of levelling.
- HEIGHT OF COLLIMATION – height of line of sight.
6.0 Booking and reducing the readings
As in all aspects of surveying, the fieldwork must be properly recorded and calculated. In levelling, two methods are available.
- Rise and fall method (RF method)
- The height of the plane of collimation method (HOC method)
- Height of Collimation Method
- This method is simple and easy.
- Reduction of levels is easy.
- Visualization is not necessary regarding the nature of the ground.
- There is no check for intermediate sight readings;
- This method is generally used where more number of readings can be taken with less number of change points for constructional work and profile levellings.
- Rise and Fall Method
- This method is complicated and is not easy to carry out.
- Reduction of levels takes more time.
Visualization is necessary regarding the nature of the ground.
Complete check is there for all readings.
This method is preferable for check levelling where number of change points are more.
REDUCED LEVEL
Obtaining the reduced level is a process of calculating the reduced level at each point.
This can be done by rise and fall or height of collimation method.
CALCULATING REDUCED LEVELS BY RISE AND FALL METHOD
In this method, reduced level at each point is calculated based on the difference of heights between two points.
7.0 CONTOUR
Defined as the line of intersection of a level surface with the surface of ground.
Lines drawn on the map to join points of the same height on the earth surface.
The best method of representation of features such as hills etc.
8.0 CONTOUR INTERVAL
The constant height difference between two contour lines.
Depends upon the following factors:
- The nature of the ground
- The purpose and Extent of the survey
- The scale of the map
- Time and Expense of Field and Office work.
9.0 Example of contour interval
For large scale maps of flat country, for building sites for detailed design work
0.2m to 0.5m
For reservoirs and town planning schemes
0.5m to 2m
For location surveys
2m to 3m
For small scale maps of broken country and General Topographical work
3m, 5m, 10m or 25m.
11.0 USE OF CONTOUR MAP AND CONTOUR PLAN
Study original shape of the earth surface
Identify most suitable site and saving cost for engineer works
Identify water catchments area
Calculate capacity or volume of a pond
Identify volume of cut and fill
Identify intervisibility between two points
Identify the slope of road
Draw longitudinal section and cross section to view the shape of earth surface.
Calculate horizontal distance between contour lines.
12.0 METHOD OF CONTOURING
a. DIRECT METHOD
The contours to be located are directly traced out in the field by locating and making a number of points on each contour
b. INDIRECT METHOD
The points located and surveyed are not necessarily on the contour lines but the spot levels.
GRID LEVEL METHOD
CROSS SECTIONING METHOD
SPOT HEIGHT METHOD
13.0 GRID LEVEL METHOD
This method is most systematic and favoured by many because the contouring process is easy to understand.
Suitable for flat and open survey area
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