Introduction to Levelling.
Levelling is the process of determining the difference in elevation between two or more points on the earth's surface. It is of particular importance to engineering works, both in the design stages and during construction operations.
Methods of obtaining difference of height
There are many different ways of obtaining differences in height.
Spirit Levelling
Spirit levelling is any of levelling that uses a spirit level (bubble), similar to the bubble in a carpenter’s level, as part of the procedure. It is the term usually applied to the traditional use of level and staff in obtaining differences in height.
The term Differential Levelling is also used to describe the same process. The telescope of a level provides a horizontal line (line of collimation or a line of sight). The height of points are determined by measuring downwards from this line, by sighting on to a graduated staff held on each point in turn.
The term Differential Levelling is also used to describe the same process. The telescope of a level provides a horizontal line (line of collimation or a line of sight). The height of points are determined by measuring downwards from this line, by sighting on to a graduated staff held on each point in turn.
Barometric Heighting
Barometric Heighting is the determination of differences in height based on the premise that atmospheric pressure decreases as altitude increases. The difference in atmospheric pressure is obtained by using an aneroid barometer. This method is suitable for exploratory surveys where portability, compactness and time are important considerations, and a high degree of accuracy is not required, although careful observation can obtain results of the order of ±0.1m.
Trigonometrical Heighting
Trigonometrical heighting is the determination of difference in height by measuring vertical angles and distances. The term often relates to long sights where allowance must be made for the curvature of the earth. To obtain an accurate difference in height between the two points, it is essential that both the distance and the vertical angle is measured from both the observation station to the target station and from the target station to the observation station.
Electronic Levelling
This is a general term used to describe not so much the method, but the type of equipment used. It includes alignment lasers, rotating head lasers and digital readout levels.
1. Level line: A level line is a line lying in a level surface. It is therefore to the plumb line at all the points.
2. Horizontal plane: Horizontal plane through a point is a plane tangential to the level surface at that point.
3. Horizontal line: It is a straight line tangential to the level line at a point
4. Vertical line: It is the line normal to the level line at a point .
5. Datum: Datum is any surface ro which elevation are referred. The mean sea level affords a convenient datum world over, and elevation are commonly given as so much above or below sea level. It is often more convenient however, to assume some other datum, specially if only the relative elevations of points are required.
6. Elevation: The elevation of a point on or near the earth surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum. The difference in elevation between two points is the vertical distance between the two level surfaces in which the two points lie.
7. Vertical Angle: is the angle between two intersecting lines in a vertical plane. Generally one of these lines is horizontal.
8. Mean Sea Level : Mean sea level is the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over along period of 19 years.
The Level
The level is the instrument used to obtain height differences of points above or below a horizontal line. If this horizontal line is at a known height with respect to a known datum, then the reduced level of points may be determined. Although the fundamental principle of a levelling instrument is to establish a horizontal line, accuracy requirements vary widely.
For this reason many different types of levels have been designed, each of which provides optimum performance within its given accuracy classification.
Levels are essentially a telescope, with a compensator for setting a line of sight horizontally. It will provide a magnification of the levelling staff graduations, and at the same time it must be free of parallax.
Types of Levels
1. Dumpy levels
2. Tilting levels
3. Automatic levels
The dumpy level originally designed by Gravatt, consist of a telescope tube firmly secured in two collars fixed by adjusting screws to the stages carried by the vertical spindle. The modern dumpy level has the telescope and vertical spindle cast in one piece and a long bubble tube is attached to the top of the telescope. This form is known as solid dumpy.
The dumpy level originally designed by Gravatt, consist of a telescope tube firmly secured in two collars fixed by adjusting screws to the stages carried by the vertical spindle. The modern dumpy level has the telescope and vertical spindle cast in one piece and a long bubble tube is attached to the top of the telescope. This form is known as solid dumpy.
Both Dumpy and Tilting levels have the line of collimation (sight) set horizontal by means of a levelling tube (bubble). This will set the line of sight at 90° to the vertical axis ie to the direction of gravity.
Automatic levels will still produce a horizontal line of sight if the telescope is almost horizontal which will put it in the range of the compensator. To level these instruments, the vertical axis is set vertical by centring the circular bubble.
Cross hairs.
When sighting through the telescope of a level the observer will see one vertical cross hair and usually three horizontal crosshairs.
The long central horizontal crosshair is the crosshair used to obtain a reading from the staff during levelling. The two smaller horizontal crosshairs are called stadia lines and are used by a surveyor to measure the distance between the level and the staff.
Parallax
With all optical instruments, the image of the target must be sharply focused on the crosshairs before any observations are made. If this is not so, parallax will occur, and could result in staff readings being taken which are not the ‘true’ readings. The phenomenon of parallax can be observed when the observer's head is moved up or down or from side to side when looking through the eyepiece, and the crosshairs appear to move with respect to the image behind them. Accurate sighting is therefore impossible. The diagram below shows parallax as the target image is formed slightly in front of the crosshairs (it could also be formed behind the crosshairStaff.
The Levelling Staff may be rigid, telescopic or hinged, and is usually made of metal or fibreglass. Most are either 3m or 5m in length when extended. It is essential that a staff has a solid ‘foot’ or base. A staff must have some provision for attachment of a levelling bubble to ensure that the staff is held vertical. There are two types in common use:
i. Direct reading staff with 5mm graduations.
ii. The 'E' type, with 10mm graduations and alternate metre graduations in black and red.
2. Change Plates
Change Plates are used to provide a solid base upon which to place the foot of the staff when in soft ground such as on grass or sand, or on smooth, hard surfaces, such as concrete. They may be of two types:
Since a vertical distance is required, it is necessary to hold the staff vertically. The staff bubble assists the staff person to hold the staff vertical. It consists of a circular bubble and stock and is held against the side of the staff.
Other Equipment
1. Levelling Staff:The Levelling Staff may be rigid, telescopic or hinged, and is usually made of metal or fibreglass. Most are either 3m or 5m in length when extended. It is essential that a staff has a solid ‘foot’ or base. A staff must have some provision for attachment of a levelling bubble to ensure that the staff is held vertical. There are two types in common use:
i. Direct reading staff with 5mm graduations.
ii. The 'E' type, with 10mm graduations and alternate metre graduations in black and red.
2. Change Plates
Change Plates are used to provide a solid base upon which to place the foot of the staff when in soft ground such as on grass or sand, or on smooth, hard surfaces, such as concrete. They may be of two types:
- A long spike with a metal plate on top
- A triangular plate with small feet to push into the ground. In both cases a raised centre section to place the foot of the staff on is necessary.
Since a vertical distance is required, it is necessary to hold the staff vertically. The staff bubble assists the staff person to hold the staff vertical. It consists of a circular bubble and stock and is held against the side of the staff.
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