levelling
LEVELLING
Aim: to learn the basic levelling principles, theory and applications and to be able to book and reduce levelling data.
Levelling refers to height measurements for representing the relative difference in height (altitude) between various points on the earth’s surface.
Basic equipment
(a) A device which gives a truly horizontal level (the Level).
(b) A suitably graduated staff for reading vertical heights (the Levelling Staff).
Types of levels
(a) Tilting – adjustment of level bubble needed before every measurement.
(b) Dumpy – adjustment of level bubble needed only once after level set up.
(c) Automatic levels – self levelled instruments.
Bench Mark and Reference Datum
In order to calculate the heights of points a datum is required, i.e. a reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows:
Bench Mark (BM) – a point with known height above mean sea level (or other reference datum).
These are permanent points (e.g. unchanged by weather conditions) and are provided by the
Department of Lands and Surveys.
Temporary Bench Mark (TBM) – a point of known height above a pre-defined level. This level is not absolute and is defined locally by the surveyor for the purpose of the survey. Based on the TBM the survey may then later be reduced to absolute levels if the level of TBM is known.
The height of any target point is referred to as Reduced Level (RL), because it is reduced to a known datum.
Fig.1 Levelling line. For large areas a correction is required for curvature of the earth.
Common sources of errors in levelling
1.
2.
3.
4.
5.
Instrument not correctly levelled.
Telescope not correctly focused.
The wrong cross-hair reading recorded (e.g. top instead of middle).
Staff incorrectly read or not held vertical.
Staff incorrectly booked.
All the above are mistakes (blunders) and cannot be corrected unless the work is repeated. A
Aim: to learn the basic levelling principles, theory and applications and to be able to book and reduce levelling data.
Levelling refers to height measurements for representing the relative difference in height (altitude) between various points on the earth’s surface.
Basic equipment
(a) A device which gives a truly horizontal level (the Level).
(b) A suitably graduated staff for reading vertical heights (the Levelling Staff).
Types of levels
(a) Tilting – adjustment of level bubble needed before every measurement.
(b) Dumpy – adjustment of level bubble needed only once after level set up.
(c) Automatic levels – self levelled instruments.
Bench Mark and Reference Datum
In order to calculate the heights of points a datum is required, i.e. a reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows:
Bench Mark (BM) – a point with known height above mean sea level (or other reference datum).
These are permanent points (e.g. unchanged by weather conditions) and are provided by the
Department of Lands and Surveys.
Temporary Bench Mark (TBM) – a point of known height above a pre-defined level. This level is not absolute and is defined locally by the surveyor for the purpose of the survey. Based on the TBM the survey may then later be reduced to absolute levels if the level of TBM is known.
The height of any target point is referred to as Reduced Level (RL), because it is reduced to a known datum.
Fig.1 Levelling line. For large areas a correction is required for curvature of the earth.
Common sources of errors in levelling
1.
2.
3.
4.
5.
Instrument not correctly levelled.
Telescope not correctly focused.
The wrong cross-hair reading recorded (e.g. top instead of middle).
Staff incorrectly read or not held vertical.
Staff incorrectly booked.
All the above are mistakes (blunders) and cannot be corrected unless the work is repeated. A