Technical Terms Used in Levelling

1. Level surface: A level surface is any surface parallel to the mean spherical surface of the earth. It is a curved surface, which at each point is perpendicular to the direction of gravity at that point. Every point on a level surface is equidistant from the centre of the earth (Figure 27.8).
2. A level line: A level line is a line lying in a level surface. It is normal to the plumb line at all points (Figure 27.9).  Figure 27.8 Level surface  Figure 27.9 Figure showing level line, datum, mean sea level and ground surface
3. A horizontal plane: A horizontal plane through a point is a plane tangential to the level surface at that point.
4. A horizontal line: A horizontal line is any line lying in the horizontal plane.
5. A vertical line: A vertical line at any point is a line normal to the level surface through that point.
6. Elevation: The elevation of a particular point is the vertical distance above or below a reference surface. Usually the mean sea level is used as reference.
7. Mean sea level (MSL): Mean sea level is obtained from the average height of the sea’s surface for all the stages of the tide, over a period of 18.6 years.
8. Datum: Datum is any arbitrarily assumed level surface to which elevations are referred.
9. Reduced level (RL): The reduced level of a place is its elevation or vertical distance above or below the datum or any fixed point.
10. Line of sight: It is the imaginary line joining the intersection of the cross hairs of the diaphragm to the optical centre of the object glass and its continuation.
11. Back sight (BS): A back sight is the first staff reading taken after setting up the instrument at any position. This will always be a reading on a point of known elevation. It ascertains the amount by which the line of sight is above or below the elevation of the point. Back sight enables the surveyor to obtain the height of the instrument.
12. Fore sight (FS): A fore sight is the last staff reading taken before shifting the instrument. This will always be a reading on a point whose elevation is to be determined. This reading indicates the shifting of the instrument. It is also generally known as minus sight as the fore sight reading is always subtracted from the height of the instrument (except when the staff is held inverted) to obtain the elevation.
13. Intermediate sight (IS): An intermediate sight is any staff reading, taken on a point of unknown elevation, after the back sight and before the fore sight. This is necessary when more than two staff readings are to be taken from the same position of the instrument. It may be noted that for one setting of a level there will be only a back sight and a fore sight but there can be any number of intermediate sights.
14. Change point (CP): This is an intermediate staff position and it is used for the purpose of shifting of the instrument. Both back sight and fore sight are taken from this intermediate staff position. Great care is necessary in taking readings at the change point since an error in reading affects every succeeding point of observation (elevation). Any firm point, which can be easily found, may serve as a change point.
15. Bench mark (BM): A bench mark is a fixed point of known elevation and the elevation of this point will be noted on it with respect to a datum. The reduced level of bench mark is used to determine the reduced levels of other points. The bench mark is usually taken as the plinth of an important building, top of parapets of bridges, etc. It will be marked by an identifying mark like cross mark or a circle with a dot at its centre.

There are four kinds of bench marks.

1. Great trigonometric bench marks (GTS)
2. Permanent bench marks
3. Arbitrary bench marks
4. Temporary bench marks

Figure 27.10 Figure showing a permanent bench mark

Great trigonometric bench marks (GTS) are established by the Survey of India Department. They will be marked with very high accuracy at intervals, all over the country by this department. Their positions and elevations above the standard datum are given in the catalogue published by that department.

Permanent bench marks are established when the distance between the GTS bench marks is very large. Hence, fixed reference points are established at closer intervals. They are established by the Government agencies such as the local surveying departments, on clearly defined and permanent points such as the top of a parapet wall of a bridge or culvert, a corner of the plinth of a building, a gate pillar and a kilometre stone. On a vertical surface there will be a broad arrow and a horizontal groove, the centre of which is the point of reference (Figure 27.10).

In small levelling works, the surveyor can assume a well-defined point for reference. The reduced level of that point is arbitrarily assumed (say as 100.00, 50.00 etc.). These are called arbitrary bench marks.

Temporary bench marks are the reference points established at the end of a day’s work or when there is a break in the work. This becomes necessary when the whole levelling work cannot be completed at a stretch. The work when started again is continued with reference to these bench marks. They should be carefully established on definite and comparatively permanent objects, which can be easily described and found, such as the top of a stone gate, posts, spikes in the roots of the tree and highest point of solid rock.