A horizontal plane: A horizontal plane through a point is a plane that is tangent to the surface of the surface at that point.
Horizontal Line: A horizontal line is any line lying in a horizontal plane.
Vertical Line: A vertical line at any point is a line normal to the surface through that point.
Elevation: The elevation of a particular point is the vertical distance above or below a reference surface. Usually mean sea level is used as a reference.
Mean Sea Level (MSL): The mean sea level is derived from the mean height of the sea surface over a period of 18.6 years for all tide phases.
Datum: A datum is any arbitrarily assumed level to which elevations are referenced.
Reduced Level (RL): The reduced level of a place is its elevation or vertical distance above or below a datum or a fixed point.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.
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.
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.
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.
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.
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.
Temporary bench marks
Great trigonometric bench marks (GTS)
Permanent bench marks
Arbitrary bench marks
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