Datum Surface
A datum surface or line is any arbitrarily assumed level surface or line from which vertical distance are measured. Mean see level is usually used as standard datum, however assumed datum may be used for minor surveys.
Bench Mark
It is permanent established mark, the elevation of which is known to use as reference point in obtaining the elevations of subsequent stations. The elevation may be assumed in case it is not known.
Back Sight (B.S.)
A back sight also termed as a back sight reading, is a staff (or rod) reading taken on a point of known elevation as on a bench mark or a change point. It is also called a plus sight. It is the first reading taken after the level is set up and leveled.
Foresight (F.S.)
A foresight also called foresight reading, is a staff (or rod) reading on a point whose elevation is to be determine as on a change point. It is also termed as a minus sight. It is the last staff reading with the same setup of instrument denoting the shifting of the instrument. It must be remembered here that the foresight does not imply any direction , but it denotes the reading.
Intermediate Sight (I.S.)
An intermediate sight is any other staff reading taken on a point of unknown elevation from the same set-up of the level. All sights taken between the back sight and foresight are intermediate sights.
Elevation
The vertical distance of point from the datum is called elevation. It is also known as the reduced level (R.L.) The elevation of a point is plus or minus accordingly as the point is above or below the datum.
Elevation = H.I. – I.S. or F.S.
Line of Collimation
The line of collimation is the line joining the intersection of the cross-hairs to the optical center of the object glass and its continuation. It is also called the line of sight.
Axis of the telescope
An axis of the telescope is a line joining the optical center of the object glass to the center of the eyepiece.
Change point or Turning point (T.P.)
A change point is a point denoting the shifting of the level. It is a point on which the fore and back sights are taken. Any stable and well defined object such as boundary wall , rock, etc is used as a change point. A bench mark may also be taken as change point.
Station
A station is a point whose elevation is to be determined or a point which is to be established at a given elevation. It may be noted that it is point where the staff is held and not the point where the level is set up.
Height of Instrument. (H.I.)
The height of horizontal line (line of sight) from the datum is called height of instrument.
H.I. = B.S. + Elevation
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