Explanation

 Taken on a known point (like a benchmark).

It’s the starting reading for a new instrument setup.

Used to determine the height of the instrument (H.I.).

Explanation

 Taken on a Change Point (CP)

Marks the end of a level setup.

Used to calculate the reduced level before moving the instrument.

Explanation

 It's the location where the level is set up to take readings.

Chosen for clear sightlines to staff points.

Must be stable and roughly level for accuracy.

Explanation

 Used when shifting the level instrument to a new position.

Ensures continuity in levelling.

Acts as a reference between two setups of the instrument.

Explanation

 One central cross-hair and two stadia hairs (one above and one below).

Used in tachometry to measure distance indirectly.

The distance between the stadia hairs helps calculate the horizontal distance to the staff.

Explanation

Measured from a reference level, usually mean sea level.

Above datum: positive elevation; below datum: negative elevation.

Used in surveying, mapping, and construction.

Explanation

 It is the ground location where the levelling staff is placed.

Readings are taken from this point using a level instrument.

Examples: Benchmark (BM), Change Point (CP), Back Sight (BS), Fore Sight (FS) locations.

Explanation

 It just touches the level surface at one point.

Perpendicular to the vertical (plumb line) at that point.

Used in geodesy and mapping for local approximations of Earth's surface.

Explanation

 It is a surface where the gravitational potential is constant.

Always perpendicular to the direction of gravity.

The geoid is the most well-known level surface, representing mean sea level.

Explanation

 This traditional surveying method uses a horizontal telescope and a graduated rod to measure vertical distances between points. 

It's highly accurate and often employed in geodetic surveys and engineering projects.