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.

Explanation

 A through compass is used to orient the plane table by aligning the drawing sheet with the magnetic meridian, ensuring that the table is correctly positioned relative to the Earth's magnetic north.

Explanation

 A compass is used in plane tabling to orient the table by aligning it with the magnetic north, ensuring accurate plotting of directions and angles on the sheet.

Explanation

 An alidade is a straightedge with sighting devices (like vanes or a telescope) used in plane table surveying.

It helps in aligning and drawing lines of sight from the station point to various objects in the field directly on the drawing sheet.

Explanation

This  angle measures how much one line deviates from another, often used

in surveying and navigation to establish the direction of a new line relative to a reference line.

Explanation

 Chain surveying is recommended when the area is relatively small and flat.

It is suitable for areas where precise measurements are required but complex terrain and large distances are not a factor.