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what is bench mark and datum line
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Definition of Datum Line:

An assumed surface used as a reference for the measurement of heights and depths. A datum (plural datums or data) is a reference point from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the Earth's surface against which position measurements are made. Horizontal datums are used for describing a point on the Earth's surface in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths.

Definition of Benchmark:

Any reference point on site which has had a height assigned to it is known as a bench
mark. For most surveys and construction work, several bench marks would normally
be established by levelling from the datum. If heights are based on an arbitrary
datum these are known as Temporary Bench Marks or TBMs.
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