leveling,
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| reason for leveling | the measurement process to determine height hight on maps ensures construction at correct gradient enables horizontal distance to be calaculate
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| principles of leveling | uesd to measuer the difference in height between two points if the hight of one point is known the the hight of another can be determined
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| level line | one that is at a constant height- realitive to the mean sea level and because it follows the mean surface it must be curved itself
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| Datum | a level line is chosen to which the elevation of all points are related in the uk chosen datum is msl at newlyn cornwall
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| Horizontal line | line tangential to the level line at any particular point. it is perpendicular to the direction of gravity at any point
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| line of colimation | the line of site of the instrument
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| reduced level | height of a point realitive to the chosen datum
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| Benchmarks | reference marks or points, the reduced level of which have been accurately determined by levelling
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| Ordnance Bench Marks (OBM) | established on Ordnance from datum at Newlyn, Cornwall
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| Temporary Bench Marks | set up on stable points near construction sites where no OBM is found
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| collimation error | due to the line of sight not being horizontal
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| instrument set up midway between two points | due to curvature of the earth
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