Surveying
On vertical photographs, height displacement is

Negative for points below datum
Zero for points vertically below the air station
Positive for points above datum
All listed here

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Surveying
Systematic errors

All listed here
Always follow some definite mathematical law
Can be removed by applying corrections to the observed values
Are also known as cumulative errors

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Surveying
The rise and fall method

is not suitable for levelling with tilting levels
is less accurate than height of instrument method
quicker and less tedious for large number of intermediate sights
provides a check on the reduction of intermediate point levels

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