Surveying
The hour angle of the heavenly body for Greenwich meridian equals the hour angle of the body for any other meridian + longitude:

Vernal equinox
All listed here
True sun
Mean sun

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Surveying
Contour interval is

Inversely proportional to the scale of the map
Larger for accurate works
Larger if the time available is more
Directly proportional to the flatness of ground

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