CH Tides, Water Levels & Vertical Datums 2

Question 1

What is the primary cause of the diurnal inequality observed in mixed semidiurnal tides?

Accepted Answer

The declination of the Moon and Sun relative to the equator

Answer

Diurnal inequality occurs because the Moon and Sun are not always above the equator, causing the two daily high tides to differ in height.

Question 2

The National Tidal Datum Epoch (NTDE) currently used by NOAA spans which 19-year period?

Accepted Answer

1983–2001

Answer

NOAA's current NTDE is 1983–2001, a 19-year period selected to average out the full lunar nodal cycle.

Question 3

When reducing soundings to a tidal datum, a hydrographer must apply a 'tide correction.' What does this correction represent?

Accepted Answer

The difference between observed water level and the chart datum at time of sounding

Answer

The tide correction is the vertical difference between the actual water level at the time of sounding and the chart datum, applied to reduce depths to datum.

Question 4

Which vertical datum is commonly used as the reference for land surveys and topographic mapping in the contiguous United States?

Accepted Answer

North American Vertical Datum of 1988 (NAVD 88)

Answer

NAVD 88 is the official vertical reference datum for land surveys and topographic mapping throughout the contiguous US.

Question 5

A subordinate tide station's relationship to a control tide station is described by which pair of parameters?

Accepted Answer

Time difference and height ratio (or difference)

Answer

Subordinate station predictions are derived from the control station using a time difference (arrival offset) and a height ratio or difference.

(CH) Certified Hydrographer Practice Test

(CH) Certified Hydrographer Practice Test

CH Tides, Water Levels & Vertical Datums 2