CH Tides, Water Levels & Vertical Datums 2 — Questions and Answers
Question 1: What is the primary cause of the diurnal inequality observed in mixed semidiurnal tides?
- Variation in solar radiation intensity
- The declination of the Moon and Sun relative to the equator (Correct answer)
- Differences in ocean basin depth
- Atmospheric pressure fluctuations
Correct answer: The declination of the Moon and Sun relative to the equator
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?
- 1960–1978
- 1983–2001 (Correct answer)
- 1991–2009
- 2001–2020
Correct answer: 1983–2001
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?
- The difference between observed water level and the chart datum at time of sounding (Correct answer)
- The horizontal offset caused by tidal currents
- The barometric pressure effect on water surface elevation
- The salinity-induced density difference in the water column
Correct answer: The difference between observed water level and the chart datum at time of sounding
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?
- Mean Lower Low Water (MLLW)
- Mean Sea Level (MSL)
- North American Vertical Datum of 1988 (NAVD 88) (Correct answer)
- International Great Lakes Datum (IGLD 85)
Correct answer: North American Vertical Datum of 1988 (NAVD 88)
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?
- Epoch correction and seasonal offset
- Time difference and height ratio (or difference) (Correct answer)
- Phase lag and amplitude gain
- Range ratio and datum offset
Correct answer: Time difference and height ratio (or difference)
Subordinate station predictions are derived from the control station using a time difference (arrival offset) and a height ratio or difference.
Question 6: In the context of tidal analysis, what does the term 'spring range' refer to?
- The tidal range during springtime months
- The difference between MHHW and MLLW
- The mean range of tide at times of new and full Moon (Correct answer)
- The maximum tidal range recorded in a given year
Correct answer: The mean range of tide at times of new and full Moon
Spring range is the mean tidal range during spring tides, which occur near new and full Moon when solar and lunar tidal forces reinforce each other.
Question 7: What is the significance of the 'node factor' (f) used in harmonic tidal prediction?
- It corrects for the 18.6-year lunar nodal cycle's effect on tidal amplitude (Correct answer)
- It accounts for the elliptical orbit of Earth around the Sun
- It adjusts predictions for local atmospheric pressure variations
- It represents the phase difference between solar and lunar constituents
Correct answer: It corrects for the 18.6-year lunar nodal cycle's effect on tidal amplitude
The node factor f adjusts constituent amplitudes to account for the 18.6-year regression of the lunar nodes, which modulates tidal amplitude.
CH Tides, Water Levels & Vertical Datums 2
What is the primary cause of the diurnal inequality observed in mixed semidiurnal tides?