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Scottish Qualification Authority, Officer Of Watch, Navigation Question 2 ANSWERS:-
05 June Explain, with the aid of diagrams, the cause of Spring tides and Neap tides. 06 March Explain, with the aid of a diagram, how the relative positions of the earth, sun and moon influence the tidal ranges experienced. 08 November Explain, with the aid of a diagram, how the relative positions of the earth, sun and moon influence tidal ranges.
The diagram forms most of the answer to the question put in Jun 05. However, for most of the world, the largest spring ranges come about one to two days after new and full moon; and that the smallest neap ranges come about one to two days after quadrature. An exception to this is in Hawaii, where the largest spring tide comes before syzygy. With due alteration of details, what I have said about range also applies to the strength of the tidal stream.
Then the next two similar questions involve a little more: At perigee, when the moon is closest to earth, its increased effect will increase tidal range; and at apogee decrease tidal range. At perihelion, when earth is closest to the sun, its increased effect will also increase tidal range; and at aphelion decrease tidal range. Near each equinox, when both the moon and sun stand in the same declination, this alignment will also increase tidal range; and at the solstices decrease tidal range. -------------------------------------------------------------------------------------------------------------------- Oct 05 Discuss the reliability of the tidal predictions contained in the Admiralty Tide Tables, giving reasons for discrepancies between predicted tidal heights and actual tidal heights. Mar 06 Explain the reliability of the tidal information contained in the Admiralty Tide Tables. State reasons for possible discrepancies between published tidal heights/times and actual tidal heights/times experienced. Nov 08 Explain the reliability of the tidal information contained in the Admiralty Tide Tables. State reasons for possible discrepancies between published tidal heights/times and actual tidal heights/times experienced. Jul 09 State SIX possible reasons for differences between the published tidal heights/times and the actual tidal heights/times experienced.
While phrased differently, these questions all ask the same thing. However the last asks for six reasons and I think there are only three: 1 human errors in prediction and publication, 2 seismology a, altering tide gauge or chart datum elevation & b, producing tsunami, 3 abnormal meteorology. The meteorology then divides into four subdivisions: i Variations in tidal heights are mainly caused by strong ii or prolonged winds iii and by extreme barometric pressure. v Abrupt changes in meteorological conditions can oscillate the sea’s level. The tables of predictions are made for conditions of normal or average meteorology. This subject is discussed best on pages i & ii of your UK tide table – and you have that with you in the exam.
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Oct 06 With reference to the Pacific and Atlantic Oceans Admiralty Tide Tables, state the circumstances when intermediate tidal heights may not be found by use of the Standard Curve and Part II correction tables. Nov 07 State the instances when intermediate tidal heights may not be calculated for Pacific ports. Mar 08 Cape Cod 2787.
When, at the bottom of the daily pages, as in Balik Papan and Halifax, there is a note saying FOR INTERMEDIATE HEIGHTS USE HARMONIC CONSTANTS (SEE PART II), then the Standard Curve and Part II correction tables may not be used. Intermediate tidal heights may also not be calculated when the duration of rise and fall is less than five hours or more than seven hours; nor may they be calculated when there is a Shallow Water Correction. There are two notes to this effect in the front of the Pacific tide tables.
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Jul 07 State any additional specific information that should be brought to the attention of the Master. [Monggum P’o 7468]
The t at the end of the Monggum P’o entry says as a footnote that Time differences are approximate. You must read all of these footnote. The c’s, rather common in the UK tables, say that FOR INTERMEDIATE HEIGHTS USE HARMONIC CONSTANTS (SEE PART II).
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Nov 08 State the criteria used to determine which Standard Port a Secondary Port is linked to when the Part II correction tables were compiled.
Quoting from page i of either tide tables: The data on which the secondary port differences are based are extremely variable in quality. In order for the Part II differences to be as reliable as possible, it is necessary that the standard port (selected by the Hydrographer) should have tidal characteristics which are similar to those of the secondary port. In some cases there is no local standard port with similar characteristics and it is necessary to choose a standard port some distance from the secondary port concerned.
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Jul 09 Explain the term Lowest Astronomical Tide.
Page 4, Table V, of the Pacific tables gives several definitions. LAT (Lowest Astronomical Tide) is the lowest level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions; these levels will not be reached every year. LAT is not an extreme level that can be reached, as storm surges may cause considerably lower levels to occur.
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