Tide Tables and Time
Tide tables predict the state of the tide and are published in almanacs, locally produced booklets and some newspapers. Depending on your geographical location the heights and times of the tides vary significantly and formal tide tables are published for a number of important Standard Ports. Secondary ports generally do not have there own published tide tables but do have corrections that can give accurate calculation of tidal times and heights.
Fig 5.18 shows page 33 from the RYA Training Almanac and tide tables for Victoria which is the primary ‘Standard Port’ used in the RYA Training Materials. These tables predict the heights and times of high and low water.
The tide tables are displayed in monthly columns and then grouped into individual days. Each column gives the predicted heights in metres and times of high and low water in 24hr clock format. The time zone box at the top left of the page indicates to us which time zone the table relates to. Remember that UT (Coordinated Universal Time) time is very similar and we regularly interchange it with Greenwich Mean Time (GMT) .
Like the UK, the Northern Territories in the RYA training material use UT time and therefore are in Zone 0. All the times given in the tide tables are in UT (Zone 0) time. In Uk we are familiar with putting the clocks forwards during summer time, we can do the same with the times on tide tables. The time zone box at the top left hand corner of the page also tells us that to get Summer Time (NT DST) we can add one hour to the times in non-shaded areas. So from March to late October we would add an hour to get UT Summer Time (NT DST) which is the local time you would have on your wrist watch.
Please remember that in this course we may refer to NT (northern territories time) which is exactly the same as UT (coordinated universal time UTC) or our GMT time in the UK. If we add an hour to the times in the non-shaded areas we get NT DST (Northern Territories Daylight Saving Time) which is equivalent to the UK BST British Summer Time.
The second column of numbers indicated to us the predicted height of tide for any given date and time. These are the heights of high and low tide and measured from chart datum CD. If you examine these numbers you will notice that measurements of high and low tide change considerably over a lunar cycle (29.5 days) as we move from periods of spring to neap tides.
If we take an example of Saturday February 2nd, we can see that at 0115 hours the sea level is 5.5m above chart datum (Fig 5.19). At 0714 hrs the tide has gone out and and sea level is at 0.8m. By 1333 the tide has flooded with a high tide level of 5.8, and then ebbs to the low tide level of 0.6m. We previously mentioned the daily tides being slightly different heights (Fig 5.4), this is numerically indicated on the tide tables and can see that the first high tide was 30cm lower than the second high water that day.
When you look at Sunday February 3rd you can see that there is another high tide at 0204hrs followed by a low tide as the pattern of flooding and ebbing repeats itself. Even with the limited amount of tidal information presented in Fig 5.19 we can see that we are moving from Spring Tides towards Neap Tides which is indicated by lessening tidal ranges.