Height of Tide using the Tidal Curves
Tide tables are great for calculating the height of tide at a specific time. However, if we want an accurate calculation of the heights at a specific time other than low or hight water we have to employ the use of the admiralty tidal curves. These curves are really easy to use and provide us with an excellent tool to accurately predict tidal heights at any given time, or the time of any given height of tide.
Using the tide tables and tidal curve for Victoria in the RYA Training Almanac, what will be the height of tide above chart datum expressed in metres on Wednesday 24th april at 1929 NP DST.
Step 1 – Tide Table Times and Range
In the question we are given the time in Daylight Saving Time and therefore need to take 1 hour off to get Northern Territories Standard Time (1929 hrs NP DST -0100 = 1829 NP). Remember, we always work in standard time with the tide tables and tidal curves. So our target time is 1829 NT and can now identify the tidal levels and range we are interested in.
When we consult the Victoria tide table on 24th April (Fig 5.27) we see that our target time is between a low tide at 1412hrs 0.7m and the following high tide at 2030hrs 5.3m. We can calculate the tidal range between low and high tide as 5.3m – 0.7m = 4.6m.
When we compare our tidal range of 4.6m on the 24th April with the Mean Ranges box (Fig 5.28) on the Victoria Tidal Curve page we can see that we are close to the Spring Tide Mean Ranges (MHWS). This means that in this example we can use the continuous red mean springs curved line on the tidal curve graph rather than the dashed blue neaps line.
Step 2 – Marking up the Tidal Curve
Now we mark the Tide Table heights of High and Low water for the 24th April on the High water (‘A’) and Low water (‘B’) axes on the Victoria tidal curve (Fig 5.29) and draw a straight line between them. To help avoid errors, its worth while inspecting the scale of the High and Low water axes as different curves often have different increments on the scales.
If you look at the time axes on the bottom of the curve you will see that the high water box is below the highest point of the graph with an hour difference on each side of the HW time box (Fig 5.29 ‘C’). The boxes to the left of the HW time box represent hours before High Water and the boxes to the right, hours after High water. The flood and ebb is visually presented in the shape of the tidal curve above the boxes.
The HW box is our reference point for using this graph and can fill in the time of High Water (2030hrs) from our tide tables on 24th April. We can also enter the time of Low Water from the tide tables in the box on the tidal curve time bar. To avoid mistakes it is worth double checking that the time we enter in the HW box is the nearest High-water to our target time in the question, 1829hrs NP and
In this case our target time 1829hrs precedes the High Water time at 2030hrs and therefore interested in filling in the boxes to the left of the HW box.
Step 3 – Entering the Time on the Tidal Curve
We have entered 2030 NP into the HW box of the tidal curve time bar and can now work towards the left progressively taking 1 hour off at a time (Fig 5.30). As we fill the values of the time bar, it becomes easier to visualise what is happening with water level in relation to time.
With 1412UT entered as the time of LW and 2030UT entered as the time if HW it is easy to see how time progresses from left to right. So the boxes should have a range from LW 1412 UT through HW at 2030 UT. We don’t worry too much if the LW time (1412 UT) isn’t exactly 1 hour from the next -5HW box, as it rarely does.
You will notice that there are little division marks along the time axis, and between each hour there are 6 sub divisions (Fig 5.30 ‘A’). Each of these subdivision represent 10 minutes of time. We can now identify and mark our target time (1829 NP) along the axis that we want to determine the height of water for (Fig 5.30 ‘B’).
Step 4 – Reading the Tidal Curve
To find our height of tide for 1829hrs we draw a vertical line upwards from our target time (1829hrs) to the red solid line of the tidal curve (Springs), then horizontally ‘B’ to meet our tidal range line drawn earlier and up or down towards the scale on tidal range scale bar ‘C’. Its best to use the side of the plotter to get all the lines straight and parallel.
In this example we have disregarded the 1minute difference between our target time 1829hrs and -2hr HW time 1830 due to the small scale increments of the tidal curve time boxes.
Step 5 – Correct for DST and put Answer into Context
We have now calculated the height of tide to be 4.3 metres at 1830 hrs UT from the Victoria tide tables and tidal curve. Remember that we have to add 1 hour to Universal Time to get Daylight Saving Time in the Northern Peninsula, so at Victoria harbour at 1930 NP DST the height of tide will be 4.3 metres above chart datum.
In the example above we use the tidal curve to determine the height of tide at a point in time before high-water. It is just as easy to determine height of tides after high water. The main thing to remember is to identify your target time take a methodical approach and you shouldn’t encounter too many problems.