What is Height of Tide above Chart Datum?

Worked Example

Using the tide table and tidal curve for Hamilton Sound in the RYA Training Almanac, what will be the height of tide above chart datum on Sunday 4th August at 1147 DST.

Step 1 – Tide Table Times and Range

We know from the question that we are in the southern peninsula (zone -0100) and in summer time (Equivalent to French Summer Time). We must convert our summer time (DST) to standard time (SP) to work with tide tables and curves This means that 1147hrs SP DST (zone -0100) must be converted to 1047 SP (zone -0100).

Fig 5.32 is the tide table for 4th August and shows the range of tides that includes our time that we need to work 0807 hrs 4.7m – 1415 hrs 1.7m = 2 metres tidal range. We can see that this is nearer the mean neap range form the Mean Tidal Range box on the tidal curve page and means that will be working from the dashed blue tidal curve line rather than the solid red Spring curve line (Fig 5.33).

Fig 5.33 – Mean ranges information box found on Hamilton Sound tidal curve.
Step 2- Marking Up The Tidal Curve with Tide Table Information
Fig 5.34 – We must mark up the the heights of HW and LW and the corresponding times on the tidal curve, making sure that we are working in standard time.

We can now mark the heights of LW before and HW after our target time of 1047 SP for the 4th August on the HW and LW axes of the tidal curve ‘A’ & ‘B’ . We can then draw a diagonal line between them ‘C’ (Fig 5.34).

Step 3 – Filling in the Tidal Curve Time Bar
Fig 5.35 – We first enter the time of HW and LW form the tide tables in the time bar. We then fill-in the missing boxes and note where our target time is along the axis.

The timeline at the base of the curve runs from left to right and each box increases by an hour. The centre HW box is our reference point and where we write the time for HW that is given to us in the tide tables. This HW will be 0807 SP as it is the closest HW to our target time in the question (1047 SP). SInce we are interested in a time after high water we are concentrating on the times to the right of the HW box in the time line. We can see from the curve above the time bar that is is sloping downwards so we know that the tide is ebbing towards low water. We can now fill out the boxes in the time line increasing 1 hour to the right of our central reference HW box. We must be sure that all the times we enter in the boxes are in standard time for southern peninsula and not Daylight Saving Time, this means that all times on the curves must be in SP (zone -0100) not in SP DST (-0100). We can enter the time of LW directly from the tide tables on the last box on the right hand side of the tidal curve time bar.

We fill in the boxes from HW then work right +1 hr at a time from our reference point. So in the HW+2 box we add two hours after High Water, then complete the rest of the boxes. Don’t worry if the last box you fill in before LW isn’t exactly an hour different from the time of LW on the tide table, it rarely is. If you look at the trailing end of the curve you will see that it gets to the level of LW before reaching the final LW box marked on the time axes of the curve.

Step 4 – Reading the Curve
Fig 5.36 – After we have prepared the curve we enter at our target time, draw a line to the correct tidal curve, then along to our tidal range line and to the scale to read the height of tide in metres.

We have already adjusted the time 1047 SP (-0100) so can look for this time along the time axes at the base of the curve (Fig 5.36). The time we are looking for lies between +2 (1007hrs) and +3 (1107hrs) hrs after High Water. Our target time is 2hrs and 40 minutes after HW.

As soon as found the right time we can draw a vertical line upwards to meet the dashed blue neaps curve (which is the same as the red curve in this example) then horizontally to meet the diagonal tidal ranges line then up or down towards the tidal range scale axes. To help with accuracy use a straight edge, draw lines parallel to the printed lines and have a sharp pencil.

Step 5 – Correct for time and put answer in Context

We have now calculated the height of tide to be 3.3 metres above chart datum at 1047 SP standard time from the Hamilton Sound tide tables and tidal curve. Remember that we have to add 1 hour to SP Standard Time to get SP Daylight Saving Time. At Hamilton Sound at 1147 SP DST the height of tide will be 3.3 metres above chart datum.

So in Hamilton Sound on 4th August, when when your wrist watch has a time of 1147hrs we can say that any depth given on the chart in the vicinity of Hamilton Sound will have an extra 3.3 metres of water over the top.