If you need the height of the tide for a particular time, for example for 13:00, just enter the required time at the timescale grid.

13:00 is roughly 3 hours after high tide.

Then check whether the tidal range is close to a spring range or neap range.

In our case we have a tidal range of 3.8 meters; 4.2 – 0.4 equals 3.8 meters.

As we can see from the mean ranges, 3.8 meters is close to a spring range.

Next we draw a line from 3 hours after high water to the spring curve.

Here we can interpolate by eye.

From the intersection of the line and the spring curve we draw a horizontal line to the high and low water line. From here we draw a line to join the top scale.

The number at this point is the height above chart datum of the tide at the required time.

To calculate the total depth of water in the Port of Portsmouth, we add the height of tide to the depth shown on the chart.