36 
Instructions and Examples 
Finding mean tidal levels at subordinate stations 
Example 5: 
What are the heights of mean high water springs and neaps (MHWS, MHWN) and of mean low water springs and neaps 
(MLWS, MLWN) at Whitehaven relative to Chart Datum? 
Apply the height differences for the subordinate station to the tidal constants of the reference station. In this way, you obtain 
the tidal constants for the subordinate station relative to Chart Datum 
No. 
Place 
MHWS 
MHWN 
MLWS 
MLWN 
Tidal Constants 
Part II, page 193 
1509 
Liverpool 
9.4m 
7.5m 
1.1 m 
3.2m 
Differences 
Part II, page 193 
1496 
Whitehaven 
- 1.4m 
- 1.2m 
-0.1 m 
- 0.8m 
Result 
1496 
Whitehaven 
8.0 m 
6.3 m 
1.0m 
2.4 m 
Finding the height of tide at any time between high and low water 
A graph of the mean spring and neap tide curves 
at each reference station is shown on the page 
following the daily predictions of high and low 
water, which can be used to determine the height 
of tide at any time at the reference station or one 
of its subordinate stations. For that purpose, the 
duration of rise and fall of the relevant high water 
have to be determined from the predictions. Insert 
the predicted height of the high water at zero hour 
in the diagram. Enter the height of the predicted 
low water at a point corresponding to the rate 
of rise before the high water. Likewise, enter 
the height of the following low water at a point 
corresponding to the rate of fall after the high 
water. Then draw a current tide curve through 
these points of high and low water in accordance 
with the given mean spring and neap tide curves. 
The heights of tide at any time of the day are easily 
read from the curve. 
The method is simple, has few sources of error, 
and yields appropriate results even in case of, 
e. g., uncertain ETA. Its accuracy is adequate for 
practical navigation because there always is a 
possibility of meteorological conditions causing 
additional differences between predicted and ac 
tually occurring tides. 
Co-Range Charts 
Determination of high and low water using co-tidal and co-range charts 
The mean high water time difference between 
two positions is computed by taking the mean 
high water lunitidal intervals for the two positions 
from Chart 1 or 5 and calculating the difference. 
The same applies to the mean low water lunitidal 
intervals in Chart 2. The difference between the 
mean high or low water lunitidal intervals at a 
reference station and at a position at sea is the 
high or low water time difference for that place as 
a subordinate tide station of the reference station. 
The mean lunitidal intervals of reference stations 
are given in hours and minutes on the co-tidal 
charts. The closest reference station should be 
used in each case. It should be kept in mind that 
it is the co-tidal lines 12 h 25 min and 0 h 00 min 
which overlap, not the co-tidal lines 12 h 00 min 
and 0 h 00 min, because the mean interval of the 
moon’s transit (upper or lower) over the Greenwich 
meridian is 12 h 25 min. 
Chart 3 and Chart 6 show mean spring co-range 
lines. Using these mean values, the range for a 
position at sea on a particular day can be estimated 
by multiplying the spring tidal range at the particalu- 
ar place with the quotient of the tidal range divided 
by the spring tidal range of the reference station.