Heights of Charted Objects and Tidal Levels Table
Clearances under bridges, pylons, overhead cables etc., are measured from the Highest Astronomical Tide level, all other non-clearance height measurements, lighouses, churches, hills etc., are measured from the High Water Springs tidal level MHWS. On the last page of the Training Almanac there is a Distances Off lights table that can be used at night to determine how far from the lighthouse we are. To use this table we need to know the height of the centre of the light is above sea level.
We are in the sailing club bar at Endal Marina having a bet with a fellow member. Using binoculars we are interested to know how far out to sea we will we see the Gamp Holm lighthouse before it disappears over the horizon (Fig 5.16). So we need to know the height of the Gamp Holm light, just north of Endal Marina and if there is any difference in observation between High and Low Spring tide.
As always the first thing to do, is look at the chart and identify the light. The information beside the light reads Fl(2)6s30m5M(U). This tells us that it flashes twice every 6 seconds, the centre if the lighthouse light is 30 metres above MHWS and the light can be seen for 5 miles under normal atmospheric conditions. Intuitively we know that the higher up something is the further we can see it.
Obviously, if we are on a boat and the tide goes out, the lighthouse would seem relatively higher to us. To work out this new height or what the lighthouse has grown to we need to consult the Tidal Levels information on the chart for Endal Bay and find that the MLWS level is 0.4m.
Next we need to determine how much fall of tide there s between the Mean High Water Spring level, where we measure the heights of charted objects from, and Mean Low Water Spring, the level we are viewing the lighthouse from. fall of tide = MHWS- MLWS = 4.6m – 0.4m = 4.1m
This fall of sea level is akin to how much our lighthouse has gown as the tide ebbs to low spring tide level. Our lighthouse has a new height of 30m + 4.1m = 34.1m. We can now enter the Distance Off Lights table with our new lighthouse height of 34.1m
When we look at the table and enter the table we can see that we should be able to see the lighthouse, albeit with binoculars, from 15 miles away.
This is contrary to the information given on the chart which tells us the light is visible from 5 miles away under normal atmospheric conditions. Perhaps they need a stronger lantern in the lighthouse as the lighthouse cannot be seen on a night passage.