How far can you see?

How far can you see whilst sitting in your kayak?  Knowing how far away you can see an object whilst paddling is a useful technique and a valuable aid to navigation.  As a simple rule the higher up you are the further you can see.  Standing on top of the Empire State Building, with good visibility it is amazing how far away the horizon is.  Also the taller the object you are looking at the further away that it can be seen.
As paddlers it is not that easy to raise our eye level, our eyes are generally just less than 1 metre above sea level.  At times in rough weather or when there is a swell running it is possible to take advantage of the extra elevation that results from being on top of the swell to increase how far we can actually see.    Due to the movement up and down of the kayak, on the water, the distance off an object which is obtained should be seen as an approximation.
Clearly a further problem is caused by the rise and fall of the tide, which may well be significant in certain areas.  For example on a chart a lighthouse’s charted height is given above MHWS.  In certain areas of the world with a large tidal range the height above water of the light may vary by more than 10 metres, considerably affecting the distance away that the light may be seen from.  If you want to be really accurate it is necessary to add the estimated height that the tide is below MHWS to the height of the land or the light before referring to the table.
An example of the effect of this from a trip to the Ecrehous, on a large spring tide, is as follows:
Maitre Ile at the Ecrehous has a height of 8 metres at MHWS, which in Jersey is 11.1 metres, but the tide on Saturday was 11.8 metres, it was bigger than a mean spring.  This meant that at high water the maximum height of Maitre Ile was not 8 metres but 7.3 metres.
When the eye of the observer is 1 metre above water level an object 8 metres high is visible from 7.8 nm away but when the height of the object drops to 7 metres  it is not visible until you are within 7.4 nm.  When we left La Rocque, Maitre Ile was 8.3 nm away.  This meant that even in excellent visibility we would have not been able to see our destination when we left.
On a spring tide when the water level may drop by as much as 11 metres the highest point on Maitre Ile is now 19 metres above the water level.  This means that the island is now visible from 10.9 miles away for a paddler whose eye is 1 metre above the water.
The lesson is that objects will be visible from much further away when you approach them at low water, particularly in an area with a large tidal range, such as the Channel Islands.

Visibility
Approaching Maitre Ile, early on a Saturday morning. We had been paddling for some time before our destination came into view. The following table gives an approximate relationship between the distance from which an object is visible and its height above sea level. It is assumed that the height of the paddlers eyes above water is 1 metre.

The table below shows the distance at which, an item becomes visible depending upon its height above water.  This is based upon the observers eye being 1 metre above the level of the water.

Clearly there are number of variables which impact upon the accuracy of the above table such as sea state, the exact height of the paddlers eye above sea level and the height of the tide but it is a useful tool in helping the sea paddler to locate their position.  For example 12 nautical miles to the south of my nearest beach is the superb reef of the Minquiers.  The tallest rock on the northern edge is only 3 metres high, which according to the table means that they only become visible when they are 5.5 nautical miles away.  Therefore there is no point in even starting to look for the reef until you have paddled for over 6 nautical miles or have been underway for over an hour and a half.
Using this technique as a way of assisting navigation is particularly satisfying but it is a method which is gradually slipping into obscurity.  Today the vast majority of us simply turn to the switch on the GPS to receive far more accurate information about our position than we could ever obtain by using the above method.  That said there is a degree of satisfaction from being able to navigate using the more traditional methods and you never know if the batteries are going to run out!

Visibility
The photo is of La Conchiere Beacon. At MHWS it is 2 metres high and so is visible from 4.9 nautical miles away from an observer whose eye is 1 metre above sea level. Clearly an object this narrow would be more difficult to sea that something more substantial, such as a reef but of similar height.

2 thoughts on “How far can you see?

  • March 15, 2012 at 15:13
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    Thanks. I hope that you find it useful. Sometimes I think we fail to appreciate more traditional techniques in our rush to embrace new technology. I use my GPS but still enjoy using the older methods.

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  • March 14, 2012 at 22:51
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    Now I normally go all glassy eyed reading stuff like that, but as someone who has so far refused to buy a GPS, it was actually a useful, interesting post and is a technique I'll adopt.
    Thanks
    W

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