Range has three distinct meanings a sailor needs to keep straight. First, in navigation, a range is a pair of fixed marks – beacons, lights or daymarks – set up so that when they appear in line you are on the safe centreline of a channel; British sailors call the same thing a transit or leading line. Second, range is distance: the range of a light is how far it can be seen, radar range is how far away a target is, and a bearing and range from a landmark give a position. Third, tidal range is the difference in height between high water and low water on a given tide. Experienced skippers know that a natural range – any two charted objects in line – is the most accurate position line there is.
Ranges as leading marks
A leading range is two marks, a front one lower and a rear one higher, placed so that keeping them in line leads a vessel safely through a channel. On the chart they appear joined by a dashed leading line with its bearing written along it. Steer so the rear mark sits directly above the front one. If the rear mark appears to move left of the front, you are left of the line and must steer right to bring them back together, and vice versa; the picture corrects itself as the boat comes back on the line, so a range gives instant and continuous feedback that no compass course can.
The beauty of a range is that it needs no instrument and has no error. Compass bearings carry deviation and variation and the ordinary scatter of reading a swinging card; a range is geometry. At night, leading lights do the same job, and a sectored light shows white in the channel and red or green when you drift out of it.
Natural ranges in piloting and racing
You do not need an official range to use one. Any two charted objects in line – a headland and a distant hilltop, a buoy and a church tower – give you a position line as accurate as the chart. Racing sailors live by this. At a start, sighting from the committee boat through the pin to a mark on the shore gives a line range; if that shore mark is visible past the pin, you are behind the line, and if it is hidden you are over. Round a mark, a range on the shore behind it shows instantly whether the tide is setting you on to it or off. Good tacticians collect ranges the way navigators collect fixes.
Range as distance
The second meaning is distance off, and it comes in several forms:
- Luminous or nominal range: the distance at which a light can be seen in clear visibility, as printed on the chart or in the light list. Geographic range is the distance at which the curvature of the earth allows the light to be seen from a given height of eye, and the lesser of the two is what you will actually get.
- Radar range: the distance to a target, read from the range rings or a variable range marker. Radar range is more accurate than radar bearing, so a radar range combined with a visual bearing makes a fine fix.
Tidal range
Tidal range is the vertical difference between a high water and the following or preceding low water. It varies through the fortnightly cycle: large springs ranges around new and full moon, smaller neaps ranges in between. Range matters in two ways. It sets how much water you will have over a bar or a sill at a given time, which is a question of height of tide and therefore of the day’s range and how far through the tide you are. And it drives tidal streams: big range, strong streams, because more water has to move in the same six hours. Tide tables list the range for each day.
Frequently Asked Questions about Range
What is the difference between a range and a transit?
Nothing, in this sense: range is the North American word and transit the British word for two marks in line giving a position line or a leading line through a channel.
Why is a range more accurate than a compass bearing?
A range is pure geometry between two fixed objects and carries no compass error. A bearing carries variation, deviation and reading error, typically a degree or two, which at a mile off is a position error of some tens of metres.
How do I use tidal range to work out depth?
Take the height of low water, add the fraction of the range that has risen by the time you arrive (the rule of twelfths gives a quick estimate), and add the result to the charted depth. Subtract if the chart shows a drying height.
Whether it is two beacons in line leading you into a harbour, the distance a light can be seen on a landfall, or the metres of water the tide will add over a bar, range is a word about knowing where you are and how much room you have. Learn to see natural ranges everywhere, respect the spring range when you plan a passage, and you will pilot and race with the confidence of someone who has more information than the compass alone can give.