Tools · reverse light identification · NGA List of Lights, Pub. 110-116

What Light Is That?

You are on a coast at night and something out there is flashing. A chart answers the forward question, what does that named light do. This answers the backward one: given what you can actually see from where you are standing, which real light is it? Tell it the colour, how many flashes come in a burst, and tap out the period while you watch. It searches 40,559 real navigation lights, keeps the ones that are above your horizon for your eye height and inside their own advertised range, and blinks each candidate on its own printed schedule so you can hold the screen up and compare. It also keeps a second, clearly labelled tier: lights out to 1.25 times their printed limits, which are by construction outside their advertised range and are listed below the rest because cutting hard at the printed figure was measured to lose real answers.

Not for navigation. This is a snapshot of a published list, parsed and served offline. Lights change, are extinguished, and are re-charactered, and per-record notices in these volumes go back years. Never use it to fix a position or clear a hazard. Use the current official list and current Notices to Mariners.

The United States is almost entirely missing, and this is not a bug. Pub. 110-116 is NGA's list of lights in foreign waters. US aids to navigation are published separately by the Coast Guard in its own Light List and are not in these volumes. The whole corpus holds 9 records under a UNITED STATES heading. If you are on a US coast, this tool tells you exactly what it holds where you are, rather than handing you an empty list that reads like an empty sea.

1 · where are you, roughly?

latitude

longitude

eye height above water, m

Rough is fine: a few kilometres out changes almost nothing, because a light either clears your horizon or it does not. Eye height matters more than you expect. Standing on a beach your eye is about 2 m up; on a small boat about 3 m; on a ship's bridge 15 m or more. The higher your eye, the further the horizon, and the tool recomputes every geographic range from the number you put here.

2 · what can you see?

colour, as it looks to you (leave all off if you cannot tell)

how many flashes in one burst, before it repeats?

A group of two flashes then a gap is 2. A light that goes out briefly rather than flashing on is still counted the same way: one eclipse is 1. If it is blinking far too fast to count, that is many, which is the quick and very quick classes. If it never goes out at all, that is steady. The counts stop at 10 because 10 is the largest group any record in this book prints, counted from the corpus rather than chosen.

3 · how long is one full pattern?

period, seconds

Watch the light. Press the button the instant it starts each burst, four or five times. The period is the mean gap between your presses, and the spread of your own presses is shown too, because that spread is the real precision of your answer.

timing tolerance 20%

A hand on a button is wrong by a few per cent. The tolerance is the window around your figure that a printed period has to fall in. At the default, simulated observers found their light 98.4% of the time with a median shortlist of 1; the whole sweep, and the observer model behind it, is under the hood below.

Under the hood: how often this actually finds the light

Under the hood: what the list does not print

The URL grammar: every query is a link

The entire query lives in the address bar as readable parameters, and loading that address restores it exactly. Nothing is stored anywhere else, and there is no server: the corpus is downloaded once and every search runs in your browser.

Every range in this table is the range the decoder enforces and the range the control on the page can reach, read from the same constants, so a link cannot ask for something no control can show. A value outside a range is rejected and named on screen, never quietly rounded into range, because a slider that clamps while the label reads otherwise is a tool answering a different question from the one it was asked.

parameter	value	meaning
`lat`	-90 to 90	your latitude, decimal degrees, north positive
`lng`	-180 to 180	your longitude, decimal degrees, east positive
`colour`	W R G Y Bu Or Vi	colours you can see, comma separated; the light must print all of them
`flashes`	1 to 10, or many, or steady	flashes in one burst; the top of the range is the largest group printed anywhere in this corpus
`period`	seconds, above zero	length of one full pattern
`eye`	1 to 30 metres	your eye height above the water; sets every geographic range
`tol`	5 to 30 per cent	timing tolerance, default 20
`r`	km, above zero	search radius; the default is the widest any light in this corpus reaches, and whatever is used is printed in the result

worked example · a white light off the Cornish coast, a single flash about every three seconds, from a small boat with your eye 3 m above the water:
/tools/what-light/?lat=50.152&lng=-5.064&colour=W&flashes=1&period=3&eye=3
That link resolves, out of 40,559 lights, to exactly one: Lizard Point, the light at the southern tip of Britain. The verifier asserts that this sentence stays true.