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How to interpret S-57 LIGHTS objects: character, colour, range, height, sectors, and source information.

LIGHTS Object Class

This document describes the LIGHTS object class as it is presented in the chart viewer, and explains how a mariner should interpret each property of a LIGHTS object.

1. Overview

A LIGHTS object represents a light: a luminous or lighted aid to navigation such as a lighthouse, a lit beacon, or the light fitted to a buoy, beacon, or other structure.

In the underlying chart data (IHO S-57 ENC), the object class is named LIGHTS and its friendly label is "Light". When you tap or select a light on the chart, the viewer shows a popup that begins with:

LIGHTS - <LNAM>
Light (<character>) - <colour>

For example:

LIGHTS - 001:00004567:002
Light (flashing) - red

The two most important properties for identifying a light at sea — its character (rhythm) and its colour — are shown in the heading. The remaining properties are listed as individual bullet points and are described below.

2. Property reference

All properties are optional; an individual light may not carry every attribute. Properties that are missing are simply omitted from the popup.

3. Detailed property descriptions

3.1 COLOUR — Colour of the light

The colour of the light itself (not of the structure carrying it). This is shown in the heading after the character.

Possible values (from the S-57 COLOUR lookup):

Mariner interpretation: The colour, combined with the character, is how you identify a light at night. A single light may alternate between two colours (for example, white/red); the viewer currently shows one primary colour in the heading. Always cross-check the colour against the chart symbol and the published light list.

3.2 LITCHR — Light characteristic

The rhythm, or "character", of the light: whether it is steady or interrupted, and, if interrupted, the pattern of flashes or occultations. Shown in the heading as the character.

Possible values (from the S-57 LITCHR lookup):

Mariner interpretation:

• Fixed (F) — a steady, continuous light.
• Flashing (Fl) — a light in which the total duration of light in each period is shorter than the duration of darkness, and the flashes repeat at a regular rate.
• Occulting (Oc) — the opposite of flashing: the light is on longer than it is off.
• Isophased (Iso) — equal periods of light and darkness.
• Long-flashing (LFl) — flashes of at least 2 seconds.
• Quick (Q) / Very quick (VQ) / Ultra quick (UQ) — rapid flashing used for special marks and some cardinal marks.
• Group (Gp) — the flashes/occultations are combined in groups (see SIGGRP for the number in each group).
• Morse (Mo) — a Morse-code letter (typically "A") used on safe-water marks.
• Fixed/flash, alternating, etc. — combinations used on specialised or sector lights.

The characteristic is the single most important identifying feature of a light; match it exactly (character + colour + period) before using the light for a fix or for passing a hazard.

3.3 SIGSEQ — Signal sequence

The precise timing sequence of the light, written in the standard signal sequence notation (for example 0.3+(2.0)+0.3+(5.0) for a group-flashing light). It records the duration of each flash and each eclipse.

Mariner interpretation: This is the exact rhythm you would observe with a stopwatch. Use it to confirm a light's identity when the general character alone is ambiguous. The notation lists light durations followed by darkness durations in seconds; parentheses group the pattern.

3.4 SIGGRP — Signal group

The number of flashes or occultations in each group of a group characteristic. For example, a "group flashing (2)" light has SIGGRP = 2.

Mariner interpretation: Combined with the character, this tells you how many flashes to count in each group — e.g. Gp Fl (2), Gp Oc (3). Count the flashes, then note the dark interval between groups.

3.5 SIGPER — Signal period

The time, in seconds, for one complete cycle of the light's characteristic (from the start of one pattern to the start of the next).

Mariner interpretation: The period is a key identification value published alongside the character (for example, Fl(3) 15s). Time several cycles and divide to confirm the period. A mismatch between the observed period and SIGPER may mean you have identified the wrong light.

3.6 VALNMR — Nominal range

The nominal range of the light, in nautical miles (NM). This is the luminous range of the light under standard atmospheric conditions (meteorological visibility of 10 NM).

Mariner interpretation: The nominal range tells you how far the light can be seen by its intensity alone, in clear weather. In reduced visibility the actual luminous range is less than the nominal range. The true maximum range at which you can see the light is the lesser of:

1. the luminous range (nominal range adjusted for current visibility), and
2. the geographic range (determined by the light's focal height and your height of eye).

Use both VALNMR and HEIGHT together to work out when you should expect to sight the light.

3.7 HEIGHT — Focal height

The height of the light's focal plane above the vertical datum (typically the level of mean high water springs or chart datum), in metres.

Mariner interpretation: Focal height is used to compute the geographic range of the light — how far the light's beam can "see" over the horizon for a given observer height of eye. A taller light is visible from farther away, independently of how bright it is. If you know your height of eye, you can estimate when the light will rise above the horizon.

3.8 SECTR1 / SECTR2 — Sector limits

The start and end bearings, in degrees, of a sector in which the light shows a particular colour or characteristic. They are displayed together:

Visible sector: <SECTR1>° to <SECTR2>°

Sectors are only shown when both values are present.

Mariner interpretation: A sector light shows different colours (or characters) over different arcs, used to keep you in a safe channel or warn you when you are on a dangerous bearing. The bearings are typically expressed in degrees true as seen from seaward. When you observe the light, your compass bearing identifies which sector you are in:

• Stay in the white (safe) sector when following a leading line.
• Entering the red or green sector usually means you are too far to one side and may be standing into danger.

Always correct your compass bearing for variation and deviation before comparing it with the sector bearings.

3.9 EXCLIT — Exhibition condition

Indicates when the light is exhibited (shown). The viewer labels this "Exhibited light".

Possible values (from the S-57 EXCLIT lookup):

Mariner interpretation:

• 1 (shown without change) — the normal case: the light operates continuously with a single character.
• 2 (daytime light) — shown only in daylight (e.g. some leading lights used only by day).
• 3 (fog light) — shown only in fog or restricted visibility.
• 4 (night light) — shown only at night.

Do not expect to see a light marked as a daytime, fog, or night light outside its stated exhibition condition.

3.10 CATLIT — Category of light

The function or physical arrangement of the light. This may be a single code or a list of codes.

Possible values (from the S-57 CATLIT lookup):

Mariner interpretation:

• Leading / directional / front / rear / upper / lower — these lights are used in pairs or as a transit line to keep you on a safe course. Keep the front (lower) and rear (upper) lights in line; if they open up, you are off the transit.
• Aero / air obstruction — aviation lights; visible from sea but not a marine aid. Do not mistake them for a navigation light.
• Fog detector — an automatic light that activates when visibility drops.
• Subsidiary — a secondary light associated with the main light.
• Moire effect — a special leading light that shows a distinctive moving pattern to indicate off-course direction and magnitude.
• Emergency — a backup light used when the main light fails.

3.11 SORDAT / SORIND — Source information

Shown in the footer as:

Source: <SORIND> (<SORDAT>)

• SORIND (source indication) — identifies the agency or organisation that supplied the information. If absent, "unknown" is shown.
• SORDAT (source date) — the date of the source information. If absent, "date unknown" is shown.

Mariner interpretation: These help you judge how current and authoritative the light's information is. An old source date is a prompt to consult the latest Notices to Mariners or the official light list.

Mariner interpretation: Tells you which chart cell contains the feature. Useful for reporting a discrepancy or cross-referencing with the official chart edition.

3.14 LNAM — Long name

The unique identifier (long name) of the feature, in the form AGENCY:FIDN:FIDS, for example 001:00004567:002.

Mariner interpretation: LNAM uniquely identifies this specific light across chart cells and updates. Quote it when reporting an error or requesting a chart correction so the exact feature can be located.

4. Reading a light — worked example

A popup showing:

LIGHTS - 001:00001234:001
Light (flashing) - green

Sequence: 0.5+(1.0)+0.5+(4.0)
Group: 2
Period: 6 s
Nominal range: 12 NM
Focal height: 25 m
Chart: US3CA52M
Source: 001 (2023-05-14)

tells the mariner:

• It is a green flashing light, group flashing (2) — two green flashes per group.
• Each flash lasts 0.5 s, separated by 1.0 s, with 4.0 s of darkness between groups; the whole cycle repeats every 6 s.
• In clear weather it should be visible up to about 12 NM by its intensity.
• With a focal height of 25 m, its geographic range for your height of eye may be less than 12 NM; sight it when the lesser of the two ranges is reached.
• It comes from chart cell US3CA52M, source agency 001, information dated 2023-05-14.

Observe the light, count the flashes, time the period, and confirm the colour before relying on it for navigation.

5. Units and conventions summary

Sector bearings are read as seen from seaward, in degrees true unless otherwise stated; apply variation and deviation before comparing with a compass bearing.

6. Notes and caveats

• Not every property is present on every light; missing values are omitted from the popup.
• A light with multiple colours (e.g. an alternating white/red light) shows one primary colour in the heading; consult SIGSEQ/SECTR1/SECTR2 and the official light list for the complete description.
• Sector limits are displayed only when both SECTR1 and SECTR2 are present.
• EXCLIT and CATLIT are coded values; interpret them using the tables in sections 3.9 and 3.10.
• Always confirm critical light information against the latest official charts and Notices to Mariners; the source date (SORDAT) helps you judge how current the data is.