Chartplotters explained: charts, AIS targets and what the screen cannot tell you

A chartplotter is a marine display that places your boat’s position, obtained from a Global Navigation Satellite System (GNSS) such as GPS, on an electronic chart. Depending on the model and its connections, it can also show routes, depth, radar and nearby vessels’ Automatic Identification System (AIS) reports. It is a display and planning tool; a chartplotter alone does not hear AIS radio broadcasts or transmit your position to other boats.

Not for navigation. AIS data on this site is informational only. It may be delayed, incomplete or absent, and it must never be used for navigation, collision avoidance, or any safety-of-life decision. A chartplotter aboard a boat has its own limits too: use current official charts, a proper lookout and the equipment and procedures required for your voyage.

What the chartplotter actually shows

The chart is a separate data product installed on the display or supplied through a chart service. The boat symbol comes from the plotter’s GNSS position source. Neither tells the plotter where another vessel is. For that, it needs AIS reports received on board, or another connected sensor such as radar. Check the chart’s coverage, edition, updates and depth datum before relying on its depiction of a channel or shoal.

An AIS target is a vessel or other AIS-equipped station represented from its last received radio message. The display may show its Maritime Mobile Service Identity (MMSI), name, position, course over ground and speed over ground. Some details arrive less often than position reports, so a new target may initially show only an MMSI. How AIS works explains what the broadcasts contain and why names can arrive later.

Many plotters draw a projected course line and calculate closest point of approach (CPA) and time to closest point of approach (TCPA): the predicted minimum separation and when it would occur if both vessels keep their reported courses and speeds. These are estimates, not a promise about either vessel’s next manoeuvre. Check the age of a target’s last report and how your plotter marks lost targets.

How AIS reaches the screen

An AIS receiver listens for nearby broadcasts. An AIS transponder listens and also transmits your own vessel’s details. Either can supply received targets to a compatible chartplotter. The plotter’s AIS support does not mean it contains a receiver, and a receive-only installation does not make your boat visible to other vessels. See AIS transponders explained before choosing that part of the system.

The usual wired interfaces are NMEA 2000, a shared marine data network, and NMEA 0183, an older point-to-point serial data connection. The AIS device sends decoded target data over one of them; the plotter reads it and draws the symbols. An NMEA 2000 socket and an NMEA 0183 port are not interchangeable. Check both manuals for the same interface, the required AIS messages, connectors and power. NMEA 0183 AIS input may require a high-speed port setting; a port that accepts other instrument data does not necessarily accept AIS at the needed rate. Some systems use a supported network or wireless gateway, but compatibility still has to be checked end to end.

The AIS radio path matters before the data cable does. The receiver or transponder needs a suitable VHF antenna and an installation with a clear radio path. If a transponder shares the voice radio’s antenna, it needs an active splitter, not a passive joiner. AIS antennas and splitters explains the range and wiring trade-offs.

A buying and installation check

• Start with the job. Do you only need to see transmitting vessels, or do you also need your boat to transmit? The second job requires a transponder, a correctly assigned MMSI and a suitable antenna installation.
• Match the data interfaces. Confirm the exact plotter and AIS unit exchange the required AIS messages over NMEA 2000 or NMEA 0183. Check port speed for NMEA 0183 and the network parts required for NMEA 2000. A generic “AIS compatible” label is not enough.
• Check the chart arrangement. Find out which chart region and updates are supplied, whether the display can use the chart data you need, and how it indicates missing or old chart coverage.
• Try the target display. Check that target age, course, speed, CPA, TCPA and lost-target warnings are understandable at the helm. Alarm behaviour, target filters and screen size vary by model; learn the settings before relying on an alert.
• Plan power and placement. Confirm the screen is readable from the helm, protected as the installation requires, and has a reliable power supply. Keep the antenna, network and display manuals together for fault-finding.

The equipment notes include a named chartplotter as an example of a networked display. The catalog notes describe published specifications, not hands-on testing; verify the exact listing, chart bundle and AIS connection before ordering.

Limits of the AIS picture

AIS shows only stations that transmit and whose broadcasts your receiver can hear. Many small craft carry no transmitter. An antenna can be blocked by terrain, a target can leave VHF range, and a reported position can be old or wrong. Display filters can hide targets too. A blank patch of screen is not proof that water is clear.

CPA and TCPA are particularly sensitive to changes of course and speed. A target turning after its last report can make the projection wrong at once. Chart, GNSS, AIS and radar layers also have separate failure modes; putting them on one screen does not make them independent. Maintain a lookout, use the required navigation equipment and follow the collision rules. This site’s shore-receiver map is informational and cannot verify what your onboard receiver can see.

Common questions

Does a chartplotter receive AIS by itself?

Usually no. It needs a built-in AIS receiver or an external receiver or transponder. Check the exact model; “AIS capable” can mean display capability only.

Will a chartplotter with AIS make my boat visible to others?

No. The display does not transmit AIS. An installed transponder with a correctly assigned MMSI does that.

Can NMEA 0183 and NMEA 2000 AIS equipment be connected directly?

No. They use different data interfaces. Choose matching connections or a compatible converter, and check the AIS messages and port settings in both manuals.

Does an AIS target alarm mean a collision will happen?

No. It is a projection from reported movement. Reports can age, vessels can manoeuvre, and non-transmitting traffic is absent. Investigate the situation with a proper lookout and the appropriate instruments.