Is Navionics or C-MAP better for fishing in the Andaman Sea?

Both have meaningful gaps in the Andaman Sea, particularly in the Mergui Archipelago and around less-visited Thai island groups south of Koh Rok. Navionics typically has better crowd-sourced depth data around Phuket, Krabi, and the Similans due to the high recreational boating density. C-MAP has historically had better coverage of deeper offshore structure. Carry both if the plotter supports dual card slots.

How do I share fishing waypoints with other anglers in Thailand?

Waypoints export as GPX files from all major chart plotter brands. Email or messaging app transfer is the standard method between Thai fishing groups. Some Phuket and Krabi charter operators share GPS marks via Line (the dominant messaging app in Thailand) to guests booking multi-day trips. Verify any shared marks against the chart before running to them in unfamiliar waters.

What does AIS show on a chart plotter and is it useful for Thai fishing?

AIS (Automatic Identification System) shows vessel positions, speeds, and headings from any vessel transmitting an AIS signal. In Thai waters this means ferry routes, cargo ships, and larger charter vessels. For fishing boats, AIS helps identify other fishing activity clusters (multiple vessels holding position in the same area often indicates a productive FAD or reef mark) and provides advance warning of ferry traffic near fishing grounds.

Do I need a chart plotter for reservoir and pay-lake fishing in Thailand?

No. Chart plotters are marine navigation tools and add no value at freshwater pay lakes. For reservoir fishing at Cheow Lan or large Thai reservoirs, a handheld GPS with marked waypoints for fishing structures and camp sites is useful. A dedicated marine chart plotter is only warranted when operating in tidal or offshore waters where navigation safety is relevant.

GPS Chart Plotters for Thai Waters: Navionics, C-MAP, and Waypoint Strategy

A chart plotter is a GPS receiver integrated with electronic nautical chart display. In Thai waters, it is the primary navigation tool for any offshore or coastal fishing, and it serves a secondary function as the data storage system for fishing marks — the waypoints that represent productive reefs, pinnacles, FAD locations, and drop-off edges that experienced anglers accumulate over years of fishing. Understanding how to use one effectively in Thai waters — and where the chart data fails you — is the foundation of safe and productive offshore fishing in both the Andaman Sea and the Gulf of Thailand.

Chart Coverage in Thai Waters

Navionics in Thai Waters

Navionics (now owned by Garmin) provides electronic chart data for Thailand through its Navionics+ subscription service, which covers the Gulf of Thailand and Andaman Sea with varying detail quality. The coverage is strongest in areas with high recreational boating density:

Good Navionics coverage: Waters around Phuket (Chalong Bay, Phang Nga Bay), the Similan Islands corridor, Ko Lanta approaches, and the Gulf coast from Hua Hin to Koh Samui. These areas have been surveyed by commercial hydrographic teams and supplemented by crowd-sourced depth data from thousands of recreational boats and fishing charters.

Variable coverage: The outer Andaman reef systems beyond the Similan chain, waters around Koh Racha Noi and further south, the Tarutao archipelago, and the Butang Island group (Koh Adang, Koh Rawi). In these areas, the chart shows the correct island outlines and approximate depth contours but the precise reef and rock positions may be inaccurate by 50 to 200 metres — enough to run a boat onto a charted clear patch that is in reality a coral head.

Significant gaps: The Mergui Archipelago north of the Thai-Myanmar border has minimal commercial chart coverage in Navionics. For operators running charters into Mergui waters, paper charts from the Hydrographic Department of Thailand and Myanmar supplemented by personal GPS marks from previous expeditions are essential.

C-MAP Coverage

C-MAP (from Furuno) historically provided better offshore bathymetric detail for the Andaman's deeper waters, particularly the canyon systems south of the Similan chain where sport fishing for sailfish and wahoo operates. The deep-water contour lines in C-MAP charts at the 100 to 300 metre depth range are typically more complete than Navionics in these specific areas.

However, C-MAP's coverage of the shallower reef systems and inshore Andaman waters is often inferior to Navionics in the recreational areas around Phuket, where the crowd-sourced depth update mechanism of Navionics+ has significantly improved coverage over the past decade.

The practical recommendation for Thai offshore fishing: run Navionics as the primary chart for coastal navigation and mark management around major ports (Phuket, Krabi, Chumphon, Samui), and supplement with C-MAP for deep-water offshore navigation where the better bathymetric data is relevant.

Chart Data Warning

No electronic chart should be used as the sole navigation reference in Thai waters without cross-checking against visual observation. Reef positions in the Thai Andaman are partially surveyed at best, and uncharted coral heads exist throughout the island groups south of Phuket. Reduce speed to below 8 knots when navigating unfamiliar reef areas regardless of what the chart shows.

Waypoint Mark Management

Establishing a Mark Hierarchy

A well-managed waypoint library is the collective record of fishing experience that makes a chart plotter genuinely valuable. Thai offshore anglers who have fished the Andaman for more than a few seasons accumulate hundreds of marks, and without a disciplined naming and categorisation system these become impossible to navigate.

A simple but effective hierarchy for Thai fishing marks:

Category prefixes:

REEF — inshore reef systems and coral heads with consistently productive fish populations
PIN — offshore pinnacles and seamounts
FAD — fish aggregating devices (with date of visit appended, as FADs move or are removed)
ANCH — safe anchoring positions with confirmed bottom type and depth
SBAIT — bait schools encountered repeatedly at a specific GPS position across multiple visits
COBIA / GT / SAIL — species-specific marks at locations where target species have been repeatedly taken

Naming format: PREFIX-LOCATION-DEPTH. For example: PIN-SIMILAN-60 (a pinnacle in the Similan region at 60 metres peak depth), or REEF-LANTA-12 (a reef system off Koh Lanta at 12 metres).

Mark Verification Before Running

When navigating to any mark received from another angler, always examine the chart context around the mark before running to it at speed. What appears as a productive reef mark may have been entered with a GPS calibration offset, or the vessel's GPS receiver may have had an error at the time of marking. Verify that the mark's depth matches what the chart shows, that there are no charted hazards between your current position and the mark, and that the approach line from your departure point avoids any shallow bank or headland.

Thai fishing communities in Phuket, Krabi, and Koh Samui operate active GPS mark sharing networks, primarily through Line group chats. Marks from these networks are generally reliable for the major known structures, but unique or remote marks should be approached with more caution.

A GPS mark is a coordinate, not a guarantee. The reef that produced a 40 kg GT six months ago may have been dynamited, fouled with discarded nets, or simply not holding fish today. Marks are starting points, not destinations.

In Thai fishing culture, sharing GPS marks of productive fishing grounds is a social currency — an expression of trust and reciprocity between anglers who know each other. Sharing marks with strangers or publicly posting coordinates of sensitive spots is regarded as inappropriate and can damage relationships within tight-knit charter fishing communities.

The accepted protocol:

Marks for known, publicly accessible reefs (the Similan Islands reef system, the Koh Haa pinnacles, the Bangkok Bight FADs) are freely shared.
Marks for privately discovered or less well-known pinnacles and structures are shared between established fishing partners only.
Marks received in confidence should not be redistributed without the giver's consent.
When a charter crew shares marks with a client, those marks are typically understood to be for the client's personal use, not for redistribution to their social network.

This etiquette is unwritten but widely understood in Thai charter fishing circles. Violating it by broadcasting shared marks publicly is the fastest way to be excluded from future mark-sharing relationships.

AIS for Charter and Safety Awareness

What AIS Shows

Automatic Identification System (AIS) transmits vessel identification, position, course, and speed from any vessel equipped with a Class A or Class B transponder. In Thai waters, AIS coverage includes:

All commercial cargo and passenger vessels above 300 gross tonnes (mandatory AIS)
Most licensed charter fishing vessels and ferries
Some recreational vessels with voluntary Class B AIS transponders

AIS does not cover local longtail boats, small fishing trawlers below the mandatory threshold, or any vessel whose transponder is switched off. In Thai fishing grounds, a significant proportion of the traffic — squid trawlers, small-scale net fishing boats — is not AIS-equipped.

Practical AIS Use for Thai Fishing

Identifying productive fishing clusters: When multiple vessels are clustered on the chart plotter's AIS overlay and are showing very low speed (0.5 to 2 knots) or stationary positions in open water, they are almost certainly fishing. Clusters of 3 to 8 vessels in the Gulf of Thailand near FAD positions or seamounts are reliable indicators of active fishing grounds, particularly for long-tail tuna and Spanish mackerel. The AIS overlay allows this to be seen from 20 to 30 km away before committing to the run.

Ferry and cargo traffic avoidance: The Phuket to Phi Phi and Phuket to Similan Islands ferry routes cross popular trolling corridors. AIS shows ferry positions and headings in real time, allowing a trolling vessel to adjust course or speed to avoid crossing the ferry's path. This is particularly relevant near the departure and arrival times published for ferry schedules.

Safety overlap with VHF monitoring: AIS and VHF Channel 16 monitoring together provide the most complete picture of nearby vessel activity. AIS shows what is there; Channel 16 tells you what it is doing if the vessel is actively communicating.

Connecting to Sounder and Network

NMEA 2000 Integration

Modern chart plotters connect to the onboard sounder via NMEA 2000 network, sharing depth, bottom composition, and GPS data between units on a single standard network bus. This allows the sounder's depth readings to appear on the chart plotter's chart display as a depth overlay, and allows GPS position from the chart plotter to annotate sounder screenshots with precise location data — invaluable when logging new marks in real time during a fishing session.

Setting up a simple two-unit network (plotter + sounder) on a Thai charter fishing boat requires NMEA 2000 backbone cable (T-connectors, terminators) and the relevant NMEA 2000 adapters for each unit's proprietary network port. Equipment from different manufacturers is cross-compatible as long as both comply with the NMEA 2000 standard, which all major brands (Garmin, Furuno, Lowrance, Simrad, Raymarine) do.

Tablet and Mobile Alternatives for Day Trips

For anglers who charter boats occasionally rather than owning their own vessel, full fixed chart-plotter installations are impractical. Navionics and C-MAP both offer mobile apps for iOS and Android that run on a tablet or smartphone and provide chart display and waypoint management. A dedicated 10-inch waterproof tablet with the Navionics+ subscription is a viable alternative to a fixed plotter for occasional use, at a fraction of the hardware cost.

Waterproof or water-resistant ratings (IP67 or above) are essential for any tablet used on Thai fishing boats, where spray and rain are routine. Mount the tablet on a RAM-style ball mount at the helm for visibility without handheld management, and carry a portable 20,000 mAh power bank to ensure the tablet lasts a full offshore day without mains charging.

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