Marine Charts

What "Marine charts" includes and what they do

Marine charts are positional maps for navigation. They depict coastlines, water depths, seabed features, aids to navigation (buoys, lights), wrecks and hazards, port details, and other information needed to plan and pass safely. The category covers traditional printed charts and modern digital chart products for chartplotters, multifunction displays (MFDs) and mobile devices. It also includes chart formats and data types such as raster charts, vector charts (including official electronic navigational charts), and specialised products like bathymetric or approach charts.

Where and how charts are used aboard a sailing boat

Charts are used for route planning before departure, for real‑time navigation while underway, and for identifying features when approaching land, harbours or anchorages. Aboard a sailing boat charts are consulted at the navigation station or on a cockpit plotter; paper charts remain a backup for electronic failure. Chart data may be shown alone or overlaid with instrument data such as GPS position, AIS targets, radar, depth sounder and weather information.

Main product types and what is excluded

• Paper charts: Printed nautical charts produced from surveyed data. Useful as offline, battery‑free backups.
• Raster charts (RNCs): Pixel images of traditional charts that display on an electronic screen as a static map.
• Vector charts: Digital charts built from datasets of discrete objects (shoreline, buoys, depth areas). These allow dynamic display, selectable layers and route guidance.
• Official ENC (Electronic Navigational Charts): Vector datasets produced under national hydrographic authority standards for use in certified ECDIS systems and by leisure chartplotters supporting the format.
• Specialised charts: Bathymetric grids, fishing charts, pilotage approach charts and harbour plans.

Excluded from this category are general-purpose recreational maps that lack hydrographic survey data and non‑navigational shore maps.

The decisions that matter when choosing charts

Selecting charts is not only about coverage but also format, update practices and how they integrate with your onboard electronics. Consider these factors in relation to your boat and sailing style.

Boat size, type and intended use

Smaller dayboats with limited electronics may rely on a tablet and a small plotter plus one or two relevant paper charts. Larger cruisers or offshore boats typically keep comprehensive chart coverage in electronic form and maintain a set of paper charts or printed extracts for critical passages. Multihulls with shallow drafts may prioritise detailed bathymetry and recent surveys for shallow approach areas.

Crew size and experience

Less experienced crews benefit from vector charts with automatic depth shading, under keel clearance alarms and route‑planning tools. Experienced navigators may prefer the unequivocal detail of a paper chart or raster chart for pilotage. Training in reading both electronic and paper charts reduces reliance on a single format.

Type of sailing and conditions

• Day sailing: convenience and simplicity matter; mobile device charts or small plotter charts are often enough.
• Coastal cruising: broader coverage, recent updates and a combination of electronic and paper charts increase safety and flexibility.
• Offshore cruising: redundancy and reliability are paramount—multiple electronic sources, official vector charts where possible and a comprehensive paper backup are recommended.

Chart format and device compatibility

Check which formats your chartplotter or navigation app supports (raster, vector, ENC, proprietary formats) and whether charts require a license key or card. Storage space, update method (SD card, USB, network download) and integration with autopilots, AIS and radar also matter.

Updates and datum

Hydrographic information changes—new wrecks discovered, beacons replaced, or depths altered by storms. Understand the chart producer’s update cycle and how easy it is to install updates. Also check the chart datum (the vertical reference for depths) and the geographical datum (e.g. WGS‑84) used; mismatched datums can cause positional or depth interpretation errors.

Scale, detail and resolution

A chart’s scale determines how much detail it shows. Large‑scale charts (showing small areas) are used for harbour approaches and pilotage; small‑scale charts cover larger regions and are used for passage planning. For inshore navigation prioritise charts with up‑to‑date soundings and contour detail.

Product types and configurations explained

Paper charts

Paper charts are printed reproductions of surveyed hydrographic data. They remain the universal, battery‑free backup and are required or recommended for many offshore passages.

• How they work: read directly, mark planned tracks, bearings and waypoints with pencil and plotters.
• Where used: navigation station, as a backup on deck, or for detailed pilotage.
• Advantages: no power needed, full situational awareness, easy sharing among crew.
• Limitations: can be bulky, require physical storage and regular replacement to stay current.
• Suitable for: all boats as a backup; indispensable for remote offshore work where electronic failure consequences are severe.

Raster nautical charts (RNCs)

Raster charts are scanned images of paper charts that display as they would appear on paper, but on an electronic screen.

• How they work: image files georeferenced to position data; plotters display them without redrawable vector layers.
• Advantages: faithful reproduction of paper chart detail and annotations; familiar symbology.
• Limitations: not scalable without loss of clarity; limited interactive features.
• Suitable for: users who want paper chart style on a screen, or when specific chart annotations must be preserved.

Vector charts and ENCs

Vector charts are constructed from object datasets rather than pixels. ENCs are official vector datasets conforming to international standards.

• How they work: objects (buoys, depth areas, coastline) are drawn from data layers so the display can be customised.
• Advantages: scalable display, selectable layers (depth shading, lights), route assistance, automatic hazard alarms and integration with navigation systems.
• Limitations: interpretation depends on chart software; not all vector charts are created to the same standard—official ENCs are more rigorous but may not cover every recreational need.
• Suitable for: those using chartplotters for live navigation, especially when integrating AIS, radar and depth sounder data.

Specialist charts (bathymetric, harbour plans)

Some charts focus on undersea detail (bathymetric grids) or offer enlarged harbour approaches for pilotage. These are useful supplements rather than replacements for coastal coverage.

Understanding specifications and terminology

• Scale: Expresses the ratio of chart distance to real distance (e.g. 1:25 000). Larger scale = more detail. Choose larger scale charts for nearshore and pilotage.
• Soundings and contour intervals: Soundings are spot depths; contour lines join equal depths. Check the units (metres or fathoms) and the vertical datum used for depths.
• Chart datum vs GPS datum: Chart datum is the zero level for depths (often a low tide reference); GPS uses geographic datums like WGS‑84 for position. Understand both to avoid misinterpreting safe clearance.
• ENC vs Raster: ENCs are data sets—dynamic and interactive—while raster charts are images. Both have uses; know which your system handles.
• Update frequency: Indicates how often the provider issues new editions or notices to mariners. Regular updates are crucial near dynamic coasts.

Compatibility and installation checks

Before buying charts, verify these practical points so the product functions aboard your specific boat.

• Device support: confirm your chartplotter, MFD or navigation app supports the chart format and file delivery method.
• Storage and transfer: ensure you have sufficient onboard storage and the means to transfer large chart files (SD card, USB, network link).
• Electrical and mounting considerations: electronic chart devices must be installed where power and signal cabling can reach, with adequate ventilation and safe access for SD cards or USB sticks.
• Integration: check how charts display with AIS, radar, autopilot and depth instruments; mismatches can produce confusing overlays.
• Paper chart storage: ensure a dry, flat, accessible space; keep charts protected from sun and salt.

Day sailing, coastal cruising and offshore differences

Day sailing

Buy or download the minimal chart coverage you need for likely local trips. Simplicity, portability and quick access matter. A tablet with offline charts or a small plotter plus one or two paper charts for the harbour and common coastal areas is usually sufficient.

Coastal cruising

Expect to navigate new anchorages and night approaches. Maintain a broader set of electronic charts for your cruising ground, keep them updated, and carry paper charts for critical coastal stretches and port approaches. Consider detailed bathymetry for unfamiliar inlets.

Offshore and remote cruising

Redundancy is the priority. Carry comprehensive electronic chart coverage on independent devices (e.g. chartplotter and tablet) and a full set of relevant paper charts. Use official ENCs where appropriate, and carry means to update charts at sea when possible (satellite comms, if available). Remember that chart errors or a lack of recent surveys are more consequential far from safe haven.

Trade-offs to consider

• Simplicity (paper or simple raster charts) versus feature richness (vector charts with layers and alarms).
• Upfront cost and storage (paper charts can be bulky) versus ongoing subscription costs for digital updates.
• Single integrated system versus multiple independent sources for redundancy.
• High detail for pilotage versus broad coverage for passage planning—both are often required and demand space or digital capacity.

Maintenance and service life

Keep charts accurate and usable by following simple routines:

• Regularly install official chart updates and Notices to Mariners relevant to your area before each cruise where possible.
• Inspect paper charts for fading, tears or water damage; replace or laminate high‑use sheets.
• Back up electronic chart files and keep copies of licence keys or activation information in a safe place ashore and aboard.
• Monitor device storage and file integrity—corrupt files can render a chart unreadable at a critical moment.
• Carry spare SD cards or external drives with essential chartsets for quick recovery.

Common mistakes and how to avoid them

• Buying charts for the wrong device format: Confirm file formats and copy protection methods; don’t assume one product will work across all plotters.
• Relying on a single source: Use at least two independent chart sources (different manufacturers or paper + electronic) to cross‑check critical information.
• Ignoring datum differences: Verify vertical and horizontal datums to avoid depth and positional misinterpretation.
• Failing to update charts: Outdated charts can miss new hazards—install updates before long cruises and after known local changes.
• Neglecting paper backups: Don’t treat paper charts as optional; they’re the final fallback in an electrical or software failure.
• Overlooking chart detail for pilotage: Using small‑scale charts for harbour approaches risks missing local hazards; obtain larger‑scale harbour plans where available.

Before you buy: practical checklist

• Define your intended cruising area and typical passages (day, coastal, offshore).
• List the navigation devices you will use and check supported chart formats.
• Measure available storage and physical card/USB access on each device.
• Confirm the chart datum and units for depths for the areas you’ll visit.
• Ask about update frequency, delivery method and the cost or licence terms for updates.
• Plan redundancy: at least one independent electronic source plus paper charts for critical areas.
• Check how charts integrate with AIS, radar, depth sounder and autopilot features you rely on.
• Prepare a maintenance plan: where you will obtain updates and how you will back up chart data.
• Ensure paper charts fit available storage and are kept dry and accessible.

Prioritise based on your worst‑case scenarios. For short local trips, convenience and current local charts make sense; for longer or offshore passages, redundancy, official vector data and comprehensive paper backups should carry more weight. A well‑considered mix of electronic and paper charts, kept current and compatible with your onboard systems, provides the best balance between modern navigation convenience and resilient seamanship.