GPS and Chartplotters
GPS transformed offshore navigation from an art practiced by a few into a skill accessible to every sailor — but understanding how it works makes you a better navigator, not just a button-pusher.
How GPS Works — Satellites, Signals, and Accuracy
The Global Positioning System operates on a deceptively simple principle: if you know the exact distance from your position to three or more satellites whose locations are precisely known, basic geometry — trilateration — pins your position to a single point on Earth's surface. In practice, your GPS receiver listens for signals from the GPS constellation of 31 active satellites orbiting at roughly 12,550 miles altitude. Each satellite broadcasts a time-stamped signal, and the receiver calculates distance by measuring how long that signal took to arrive. Four satellites are the true minimum — three for position and one to correct for clock errors in the receiver, since your handheld doesn't carry an atomic clock like the satellites do.
Standard GPS accuracy for civilian receivers is typically 3 to 5 meters under open sky with good satellite geometry. That is extraordinary compared to what sailors had before 2000, when Selective Availability deliberately degraded civilian GPS to about 100 meters. But several factors degrade accuracy in practice: satellite geometry (if all visible satellites are clustered in one part of the sky, positional dilution of precision increases error), atmospheric effects (ionospheric and tropospheric delays bend the signal), multipath errors (signals bouncing off the mast, rigging, or nearby structures before reaching the antenna), and obstructions (sailing near cliffs or in narrow fjords can block satellites).
WAAS (Wide Area Augmentation System) in North America and EGNOS (European Geostationary Navigation Overlay Service) in Europe are satellite-based augmentation systems that broadcast correction signals, reducing positional error to 1 to 2 meters. Most modern marine GPS receivers receive WAAS/EGNOS automatically — you don't need extra hardware. For cruising sailors, the practical improvement over standard GPS is modest for open-water navigation but meaningful when navigating narrow channels, approaching waypoints near hazards, or anchoring by coordinates. Many newer receivers also support multi-constellation positioning, pulling signals from GLONASS (Russia), Galileo (EU), and BeiDou (China) in addition to GPS, which improves accuracy and reliability by increasing the number of visible satellites at any given time.
What GPS cannot tell you is as important as what it can. GPS gives you position, speed over ground (SOG), and course over ground (COG) — but it cannot measure heading when stationary, it cannot detect current directly, and it has no inherent ability to warn you about obstacles or depths. A GPS receiver with no chart data is a latitude/longitude readout — useful, but not navigation. The combination of GPS position data with electronic chart data is what makes a chartplotter, and that combination is where the real utility for sailors lives.
Check your chartplotter's satellite status screen periodically — it shows how many satellites the receiver is tracking, their positions in the sky, and the resulting HDOP (Horizontal Dilution of Precision) value. An HDOP below 2.0 means excellent accuracy. Above 5.0 means your position fix is degraded, and you should not trust it for close-quarters navigation near hazards.
Standalone GPS vs Integrated Chartplotters and MFDs
A standalone GPS receiver — the kind that simply displays latitude, longitude, SOG, and COG on a small screen — was the standard marine GPS for two decades. Units like the Garmin GPS 72 or the old Magellan handhelds gave you a position fix and basic waypoint navigation, and many offshore sailors still carry one as a backup. They are cheap ($100–$200), waterproof, run on AA batteries, and do one thing well. But they don't display charts, they can't overlay your position on a map, and navigating by waypoint coordinates alone requires constant chart work — cross-referencing your displayed position against a paper chart or separate electronic chart.
Chartplotters combine a GPS receiver with electronic chart display, showing your boat as a moving icon on a detailed nautical chart. This is the standard now — even entry-level units from Garmin, Raymarine, B&G, Furuno, and Simrad display vector or raster charts with your position, track, waypoints, and routes overlaid in real time. Dedicated chartplotters range from compact 5-inch units suitable for a small cockpit to 12-inch or larger displays for pilothouse installations. They typically include built-in GPS antennas, chart card slots, and connectivity for depth, wind, and AIS data.
Multifunction displays (MFDs) take the chartplotter concept further by integrating multiple data sources onto a single screen — or split screens showing two or more functions simultaneously. A modern MFD can display your chart with GPS overlay, radar image, depth sounder, engine data, AIS targets, and weather information all at once, with the ability to switch between full-screen views or customizable split-screen layouts. This integration is powerful but creates a single point of failure: if the MFD dies, you lose everything displayed on it simultaneously. Major manufacturers have moved toward networked systems where multiple MFDs share data over NMEA 2000 or Ethernet, so a second display can take over if the primary fails.
For cruising sailboats, the practical sweet spot is a dedicated chartplotter at the helm — typically 7 to 10 inches — with a second display at the navigation station below if budget allows. The helm unit handles day-to-day navigation, and the nav station unit allows route planning, weather overlay review, and detailed chart work in comfort. Both units should be on the same network, sharing GPS, depth, wind, and AIS data. For budget-conscious sailors, a single helm-mounted chartplotter backed up by a tablet running navigation software (with its own GPS or connected to the boat's GPS via WiFi) provides reasonable redundancy at minimal cost.
When choosing between brands, pick the ecosystem your existing instruments already use. A Raymarine chartplotter talks natively to Raymarine radar, autopilot, and instruments over their proprietary network. Mixing brands is possible through NMEA 2000, but integration is never as seamless as staying within one manufacturer's family. The exception is AIS — standalone AIS receivers from any manufacturer feed data to any chartplotter via standard NMEA 0183 or NMEA 2000.
Chart Cards, Formats, and Keeping Charts Updated
Your chartplotter is only as good as the charts loaded on it, and chart data is not universal — it comes in proprietary formats tied to specific manufacturers. Garmin uses BlueChart g3 and g3 Vision, Raymarine and Simrad use Navionics (now owned by Garmin but still sold separately), Furuno uses TimeZero or C-MAP, and some units accept multiple formats. Before buying a chartplotter, verify that charts are available for your cruising area in that unit's format. Coverage for North America, Europe, and the Caribbean is comprehensive from all major providers, but once you head to the South Pacific, Southeast Asia, or remote coastlines, chart availability and quality vary dramatically.
Raster charts are digital photographs of traditional paper charts — they look exactly like the paper chart you'd buy at a chandlery. They are familiar and trusted, but they are large files, they cannot be rotated without the text becoming unreadable, and they don't scale well (zoom in too far and you just see pixels). Vector charts are built from a database of points, lines, and areas that the chartplotter renders on the fly. They scale cleanly to any zoom level, text always reads correctly regardless of chart orientation, and the plotter can query individual objects (tap on a buoy to see its characteristics). Nearly all modern chartplotters use vector charts as their primary format.
Chart updates are not optional — they are a safety requirement. Coastlines don't move (much), but aids to navigation change, new hazards are discovered, depths are resurveyed, and marinas open and close. Most chart providers offer annual update subscriptions ($50–$150/year depending on coverage area) that you download to your chart card via a computer. Navionics offers free updates for the first year with a chart purchase and paid updates thereafter. Failing to update your charts means navigating with potentially obsolete data — and if a buoy has been relocated or a new shoal charted, your outdated chart won't show it.
Carry paper charts as backup, at minimum for your primary cruising area. Electronic chart failures — dead chartplotter, corrupted chart card, software crashes — do happen, and they always happen at the worst possible time. A waterproof chart book for your region costs $30–$50 and takes up minimal space. For offshore passages, full-size paper charts of your route and destination are prudent seamanship. You should be able to plot a position from GPS coordinates on a paper chart without assistance — practice this skill before you need it.
Before a passage, download charts for your entire route plus alternates to the chartplotter's internal memory or chart card while you still have internet access. Some chartplotters allow downloading additional chart detail via WiFi when connected — but you cannot count on having connectivity offshore or in remote anchorages. Pre-load everything you might need.
Touchscreen vs Button Interfaces and Screen Sizing
The touchscreen vs button debate in marine chartplotters mirrors the same argument in every electronics category, but the marine environment adds constraints that tilt the balance differently than on land. Touchscreens are intuitive — pinch to zoom, drag to pan, tap to select — and they allow manufacturers to pack more features into a cleaner physical design. But touchscreens have real limitations on a boat: they are difficult to operate with wet hands, nearly impossible with gloves, prone to phantom touches from spray and rain, and their optical coatings degrade from UV exposure and salt crystal abrasion over time. On a pitching foredeck in rain, a touchscreen chartplotter can be maddening.
Button-controlled units (rotary knobs, directional pads, dedicated function keys) are slower to learn but far more reliable in rough conditions. Furuno has historically favored button interfaces, and many professional mariners prefer them for exactly this reason — you can operate a button-controlled chartplotter by feel, at night, with cold wet hands, while watching the horizon. Some manufacturers now offer hybrid interfaces with both touchscreen and physical buttons or knobs, which gives you the best of both worlds: touch for calm-weather chart work and route planning, buttons for heavy-weather operation.
Screen size is determined by three factors: viewing distance (how far the helmsman sits from the display), mounting space (the available panel or pedestal area), and information density (how much data you want visible simultaneously). A 5-inch screen is adequate for a tiller-steered boat where the helmsman is within arm's reach, but unreadable from 6 feet away on a center-cockpit boat. A 7-inch screen is the minimum for comfortable chart reading at typical wheel-steered helm distances. A 9 to 12-inch screen suits most cruising sailboats and allows split-screen operation (chart plus radar, or chart plus instrument data) without everything becoming too small to read.
Sunlight readability is a specification that matters enormously on a boat and barely matters on land. Marine displays are rated in nits (candelas per square meter) — the higher the number, the brighter the screen in direct sunlight. A minimum of 1,000 nits is needed for comfortable reading in full sun; premium units hit 1,500 to 2,000 nits. Cheap consumer tablets repurposed as chartplotters typically produce 400–600 nits and wash out completely in direct sunlight, which is exactly when you need to read them. If you're considering a tablet as your primary navigation display, test it in full sun before committing.
Never mount a chartplotter where you must look away from the horizon to read it. The display should be visible with a quick downward glance from a normal lookout position. Mounting a chartplotter below the companionway hatch or inside the cabin forces the helmsman to leave the cockpit or take their eyes off the water for extended periods — this has directly caused collisions with other vessels, fishing gear, and unlit objects.
Mounting Options, Power Requirements, and Backup GPS
Mounting method affects both usability and longevity. The three standard options are flush mount (the display recesses into a flat panel — helm console, nav station bulkhead, or overhead panel), bracket mount (a U-shaped bail that attaches to a horizontal or vertical surface and allows the display to tilt and swivel), and pod mount (a dedicated pedestal, typically on the binnacle or cockpit coaming, designed for that manufacturer's displays). Flush mounting looks cleanest and protects the display edges, but requires cutting a precise rectangular hole in the panel and limits future repositioning. Bracket mounting is the most flexible — you can adjust the viewing angle seasonally and remove the unit for winter storage or theft prevention.
Power requirements for modern chartplotters are modest but not trivial. A typical 7-inch unit draws 0.5 to 1.5 amps at 12V (6 to 18 watts), while a large 12-inch MFD can draw 2 to 4 amps at full brightness. On a 24-hour passage with the chartplotter running continuously, a single 9-inch unit drawing 1.5 amps consumes 36 amp-hours — which is meaningful on a boat with a 200Ah house bank. Dimming the backlight at night reduces consumption significantly and also preserves your night vision. Most units have automatic dimming, but manually setting the lowest readable level is always more efficient than the auto mode.
Installation wiring should follow the same standards as any marine electronics: run dedicated tinned marine-grade wire from the electrical panel to the chartplotter, sized for the circuit length and load with less than 3% voltage drop. Use an inline fuse at the panel rated to the manufacturer's specification (typically 3 to 5 amps for a single chartplotter). Connect the GPS antenna cable (if external) with proper marine-grade coax and connectors — the internal GPS antenna in most modern chartplotters is adequate for normal use, but an external GPS antenna mounted on the masthead or radar arch provides better satellite visibility and faster position fixes, especially on boats with carbon fiber masts or dodgers that block the sky.
Backup GPS is not optional for offshore sailing. Your primary chartplotter will fail at some point — it might be a power supply issue, a screen failure, a software crash, or saltwater intrusion through a compromised seal. A secondary GPS source can be as simple as a handheld GPS receiver ($100–$200, runs on AA batteries, fully waterproof), a smartphone or tablet with offline charts loaded in an app like Navionics Boating or iSailor, or a dedicated backup chartplotter mounted at the nav station. The handheld GPS is the most reliable backup because it is completely independent of the boat's electrical system — it works when the batteries are dead, the panel is fried, or the cabin is flooded. Carry one on every offshore passage, with fresh batteries.
Store a waterproof handheld GPS in your abandon-ship bag along with a laminated chart of your cruising area. If you ever need the life raft, knowing your position to within 5 meters and being able to communicate it to rescuers is the difference between a fast rescue and a long search. A $150 handheld GPS may be the most important piece of safety equipment you ever buy.
Summary
GPS positioning relies on trilateration from at least four satellites, providing 3-5 meter accuracy under open sky — augmentation systems like WAAS and EGNOS improve this to 1-2 meters with no additional hardware.
Modern chartplotters combine GPS with electronic chart display, and multifunction displays integrate radar, depth, AIS, and engine data onto a single networked screen — but create a single point of failure that demands backup planning.
Chart data is manufacturer-specific — verify chart availability for your cruising area before committing to a brand, keep charts updated annually, and carry paper charts as a minimum backup.
Touchscreens are intuitive in calm conditions but unreliable with wet hands, gloves, or spray — consider hybrid interfaces or button-controlled units for offshore use, and prioritize sunlight readability above 1,000 nits.
Backup GPS is essential for offshore sailing: a waterproof handheld GPS receiver running on AA batteries provides position data completely independent of the boat's electrical system.
Key Terms
- WAAS/EGNOS
- Satellite-based augmentation systems (Wide Area Augmentation System in North America, European Geostationary Navigation Overlay Service in Europe) that broadcast GPS correction signals, improving positional accuracy from 3-5 meters to 1-2 meters.
- HDOP
- Horizontal Dilution of Precision — a numerical value indicating the quality of satellite geometry for a GPS fix. Lower values mean better accuracy; below 2.0 is excellent, above 5.0 indicates degraded positioning.
- MFD
- Multifunction Display — a marine electronics screen that integrates chartplotter, radar, sounder, AIS, and instrument data into a single unit with configurable split-screen views, typically networked to share data with other MFDs and instruments.
- Vector Chart
- An electronic nautical chart built from a database of geographic objects (points, lines, areas) that the display renders in real time. Vector charts scale cleanly to any zoom level, support object queries, and allow rotation without text distortion.
- NMEA 2000
- A plug-and-play networking standard for marine electronics that allows instruments, sensors, chartplotters, autopilots, and other devices to share data over a single backbone cable using a standardized protocol.
- SOG/COG
- Speed Over Ground and Course Over Ground — GPS-derived measurements of the vessel's actual movement relative to the earth's surface, as opposed to speed through the water or compass heading, which are affected by current and leeway.