Navigation Software and Apps
The right software turns a tablet into a powerful chart plotter for a fraction of the cost — but understanding its limitations is as important as understanding its features.
Chart Plotting Software — The Options
The landscape of navigation software ranges from free, open-source programs to professional-grade suites costing thousands of dollars, and the right choice depends on how you sail and what you expect from the software. At the free end, OpenCPN is the standout — an open-source chart plotter that runs on Windows, Mac, and Linux, supports both raster and vector charts, connects to GPS and AIS through NMEA, handles weather overlays, and has an active community of developers building plugins for everything from polar diagrams to GRIB file visualization. For a free program, OpenCPN is astonishingly capable and is the primary navigation software on thousands of cruising boats worldwide.
Navionics (now Garmin) dominates the mobile chart plotting space. The Navionics Boating app runs on iOS and Android, provides vector chart coverage for most of the world at a reasonable annual subscription, and offers features like automatic routing, community-sourced depth data (SonarChart), and dock-to-dock navigation. The app is intuitive enough for beginners and detailed enough for serious coastal navigation. Its primary limitations are the small screen of a phone or tablet and the dependence on the device's GPS accuracy, which is typically good but not as reliable as a dedicated marine GPS receiver.
TimeZero is a professional-grade navigation suite used by both recreational and commercial mariners. It runs on Windows PCs and integrates deeply with radar, AIS, and instrument data through NMEA connections. TimeZero's strength is its 3D chart display, real-time weather overlay, and the ability to function as a full multifunction display replacement on a large-screen PC. Expedition occupies the high end for racing sailors and performance-oriented cruisers — it integrates wind, current, and polar data to calculate optimal routes, provides real-time performance analysis, and supports GRIB weather file routing. Expedition is not cheap, but for offshore racing and serious passage making, its routing capabilities are unmatched.
The choice between these options is not either/or. Many experienced sailors run a dedicated marine chartplotter as their primary system, a tablet with Navionics as a secondary display and backup, and OpenCPN or Expedition on a laptop for passage planning and weather analysis. Each platform has strengths the others lack: the marine chartplotter has the best screen and reliability, the tablet has the best portability and interface, and the laptop has the most analytical power. Using all three in complementary roles gives you depth and redundancy that no single system can match.
Start with OpenCPN — it's free, it's powerful, and learning it teaches you the fundamentals of electronic chart navigation that apply to every platform. Download free NOAA ENC and RNC charts, connect a USB GPS receiver, and you have a fully functional chart plotter for zero cost. Upgrade to paid software only when you've identified specific capabilities you need that OpenCPN doesn't provide.
Electronic Chart Types — Raster vs. Vector
Electronic charts come in two fundamentally different formats, and understanding the difference between raster and vector charts is essential for using any chart plotting software safely. A raster chart (RNC — Raster Navigational Chart) is essentially a digital photograph of a paper chart. Every detail, color, symbol, and annotation from the paper chart is preserved exactly as it appears. When you zoom in on a raster chart, you see the same image enlarged — eventually it becomes pixelated, just like zooming in on a photograph. When you zoom out, the detail becomes too small to read. Raster charts look familiar to anyone who has used paper charts, and they show exactly the same information.
A vector chart (ENC — Electronic Navigational Chart) stores chart information as a database of geographic objects — depth contours are mathematical curves, buoys are data points with attributes, shipping lanes are defined polygons. The chart plotter renders this data into a visual display. When you zoom in on a vector chart, the software redraws the display at the new scale using the same underlying data — it never pixelates. When you zoom out, the software intelligently removes clutter, hiding less important features to keep the display readable. This scale-independent rendering is the great advantage of vector charts.
The danger of vector charts is the illusion of precision at high zoom levels. Because a vector chart redraws smoothly at any zoom level, you can zoom in until your boat fills a harbor on screen — but the underlying data may be based on a survey at a much smaller scale. The chart looks detailed and precise, but the positions of features may be accurate only to the original survey resolution. A raster chart makes this obvious because it pixelates when you zoom past the chart's native scale, giving you a visual warning that you're exceeding the data's resolution. Vector charts give no such warning by default — you must check the chart's source scale and data quality indicators.
NOAA publishes both ENC (vector) and RNC (raster) charts for US waters, both available as free downloads. Most modern chart plotters and apps use vector charts because of their flexibility, scalability, and smaller file sizes. But many experienced navigators keep raster charts loaded as a secondary layer because they show the complete, unfiltered view of the original paper chart, including notes, cautions, and inset diagrams that vector charts sometimes omit. Using both formats — switching between them to cross-check — is the most thorough approach to electronic chart navigation.
When navigating in an unfamiliar area with vector charts, always check the data quality indicator or source scale before trusting precise positions. In many parts of the world, the underlying survey data is from the 19th century. The vector chart's smooth, detailed appearance at high zoom gives no visual indication that the depths shown may be unreliable — you must actively check the metadata. Most chart plotters can display data quality layers if you enable them in settings.
Never zoom in on a vector chart beyond the source data scale and assume the displayed positions are accurate. A vector chart will happily display a smooth, detailed harbor at 1:1,000 scale even when the underlying survey was conducted at 1:50,000 — meaning feature positions may be hundreds of meters from where they appear on screen. This false precision has grounded more boats than actual chart errors.
Route Planning, Weather Overlays, and GRIB Files
Route planning in navigation software is far more powerful than the simple A-to-B line it replaced on paper charts. A planned route in software is a series of waypoints connected by course legs, with the software automatically calculating distance, bearing, estimated time of arrival (based on your expected speed), and cross-track error during navigation. Good route planning software lets you set safety contours that highlight shallow water, define no-go zones around obstacles, and run route checking that scans the entire planned track for hazards — crossing depth contours, passing through restricted areas, or approaching too close to navigation dangers.
Weather overlay integration transforms a chart plotter into a tactical decision-making tool. GRIB files (GRIdded Binary format) are compact weather data files that encode forecast wind speed and direction, wave height and period, barometric pressure, precipitation, and current — all as a grid of data points that the software renders as arrows, color gradients, and isobars overlaid directly on your chart. You can step through the forecast hour by hour, watching how weather systems move, and plan your departure timing to take advantage of favorable winds or avoid a developing gale.
Downloading GRIB files offshore requires either a satellite connection or an SSB radio with a Pactor modem. Within cellular range, downloading from services like PredictWind, Windy, or NOAA's GFS model is straightforward over WiFi or mobile data. Offshore, the options narrow: an Iridium GO or similar satellite device can download compressed GRIB files (the data volume is small — a typical request is 50-100 KB), or an SSB radio running SailMail or Winlink can request GRIB files by email. Planning your weather data access before you leave cellular range is a critical passage preparation task that many sailors overlook until they're offshore and blind.
The most sophisticated route planning integrates weather data with your boat's performance characteristics to calculate optimal routes. Software like Expedition and PredictWind Routing takes your boat's polar diagram (speed vs. wind angle and wind speed), the GRIB forecast, and ocean current data, and calculates the route that minimizes passage time, maximizes safety, or balances both. The optimal route is often counterintuitive — sailing extra miles to catch a favorable wind shift or avoid an adverse current can save hours or days on an ocean passage. This kind of routing was once available only to professional racing teams; now it's accessible to any cruising sailor with a laptop.
When using weather routing software, always run multiple GRIB models (GFS, ECMWF, ICON) and compare the results. No single weather model is consistently the most accurate — they each have strengths in different regions and weather patterns. If the models agree, you can have high confidence in the forecast. If they diverge significantly, the forecast is uncertain, and you should plan conservatively.
AIS Display, Tracking, and Mobile Apps vs. Dedicated Chartplotters
AIS (Automatic Identification System) display is one of the most valuable features in navigation software. When your chart plotter or app receives AIS data — either from a dedicated AIS receiver connected via NMEA, a built-in AIS receiver in your VHF radio, or an AIS-sharing network — it overlays vessel targets directly on the chart display. Each target shows the vessel's name, MMSI, speed, course, and critically, the CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach). These calculated values tell you whether a vessel will pass close to you and when, allowing you to make course-change decisions minutes or hours before a close-quarters situation develops.
AIS on a mobile app has become remarkably capable. Apps like MarineTraffic, Boat Beacon, and the AIS features built into Navionics provide AIS target display using data received from internet-connected AIS networks — no hardware needed. The catch is that this internet-sourced AIS data has latency (targets may be 1-5 minutes behind real time) and requires a data connection. For real-time, zero-latency AIS in offshore conditions, you need a dedicated AIS receiver or transceiver connected to your chart plotter via NMEA 0183 or NMEA 2000. The hardware-based AIS sees targets directly via VHF radio, with no internet dependency and no latency.
The trade-off between mobile apps and dedicated chartplotters comes down to reliability versus cost. A marine chartplotter with integrated GPS, a sunlight-readable display, waterproof housing, and NMEA connectivity costs $500-$5,000. A tablet running Navionics costs $200-$800 for the tablet plus $15-$25 per year for the app subscription. The tablet has a better user interface, a more responsive touch screen, and access to the internet for updates and downloads. The chartplotter has a sealed housing rated for continuous salt spray, a display readable in direct sunlight without glare, a power supply designed for marine voltage fluctuations, and a GPS receiver optimized for marine accuracy and update rate.
The practical answer for most sailors is both. Use a dedicated marine chartplotter as your primary navigation display — the one that's on from departure to arrival, mounted in a visible location, and connected to all your instruments. Use a tablet as a secondary display for route planning, weather analysis, and a backup chart plotter. Use your phone as a third-tier backup with offline charts downloaded. Each additional layer of redundancy costs less than the first and provides proportionally greater peace of mind. The key discipline is ensuring that every device has offline charts loaded — the internet is not available when you need it most.
Before every passage, verify that your tablet and phone have offline charts downloaded for the entire route plus reasonable diversions. Do not rely on downloading charts underway — cellular coverage ends quickly offshore, and satellite data is too slow and expensive for chart downloads. Open each chart area in the app while on WiFi to confirm the data is cached locally.
Data Backup and System Resilience
The single most common cause of navigation data loss is not equipment failure — it's failure to back up. Routes that took hours to plan, waypoints accumulated over years of cruising, chart corrections, instrument calibrations, and custom chart overlays all live on a single device or SD card. When that device dies — and electronics always die eventually — the data is gone unless you've backed it up. The marine environment accelerates this risk: a splash, a power surge, a corroded connector, or a hard drive failure in humid conditions can erase years of accumulated navigation data in an instant.
Back up waypoints and routes to multiple locations. Export your waypoint and route databases from your chartplotter to an SD card, and copy that SD card to a laptop. Export the same data from your laptop navigation software to a USB drive. Store one backup aboard and one ashore. Most chart plotters can export data in GPX format — an open standard that any chart plotter or software can import. Export to GPX regularly and treat it like any other critical ship's document. If your chartplotter dies, you can import the GPX file into a replacement unit or a tablet app and have your entire route library intact.
Chart data itself requires a different backup strategy. Official NOAA charts (both ENC and RNC) are free to download, so backing them up means ensuring you have downloaded the latest editions and stored them where your backup navigation devices can access them. Commercial chart data (Navionics, C-MAP, Garmin BlueChart) is typically licensed to a specific device or SD card, which means you may not be able to simply copy it. Understand the licensing terms of your chart data — some providers allow installation on multiple devices under one license, others do not. Having a chart subscription that works on both a chartplotter and a tablet is worth the extra cost for the redundancy it provides.
System resilience goes beyond data backup to include having a complete, independent backup navigation capability. If your primary chartplotter fails, can you navigate to your destination using only your backup devices? Test this by running a practice navigation exercise using only your tablet and compass, with the chartplotter turned off. Identify the gaps: do you need to download additional charts to the tablet? Does the tablet have a GPS fix without the chartplotter? Can you access your routes on the backup device? Finding and fixing these gaps in port is the entire point of the exercise — discovering them mid-passage is a crisis.
Set a quarterly calendar reminder to export your waypoints and routes from every navigation device to GPX files, and store copies on a USB drive and in cloud storage. This takes ten minutes and protects years of accumulated navigation data. Label each backup with the date and source device so you can identify the most recent version.
Summary
Chart plotting software ranges from free and powerful (OpenCPN) to professional-grade (Expedition and TimeZero), and most experienced sailors use multiple platforms in complementary roles for redundancy.
Raster charts are digital images of paper charts that pixelate when overzomed, while vector charts are database-rendered displays that stay smooth at any zoom — but vector charts can create a dangerous illusion of precision beyond their source data resolution.
GRIB weather files overlaid on chart displays enable tactical passage planning, and advanced routing software can calculate optimal routes using your boat's polar diagram, wind forecasts, and current data.
AIS display on chart plotters and apps provides critical collision avoidance information including CPA and TCPA, but internet-sourced AIS has latency — dedicated hardware receivers provide real-time data without internet dependency.
Back up waypoints and routes in GPX format to multiple devices, and test your backup navigation capability by running a practice exercise with the primary chartplotter turned off.
Key Terms
- GRIB File
- GRIdded Binary format — a compact weather data file encoding forecast wind, waves, pressure, and current as a grid of data points. Used by navigation software to overlay weather forecasts on chart displays and calculate optimal routes.
- ENC (Electronic Navigational Chart)
- The official vector-format digital chart published by national hydrographic offices. ENCs store chart features as database objects with attributes, enabling scale-independent rendering and intelligent feature display.
- RNC (Raster Navigational Chart)
- A digital image of a paper nautical chart, preserving every visual detail of the original. RNCs pixelate when zoomed past native scale, providing a natural visual warning against exceeding the chart data's resolution.
- GPX (GPS Exchange Format)
- An open XML-based file format for exchanging waypoints, routes, and tracks between GPS devices and navigation software. The standard format for backing up and transferring navigation data across different platforms.
- CPA (Closest Point of Approach)
- The minimum distance at which a tracked vessel will pass your position, calculated from the vessel's current course and speed via AIS or radar. A key metric for collision avoidance — low CPA values demand attention and possible action.