GRIB and GPX: getting weather and routes onto your chart plotter

What a GRIB file is

GRIB is the format weather services use for gridded model data. A GRIB file holds values such as 10 m wind, gusts and air pressure for every grid point and time step. Navigation software turns them into wind arrows and isobars and plays the forecast step by step. Once downloaded, it needs no connection on board.

Grid, time step and file size

Three things determine how big the file gets:

• Area: the area you are sailing in, with some margin for approaching lows.
• Grid spacing: halve the distance between grid points and you have four times as many points. A fine grid shows wind funnelling and shelter along the coast better, but takes up space.
• Time step and forecast range: three-hour steps are enough for most trips. Halve the step and the file doubles in size.

Over satellite or a weak mobile signal, every kilobyte counts. So download the file in harbour while you have a good connection, and keep it as small as possible at sea.

GRIB from Sailseer

Sailseer cuts the file from the same model data as its weather maps and writes it as GRIB2: wind at 10 m, gusts and air pressure at sea level. There are two ways:

• In the layers panel of the map, “Download area” downloads the visible map area. You choose the forecast range (24, 48, 72 or 120 hours); the grid spacing depends on the size of the area (0.05 to 0.5 degrees), and the time step is three hours.
• Next to the GPX button, “GRIB for the passage” downloads the rectangle around your route.

With the consensus selected, the server picks the model that covers the whole area: ICON-EU for Europe, otherwise ECMWF or GFS, for example in Greenland, Svalbard or far out in the Atlantic. The note that follows gives the model, size, number of time steps and grid spacing, and says if the file only covers part of the area. Pressure comes from a wider grid, because lows often lie outside the area. Sailseer rejects a request that is too large (over three million values) and tells you what to reduce. Which model is good for what is covered in the guide to weather models.

Opening in OpenCPN and qtVlm

OpenCPN, the free navigation software, opens GRIB files with its bundled GRIB plugin: load the file, choose the time, and show wind arrows, gusts and isobars. qtVlm reads GRIB files too and can calculate a weather route from them. Whether your chart plotter displays GRIB files from a memory card depends on the device; the manual will tell you.

GPX: the route for your plotter

GPX is the common exchange format for waypoints, routes and tracks. “GPX for the plotter” produces GPX 1.1 for Garmin, Raymarine and B&G and respects their limits:

• Names are converted to ASCII (Ærøskøbing becomes Aeroeskoebing), shortened to 16 characters and made unique, so that no waypoint overwrites another.
• Older Raymarine and B&G units only accept 50 points per route. Sailseer splits longer routes into sub-routes that join at a handover point.
• The route follows the full track to within 25 m, so that no straight line cuts across a headland. Harbours, bridges and fuel stops are included as additional waypoints with symbols.
• In the multi-day trip, each day becomes its own route. Tacks are available as a separate GPX file.

The file usually gets to the plotter via a memory card or the manufacturer's app. After importing, check every route on the plotter's chart and move any waypoints that are too close to a buoy or shallows.

Loading a route back into Sailseer

It works the other way too: in the menu next to the start, “Load GPX route” loads a route, a track or waypoints from OpenCPN, Navionics, qtVlm or your plotter as a passage. The file can be up to 5 MB and is only processed, not stored. Sailseer thins it out to at most 20 stops, and start and destination snap to the nearest harbour within 1 nm. Then you check the weather as for any other passage. In navigation mode you can save your track as GPX.

Straight to plotter and autopilot

With boat instruments connected, Sailseer sends the route as NMEA 0183 (WPL and RTE) to the plotter or autopilot while you are sailing a route: in the browser via USB, and in the Android app, which is in closed testing, also via Wi-Fi and Bluetooth. Continuously sending the course to the next waypoint only happens after you confirm it. Lookout and steering remain your job.

Offline charts: MBTiles for OpenCPN

In the Chart menu, “Offline pack of this area (MBTiles)” creates a tile pack of the free layers that the server delivers itself: the depths from EMODnet (in Poland from the SIPAM surveys), the BSH chart (from Germany's Federal Maritime and Hydrographic Agency) and the Latvian chart data. OpenCPN opens MBTiles directly. The pack covers zoom level 8 up to three levels above the current one (14 at most) and has at most 5,000 tiles; it stays available for download for two days. Not included are the base map and OpenSeaMap, whose terms forbid bulk pre-loading, and the layers that the browser fetches directly from other providers.

Limits

GRIB files are model output, not observations and not warnings. Models often show thunderstorm gusts, acceleration off headlands and katabatic winds as weaker than they are, and between time steps the software only interpolates. GPX routes and tile packs are planning sketches, and every GPX file says so itself. Sailseer is not intended for navigation: official charts, marine forecasts and warnings take precedence. How Sailseer compares with other apps is covered in the sailing app comparison.