To start the Grib plugin press the image::grib.png[image,width=32,height=32]button in the tool-bar. The “GRIB Display Control” then pops up.

Hide Data at Cursor

This is a smart and compact dialog. For a quick start press the “Open File” button and navigate to your grib file directory and press the file to be displayed.
Previous/Next Go to the previous or the next record in the gribfile. This corresponds to the time “interval” chosen when the grib file was downloaded.
Data valid at The time of validity for all grib data on screen.
Open Grib file The name of the grib file in use.
Now Displays the record, nearest in time to “now” according to the Start option you have selected: (“Start at the nearest forecast to current time” or “Authorize interpolation to start at current time”).
Select Geopotential Height If grib data for geopotential height is available, selection will be between Std, 500.
Zoom to Center This button will home in on the loaded area that the loaded grib file covers.
Play Goes through all records in turn, with the animation speed controlled in the “Grib Settings” dialog.
Hide Cursor Data Hides Grib which normally appears in the data windows. Data windows are hidden.
Progress Slider Just to the right of the “Play” button is a slider that is just another way of going through the grip records.
Open File Activates the operating systems standard file selector dialog to select a grib file. The plugin remembers the directory of this grib. This is used next time you start OpenCPN and activates the Grib plugin. See settings in the “Grib Preferences” dialog above.
Settings is where to control how the different grib data will be displayed. See more below.
Request Download grib files via email. More below.
GRIB Data at Cursor. This is where the “smart” part kicks in.
Only entries for the data contained in the active grib file will be displayed. For example, if there is no wave data in the current grib file, there will be no signs of wave data at all. In the case above, where the the wave data is N/A, the grib file includes wave data, but not for the area where the cursor is. Compare with the picture below, where the grib only contains current data.
The tick boxes next to each data entry, are used to activate the data-display on the screen.
Exactly what will be shown is controlled in the “Grib Settings” dialog, which is reached by pressing the “Settings” button.
Don’t activate to many display features at the same time. The display quickly becomes very cluttered.

The “GRIB Display Control” loaded with a grib file only containing current data.

Advanced GRIB Data at Cursor. The display options for all the data shown, can be reached by right.clicking on the little squares where the “ticks” are shown. For the meaning of all these options, read on.

Right-clicking the “Air Temp” option square.

The plugin has an advanced mode were altitude data can be displayed. This mode requires some more knowledge than plain gribs.
A good start is to read "Mariner’s Guide to the 500—​Millibar Chart".
If a grib file includes altitude data, the control will automatically include more options. See below.

If the dialog looks normal except for an extra selection box under “GRIB Data at Cursor”, like this,

then the grib file contains altitude data. Selecting one of the altitudes, for example 500, changes the dialog to what you see above.
The advanced data are displayed against a yellow background.
Wind. These are the wind data valid for the selected altitude level.
Grib Data at 500 hPa. The values at the selected level.

…controls how the grib files are displayed on the screen, and units used in the Grib Display control. Things like overlay colors, animation time interval and speed and much more. The basic dialog looks like this.

Typical DATA TAB

Note the Fixed or Minimum Spacing (pixels) selection. This is a typical setting.

Typical GUI TAB

Typical PLAYBACK TAB

The Loop Mode controls what happens when “Play” is pressed in the Grib Display Control.

Loop Mode. Sets the “Play” function into a loop mode. Otherwise, playback will stop when reaching the end of the file.
Loop Start. Were to start the loop. The option “Current time forecast” makes sure that only data now or in the future are shown
Updates per Second. Controls the speed of “play”.
Loop Start. The loop can start either at the start of the grib file or from the current time forecast.
Interpolate between gribs. If you select this option, you will be able to choose your own time interval, but you have to consider that it can decrease data accuracy. To remind you, this info will pop up.

image::33gribinterpolation.png[image,width=477,height=165]
Time Interval. This is the time interval used for interpolation. The entry is only visible if “Interpolate between gribs” is ticked. This is connected tho the chosen time interval when requesting a grib file.

The lower part of the Grib Settings Dialog, under the Display heading is really 10 different controls. The controls are activated by the choosing one of the items in the selection menu below.

Many of the entries are repeated, so let's first have a closer look at the the most important one, the Wind. Entries on the same line are connected.

Units refers to the choice of units for the selected record.
For wind speed the choices are.

Barbed Arrows Display the traditional barbed arrows, showing wind direction and wind speed.
Range refers to the distance between displayed arrows (not working 3.3.1117).
Default Color/Controlled Colors Default is dark brown while “Controlled” varies from blue for very light winds to red for very strong winds.

image::4gribcontroledcolours.png[image,width=300,height=187]
Always visible This option only exists for “Wind” and “Pressure”. When ticked the object is shown regardless of what is ticked in the “Settings Control”. The idea is to be able to Show wind and/or pressure at the same time as other options. Use with Rainfall, for example, to help identifying fronts.
Display Isotachs shows contour lines connecting points with the same wind speed.
Spacing controls which wind speed Isotachs will be shown. Use knots for units and 10 for spacing and isotach contours will be shown at 10 kts, 20kts and so on.
OverlayMap Depending on your system capability,if you use “Accelerated Graphics (OpenGL)” (see General Options/Display/Advanced), the overlay will be visible at all scales. But if you don’t, the overlay will only be visible at small scales. A small yellow pop up at the bottom left of the screen will inform you. Zoom out to a scale of 1:13 millions, or smaller to see them.
Colors is another selection menu containing pre-set color schemes. The idea is to use the scheme with the same name as the control, but this is just a hint, nothing more.

Numbers shows the wind speed in small square boxes. Minimum Spacing refers to the spacing between these boxes.
Particle Map is a kind of animation of the wind field. It is created by letting “particles”, or dots, move in the wind direction and leave a trail. The trail is colored after the wind strength, and each particle has a limited “life span”.
The Density slider creates particle trails that are shorter but closer together with increased value, which is the same as further to the right. The particle Map below clearly shows a convergence zone with stronger winds (red). This is probably a cold front.
Performance Warning: Note that high density Particle Maps on Low Powered Machines and Large Files may slow your machine to a crawl. First test on small files.

Overlay Transparency works as expected.

All the controls for Wind Gust, Rainfall and Cloud Cover, have similar entries; only the units differ.

The pressure display has no overlays, only the options of isobars and numbers. Overlay Transparency is redundant.

Waves and Current have a more or less similar display. A Particle Map is available for currents. See the descripion above for wind Particle Maps. Once again different units.
There is a choice between three different display arrows shown in two sizes.
If wave height, direction and wave period is present in the grib file it will be displayed in the Display Control like this:

Air Temperature and Sea Temperature looks the same.

Convective Available Potential Energy (CAPE) has these settings

Altitude and Relative Humidity have a simple one choice entry.

The same value for COAMPS is 3 days and RTOFS 6 days.

There are two layouts of this dialog, depending on whether it is a request to SaiDocs or to ZyGrib. Some alternatives are only available from SailDocs, and some are only available from zyGrib. Alternatives not available are grayed out.

If after hitting “Send”, the “Mail” window shows this message, instead of the “send” request:

Configuration of the user’s default Mail Server for the particular Operating System is required.

There is an advanced alternative for Mac OSX, Linux (and BSD). Install and configure a mail server. Instructions are on the Grib Weather Mail Servers Page. Following these instructions to enable the mail server will enable automatic transmission of a grib request to SailDocs.

After installing and configuring a mail server, there is one more step. Open “opencpn.conf” and add the line below. As always, do not run OpenCPN when editing opencpn.conf.

Setting “SendMailMethod” to “0” restores the default.

When ZyGrib is selected as the download server, there are User and Code fields that can be completed. This requires subscription or registration to the ZyGrib Forum.

ZyGrib allows registration to the zyGrib forum and permits use of the user/password to obtain a two month code for registered zyGrib downloads. Limitations: Maximum 16 emails per user and per 24 hours. Maximum size of 4 MB per grib file.

Combine separate Saildocs GFS Wind + RTOFS Current grib files. (Note: Only applies to OpenCPN v4.2.x and forward)
Using Windows, Click on the “Open a File” to browse grib file.

There are many aspects to the use of grib files which lead to the use of other companion programs or plugins.

These are very good tools to use in routing decisions, but they all have the same limitations as all grib-data.

(These notes are from the SailDocs Grib Server)

This grib file is extracted from a computer forecast model. While such computer data can provide useful guidance for general wind flow, there are limitations which must be understood.

What you are receiving is a weather prediction generated by a computer run by NOAA/NCEP (GFS, WW3 models) or the US Navy (comaps, nogaps) and downloaded and processed by Saildocs (a service of Sailmail).

The network is complex, and any computer network is subject to hardware and software failures or human error which can effect accuracy or availability of data.

Grib data can provide useful guidance not available elsewhere. Understand the limitations and use the data carefully. Grib data should be considered supplemental to other forecasts, and not be relied upon in lieu of professionally-generated charts or forecasts.