Maxsea Grib Compatibility

This note is for Maxsea users, regarding grib-display problems with certain versions of Maxsea.

Background: Grib (GRIdded-Binary) files are packed-binary data files which contain weather-forecast data. The grib format is an international standard which is supported by a variety of systems and software. Data in a grib file is stored as a “grid” or array of numbers in a packed-binary format. The grib format allows data to be organized either by increasing latitude with a positive latitude-increment (delta-lat parameter), or by decreasing latitude with a negative delta-lat. NOAA’s GFS model, Saildocs and Raytech use the positive convention; Maxsea and OCENS/GlobalMarineNet use the negative convention. Any grib viewer must support both conventions, and most do.

However there is a problem with certain versions of Maxsea which fail to correctly display positive-convention grib-files for southern latitudes. The data image gets inverted north-to-south (not east-west). The effected releases include Maxsea version-10 PRIOR to 10.3.5 (which was the final Ver-10 release). Version 10.3.5 is OK, and Ver-9 and -11 also appear to be OK. Others are untested, if you have an earlier/later version please check it (see below) and send me a note if you see the problem.

To temporarily accommodate users of Maxsea versions with this problem, Saildocs can also supply negative-convention grib files. In the grib-request message change the delta-lat parameter to a negative number, =
for example:
send grib:30S,40S,168E,180E|-2,2|24,48,72
In this example the “-2” specifies a negative-convention file which will avoid the Maxsea problem. (Complete details on the request-format is available be sending a blank email to:

The simplest way to check whether your Maxsea program has this display problem is to request two small grib-files from Saildocs for the southern hemisphere, one normal file and one negative-convention. Use =
Airmail’s grib-request window and then modify the request message, or simply send the following two lines to:

send grib:30S,40S,168E,180E|2,2|24,48,72
send grib:30S,40S,168E,180E|-2,2|24,48,72

This will request two files, identical except for positive/negative data convention. The grib-viewer must display them identically. When you get these files, open each with Maxsea and check the weather over North Island, New Zealand. If the two files display differently then your Maxsea version is not correctly displaying the positive-convention grib file. A short-term fix is to edit each grib-request before sending to specify a negative latitude increment following the example above. Feel free to contact me with any questions at

The longer-term fix is to update your Maxsea version. Furuno now handles distribution and support for Maxsea. They have acknowledged the problem and have made ver 10.3.5 update discs available without charge for users of prior ver-10 releases which are effected. This update only applies to ver-10 users prior to 10.3.5 and won’t work without a version-10 dongle. Contact Furuno for a copy of the 10.3.5 CD.

Jim Corenman