The Tidal Routing Plugin is designed to compute DR and ETA positions along a previously planned outline route. The ETA calculation uses tidal current GRIB files, in conjunction with the Grib Plugin.
You will need to start by exporting the outline planned route as a GPX file to an easily found location on your computer. When the calculation of the tidal route starts you will be asked for the name/location of this file.
For ETA calculations, load the GRIB file containing tidal current data using the grib plugin. Next, open the Tidal Routing plugin from the main toolbar.
For DR calculations the plugin will ignore the Grib plugin and any grib data.
Enter the name for the route When ready, select "Calculate DR" or “Calculate ETA” from the plugin dialogue to compute the route.
If you decide to “Save as GPX File” this will make a file that can then be imported into OpenCPN. The advantage of this will be that the route shown on the chart will have named waypoints. Those made within the plugin are not named.
If the ETA route fails to compute this could be due to the GRIB file not covering the time period for undertaking the planned route.
The GRIB file must cover the whole area of the planned route.
By inspecting the GRIB coverage of the route, there may be legs of the route which do not lie within 4 grib points. In this case the calculation will fail because the tidal current set/rate cannot be estimated. The outline route can be adjusted so that it falls within the grib coverage and then saved as a new GPX file. This problem is most common near the coastline.
No account is taken of the ability to sail this route. Constant speeds in still water are assumed.
Further work will be needed in areas where tidal current gribs are not available.
All bearings/courses are in Degrees True. For ETA calculations the times used from the GRIB will be in the same timezone as the GRIB, often UTC.
http://www.sailingweatheronline.com is a good source for downloading tidal current gribs.
Please contact me with questions, suggestions and feedback.