I think he's asking about grib display in expedition.
if one has both a current speed and direction grib, and a wind speed and direction grib loaded.., does the TWD/TWS display in expedition (say if you mouse over a spot) give the vector sum of current and wind.., or is it just the wind - what would be called the ground wind?
 

In B&G, True Wind is effectively observed wind + observed current

this results from using HDG and BSP in the wind calculation, rather than COG and SOG, and for many reasons this is the quantity you want to look at for tactical decisions.

But, as you point out, it is not the quantity in a grib forecast, where the wind speed and direction are referred to the fixed earth rather than the moving water.

In Expedition, you can select to display ground wind direction, and ground wind speed (GWD, GWS) in the numbers boxes - As i understand it, this uses observed SOG and COG for the wind calculation, and has the effect of subtractiong current from the True Wind.

and, at the same time, you can also display in adjacent numbers boxes, TWD predicted, and TWS predicted, which are from whatever grib you have loaded - and is the forecast ground wind at your location at the time of observation

so if you are sailing in an area of a lot of current, and you want to compare a grib forecast, with your observations, that is probably the best way to do it

 

Last edited by us7070 (3/28/2023 10:23 am)

Yes, the data in the grib files in relative to the ground.

The data in the tooltips, meteogram, number box, drawn on the chart etc is all relative to ground.

When it is used in routing, it is corrected for currents if available.