Winds Tab

From VRTool wiki
Revision as of 23:39, 6 October 2014 by Mclean (talk | contribs) (In Practice)
Jump to: navigation, search

TabWinds.gif The following control panels are on the Winds tab:






Wind Grids





  • Keep the overwrite previous wind grid box ticked.
  • Do all your downloads from a single source.
  • Avoid overlapping grids.

Polars Database


Regatta Settings


Wind Interpolation

What is Interpolation

A grib file provides information for a particular time, and for discrete geographical positions at 1° intervals of latitude and longitude. In the real world the wind changes continuously with time, and also as you move between the grib ordinates. "Wind interpolation" is the method used to simulate this continuously varying wind field between grib ordinates.

In VRTool the interpolation method for both time and position is set on this control panel. You set these to give you the most realistic simulation of the weather or, in the case of a game, to match the interpolation method used by the game.



Interpolation methods


For games the situation is more complicated. In the old days games used grib weather with no interpolation at all, so the wind changed abruptly as you crossed the 1° lines of latitude and longitude. These were the old weather "grids". Most games now use the more realistic wind interpolation for position only. Some also use interpolation for time - an example is the SailOnLine game which appears to use time interpolation around the weather changes so that the wind changes gradually when the new weather is introduced.

U/V Interpolation





  • Speed: Calculate the wind speeds at the grib ordinates then interpolate the speeds for the intermediate position.



In Practice

Once you have some weather loaded, you can easily see interpolation working by loading selecting the Ruler ToolbarRuler.gif tool and moving it over the chart. The wind indicated by the tool will match the grib values displayed at the grib ordinates, but vary gradually as you move the tool between the ordinates.


  1. First make sure you are looking at the current weather in both VRTool and in the game console. Now compare the wind speed and direction at the grib ordinates. To do this in the game console you need to zoom in and place the cursor precisely on the grib ordinate. If they match closely then you have verified that VRTool is using the correct grib.
  2. Now check the speed and direction at a selected intermediate point away from the grib ordinates. If these don't match then one of the wind interpolation settings is wrong. First suspect is the U/V Interpolation for TWD setting.

A simple consequence of interpolated wind is that if you set your boat to sail with a constant true wind angle (TWA) it will sail along a curved track, due to the constantly changing wind direction.

Some Examples


Real World modelling at sea:

  • Check the [x] Interpolate winds checkbox
  • Select interpolation in time=linear and position=bicubic


Most current games, such as Virtual Regatta games:

  • Check the [x] Interpolate winds checkbox
  • Select interpolation in time=previous and position=bilinear


Old style games using wind "grids":
