Winds Tab

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Revision as of 20:16, 6 October 2014 by Mclean (talk | contribs) (Wind Interpolation)
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TabWinds.gif The following control panels are on the Winds tab:


The controls on this panel manage the appearance of the winds on the chart. There is nothing here that affects routing or track calculations, so you can safely experiment as you like to find out what each control does. The example settings shown here will give you a simple display of wind speed and direction with minimum clutter.

If you set the Wind arrow in scale control to [1] the length of each wind arrow will be the distance you will travel in one hour.

The two Wind filters options allow you to show or hide wind data according to how strong the wind is. If you set the show tws >= to say [5] then all the winds less then 5 knots will be blanked out on the chart. This is useful for highlighting holes in the wind that you need to avoid.

The wind values shown on the chart are those taken from the Grib files at the 1° Lat and Long ordinates. Remember that if you have Wind Interpolation turned on the wind speed and direction will vary between these 1° points. You can see this as you move the Ruler ToolbarRuler.gif, Route ToolbarRoute.gif, and Track ToolbarTracks.gif tools over the chart.

Wind Grids

This control panel is where the weather used by VRTool is managed. There is a lot going on in this panel!

When you download weather from any of the sources, the downloaded weather is contained in a weather "grid" and is added to list of grids in the window. The grids are automatically named - the name usually (but not always) identifies the area covered, a time-stamp and where it came from.

If you tick the overwrite previous wind grid box, a subsequent download for the same area and from the same source and will overwrite an existing grid. This means that when you repeat a download for a later forecast period an existing grid will be overwritten. On the other hand, if you alter the area boundaries of a download there will be no matching grid to overwrite.

This can get quite confusing and you can easily end up with active grids that overlap. If you find that the wind on the chart is different to the game the most likely cause is that you have multiple grids active for the location. As a general rule you should:

  • 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

The wind information in a Grib file is for a particular time, and for discrete geographical positions at 1 degree intervals of latitude and longitude. In the real world the wind changes continuously with time and as you move between the Grib ordinates. VRTool provides various ways to simulate this continuous change - this is called "wind interpolation".

These are some of the methods of wind interpolation you can use:

Interpolation methods

The interpolation method for both time and position is set on this control panel. You should 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.

In the real world the weather changes constantly in both time and position, so for use on board at sea you will get the most accurate simulation by interpolating in both time and position.

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 degree 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.

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

A simple consequence of interpolated winds 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.

To summarise, here are some typical examples of wind interpolation settings:

Real World modelling at sea:

  • Check the [x] Interpolate winds checkbox
  • Select interpolation in time=linear and position=bicubic
  • Check the [x] Use U/V interpolation for TWD checkbox

The simulated wind will vary constantly with both position and time (as it does in nature).

Most current games, such as Virtual Regatta games:

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

The simulated wind changes as you change position, but at any one position it remains fixed for the duration of the forecast period.

Old style games using wind "grids":

  • Uncheck the [ ] Interpolate winds checkbox (no interpolation needed)