Winds Tab

The following control panels are on the Winds tab:

Winds

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 , Route , and Track 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

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

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

Interpolation methods

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° 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

This has nothing to do with what sun screen you should be using....!

Contrary to what you might expect, a grib file doesn't contain the wind speed and direction. Instead the wind is presented as north/south and east/west components, called the u and v component vectors. The speed and direction scalars must then be calculated by combining these vectors. While this may seem an odd way to do it, presenting the wind as u/v component vectors is more efficient for a lot of other things that people do with grib files.

The problem is that when interpolation is applied, the answer depends on whether it's applied to the u/v component vectors or to the resulting speed and direction scalars. In other words there are two ways to interpolate and the results are different, sometimes very different!

Meteorologists generally adopt the following methods because they give a good approximation of real wind fields:

• Speed: Calculate the wind speeds at the grib ordinates then interpolate the speeds for the intermediate position.
• Direction: Interpolate the u and v components for the intermediate position then combine to get the direction. This is called u/v interpolation.

Currently (as of 2014) the Virtual Regatta games use these same methods to apply wind interpolation.

You can set VRTool to use u/v interpolation for wind direction by checking the Use U/V Interpolation for TWD checkbox.

U/V interpolation of wind direction has a pronounced effect when you are close to a coastline and one or more of the surrounding grib ordinates is on land. In the open sea it makes very little difference.

In Practice

Once you have some weather loaded, you can easily see interpolation working by selecting the Ruler , Route , or Track tool and moving it over the chart. When the tool is at a grib ordinate the wind indicated by the tool will match the value displayed on the chart; as you move the tool away from a grib ordinate the wind displayed by the tool will change gradually to an interpolated value.

For games, you can check that your wind interpolation settings are correct:

1. Make sure you are looking at the current weather in both VRTool and in the game console.
2. Compare the wind speed and direction at the grib ordinates. To do this in the game console you need to zoom right in and place the cursor precisely on the grib ordinate. If they match closely then you have verified that VRTool has the correct grib.
3. Now compare 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. If the speed matches closely but not the direction then change 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

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

Real World modelling at sea:

• Check the Interpolate winds checkbox
  
• Set interpolation in time to linear
  
• Set interpolation in position to bicubic
  
• Check the 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 Interpolate winds checkbox
  
• Set interpolation in time to previous (meaning there is no temporal interpolation)
  
• Set interpolation in position to bilinear
  
• Check the Use U/V interpolation for TWD checkbox
  

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)