Difference between revisions of "Winds Tab"
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'''Real World''' modelling at sea: | '''Real World''' modelling at sea: | ||
| − | *Check the '''[x]Use Wind cube''' checkbox<br> | + | *Check the '''[x] Use Wind cube''' checkbox<br> |
*Select interpolation in '''time=linear''' and '''position=bicubic'''<br> | *Select interpolation in '''time=linear''' and '''position=bicubic'''<br> | ||
| + | *Check the '''[x] Use U/V interpolation for TWD''' checkbox<br> | ||
The simulated wind will vary constantly with both position and time (as it does in nature). | The simulated wind will vary constantly with both position and time (as it does in nature). | ||
'''Most current games''', such as Virtual Regatta games: | '''Most current games''', such as Virtual Regatta games: | ||
| − | *Check the '''[x]Use Wind cube''' checkbox<br> | + | *Check the '''[x] Use Wind cube''' checkbox<br> |
*Select interpolation in '''time=previous''' and '''position=bilinear'''<br> | *Select interpolation in '''time=previous''' and '''position=bilinear'''<br> | ||
The simulated wind changes as you change position, but at any one position it remains fixed for the duration of the forecast period. | 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": | '''Old style games''' using wind "grids": | ||
| − | *Uncheck the '''[ ]Use Wind cube''' checkbox (no interpolation needed)<br> | + | *Uncheck the '''[ ] Use Wind cube''' checkbox (no interpolation needed)<br> |
Revision as of 19:59, 6 October 2014
The following control panels are on the Winds tab:
Winds
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
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
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 will match the displayed Grib values at the Grib ordinates, but vary gradually as you move the cursor between them.
To summarise, here are some typical examples of wind interpolation settings:
Real World modelling at sea:
- Check the [x] Use Wind cube 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] Use Wind cube 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 [ ] Use Wind cube checkbox (no interpolation needed)
