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Board » Technical Support » qtVlm thread

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Looks like it's working. The link was missing the .exe which I've updated below.

Thanks

https://www.virtual-winds.org/maitai/qtvlm_setup_5.8.6-beta4.exe

In past years we saw a quick evolution not only on weather routing software but also on sidekicks aiding tools.
One of the best weather routing programs used around the corner is qTVlm. Since the very first versions it has been upgraded in a good rhythm bringing also a plethora of innovations, sometimes using info and suggestions gathered from the community.

Anyhow, one qT aspect that has been roller coasting is the Optimizing/Simplifying algorithm processes applied to the calculated routing WPTs.

It is very simple to verify that in spite of using the “Optimum Simplify” process first, and "Optimize" after (applied only once or several times), normally we won’t get the best ETA - even before any trimming operation on the fixed calculated WPTs, the ones in “red” color generated around land/rocks/barriers and on the remaining "green" movable ones.

The cause for this is identified. Is a direct consequence of the used decimating algorithm implemented in qT for the “Simplify”/ “Optimum” processes.

In recent versions - 2018 - this issue was aggravated as the decimating operation during the “Simplify” process stopped being uniformly applied along the routed path, leaving us to the somehow paradox of having more remaining WPTs in the end of the route, precisely where the predictability of the WFs are worse, which is a contra sense.

There are ways to overcome this issue but, again with another contra sense, as you have to spend more time rehabilitating an over decimated routed path by adding new WPTs.

Every WPT over decimated is a less one degree of freedom in the generated curve, giving here the main reason for qT going in opposite direction of the optimum ETA.

Sincerely hope that qT responsible persons take once for all this issue and, at least bring back the “old” decimating algorithm, the uniform “cutter”.

João/psail
Sail Fair.
Joao, you should also mention that you are using settings that I strongly say you should NOT use...

Running a routing with 5mn isochrons can only lead to disappointment in most cases. If I allow for 1sec isos would you use it? 1ms even better? Even less?

Using such a small amount of time between isos creates way too many points that are likely to look useless to the simp process, which can only do what it is aimed to do: simplifying (i.e. remove the point if the ETA is not worst). Since there is not enough room between points it will end up by over simplifying.

Conclusion is that there is a good reason behind the default values, what else can I say? Modify them at your own risk, but do not complain if you don't like the result.

I could add a few things:
1) Routing with 5mn interval on a 3 days travel is just ridiculous knowing that the real wind is different from the grib's wind and that anyhow next grib update will just completely rebate the cards.
2) qtVlm is not specifically designed for SOL and will never be. qtVlm is designed for real sailors or racers and does not try to give an exact answer to an infinite problem. It is just a toolbox that can help to decide where to go.
3) qtVlm is also used in official scientific projects and is also used by very famous racing teams over the world. No one is using 5mn isochrons, there must be a reason.
4) We are opened to suggestions and improvements and even complains. But it need to be reasonably compatible with the real world, and concerning bugs it needs to be reproducible with reasonable settings.

Happy Xmas to all
Philippe.

--- Last Edited by maitai at 2018-12-25 14:16:38 ---

--- Last Edited by maitai at 2018-12-25 14:17:03 ---
Hi Philippe and all.
Good afternoon.

Well, the settings I’ve been using a long time (since they were available) are the same, so I don’t have any induced distortion on comparing results generated with different settings.

Those settings have proved to give results in all kind of WFs and with different boat Polars, being also more relevant with the faster boats, as you easily understand and as per example I sent you recently.

Correct, the lower the Time Step (TS) used for calculating the Isochrones, the bigger will be number of potential WPTs. That’s the price for getting precision.

The first point you mention misses rationality.
We all know that ANY GRIB file won’t be exactly what you are going to sail IRL. Under that argument - yours - I ask why do we make routings?

I totally agree with your second point and part of the third. For that I can give you one plausible answer for people not using TS = 5 minutes: I takes a lot of time to get a result, not because is less precise.

In relation to fourth point, previously I gave you a suggestion: bring back the old "cutter", which I now reiterate.

If it doesn’t have anything to do with the used TS, what’s the problem for implementing it again?

My Dear Friend have also
a Merry Xmas

João/psail
Sail Fair.

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