Tuesday, February 16, 2021

Optimum Weather Routing with qtVlm

This note on qtVlm routing is part of our sequence of articles on the Applications of Weather Data in Grib Format. We have similar demos of routing with other applications.

qtVlm is a free, donation-supported navigation and weather software program for both Mac and PC computers, as well as mobile devices.  It is an internationally popular product with versions in multiple languages. It is the leading program worldwide for virtual tracking and taking part in ocean races presented online.  It is a versatile navigation program as well as weather resource. See also notes in the Starpath Glossary.

We cover charting aspects elsewhere; we have a video playlist on several topics, as well as a cheat sheet on specific functions; here we just take a brief look at the app for routing computations, which is of course tied to its display of grib formatted numerical weather and ocean predictions.  The program includes extensive functionality, which means there are numerous nested menus, some of which are interrelated. In short, there is a learning curve, as is with any such versatile program. So we start by just focusing on what we need to create a basic route, and then we will come back to the numerous ways it offers to fine tune and optimize the results further.

We will list the steps here and then add a video demo of the process.

Step 1. When you download and install the app, be sure to load the "maps," which in this case will be the high-res base maps. 

Step 2. The default display shows a daylight terminator which is very handy when underway or when planning a voyage, but for training and practice if you find it distracting it can be toggled off at menu View/Show-Hide/Night Zones.  

Step 3. Load a Grib forecast (review earlier notes on gribs) that will cover the range of the route you want to compute, and long enough to let the vessel finish with its known polar data.  In this example, we want to do summertime ocean routing using archived wind data from July, 201o that we have stored on the computer. Thus the steps are: menu Grib/Grib Slot 1/open and navigate to the file to load it into slot 1.

Doing these historic runs, you will get a warning that the grib data is old, and consequently it will not show on the screen. Click the clock icon in the middle of the menu bar and set the grib time to match the first forecast loaded. Using live data this will not likely arise, as the newest live data is some 4 or 5 hr old at best.

The second from the right magnifier icon (with a square inside) will center the view on the active grib data.

If you do not see the grib data, but you do see elevation and rivers on the land, then you have an overlay turned on that could be hiding the wind data. Click the menu chart icon "O" (online charts)  to toggle this on and off.

Step 4. Load the polar diagram you wish to use. It can accept files in the .pol format or the .csv format, with the separators being semicolons. See related polar format discussion.  To load the polar use menu Boat/Boat Settings.  Note we are not using menu Boat/Polars. That will be used to study the polar once it is loaded.  Once in Boat/Boat Settings, add a name and or model of your boat then open the Polars tab below it.  Navigate to your polar and import it. We can leave the other settings in default choices.

Step 5. Check the polar. Menu Boat/Polars/Wind polar analysis. Check all TWS (true wind speeds) to see if it looks as expected. Then go back to just one wind, and notice that you can click on the curve to read the data. We are not getting into this now, but if you want to change anything in the polar you can do it in Menu Boat/Polars/Wind polar editor

Step 6. Set start and end points. At the desired start point, right click within the grib area and make a mark. Give it a name and click OK. We can leave all defaults as is.  Do the same with the destination point.

Step 7.  Right click anywhere on the chart and chose Create a routing.

Give the route a name 
 
Turn off Routing from boat
 
Confirm that start and finish are the points you want—from drop downs; top is start, bottom finish.  If the points are not there, cancel, add the marks (POI), and come back. 

Turn on Keep Starting Date and type in the initial time of the grib forecast installed (default is month, day, year). Later we can use other start times. Select the month, type it in, and just keep typing. 

Leave on Isochrone color based on....  Turn on Automatic parameters, and move slider ball to far right (Best accuracy).

Leave on Convert to Route using...  at the bottom should be left on; letter "R" is fine for now. 

Then Press OK to create the routing 

(If anything is not right then when done,  just say OK, then Cancel, and from menu Routings select delete routing, and do it again.)

Step 8. Convert to Route. When done, it presents the ETA, duration, and computation time. Click OK. Then we get the opportunity to convert this routing to a route.  The former is the result of an optimization using isochrones (just completed); it is a sequence of isochrone points. Unless we need to study the displayed isochrone solution in more depth (discussed later),  then the logical next step is convert it to a route, which means converting the isochrone points along the fastest path to a series of POI.

To do this, we first we Simplify the route (series of POI), and we have two ways to do that (Maximum and Optimum).  This process removes excess POI along the route, such as intermediate ones along a straight line. Maximum simplification is the quickest solution, and removes the most waypoints, but it might not leave us with the fastest route to the finish. Optimum simplification takes a more careful look at each point. It takes a bit longer; removes less points; but never loses ETA, and usually improves it. The computation time difference is rarely large, so it is usually best to do an Optimum Simplification. 

Once the route has been simplified in the optimum way, we can still Optimize this route even more. This is an important final step to get the most efficient route using the criteria we selected.  This process goes back over each point with a more sophisticated look at the best way forward. It is an improvement over a pure isochrone solution.

Thus the sequence is: 

1. Create the Routing

2. Convert to a Route

3. Simplify optimum

4. Optimize

Once this is done, the routing will be moved from the routings list (menu Routings) to the routes list (menu Routes) and we can then look at details.

Step 9. In menu Routes/Edit Route  select your route. Logbook shows the conditions at steps along the route. You choose for these to be every so many minutes or after a specified course change. Use the gear icon to select what you want to see—click it, select ones to see, then click the gear again to set them. Histogram is an interesting way to look at plots of various parameters along the route. Statistics summarizes a few parameters of the whole route.   To export the route as a GPX file, use menu Routes/Export route.

Below is a video sample of an optimum route computation.


Routing example with no special conditions [17m:33s].


qtVlm has many options and restrictions that can be placed on the computation. Like most other apps, you can define boundaries to block the route from certain areas or passes, plus qtVlm can route around a course of marks, or through specific gateways, which adds a layer of versatility to the solution.  This is accomplished by optimizing along a pathway.  In qtVlm, a pathway is a series of waypoints, what might be called a route in other programs. Routings, routes, and pathways have distinctions in qtVlm. 

Other routing adjustments you can make include:

• With gust data in the grib, you can reduce the polar efficiency if the gusts are a certain percentage above the wind speed. 

• You can avoid areas with wind greater than some value

• With wave data in the grib (now part of the GFS forecasts) you have several options

    — Avoid areas with wave height greater than a specified value

   — Use a wave polar that reduces or enhances wind polar values depending on the true wind speed, height of waves, and angle of the waves relative boat heading

    — Engage a cross seas correction to the wind polar based upon height of the swell and the angle between the wind waves and the swell.

• You can limit the TWA to keep the routing from heading closer to the wind than you want or more downwind than you want, thus overriding the polar diagram.

• You can adjust the minimum time between successive tacks or jibes as well as specifying a time penalty for each.

• When cruising, you can set the minimum sailing speed before the engine comes on, and then what boat speed should be used when under power.

• You can also chose a reduced polar efficiency when sailing at night.

• Without changing the polar itself, you can temporarily enhance or diminish polar performance by a percentage factor for upwind or downwind sailing.

• If the present observed wind is not matching the forecast winds in the grib being used, you can scale the grib winds to match them.  This factor can be set to gradually fade into the pure grib wind over a fixed period as you anticipate the forecast improving.

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References:  

Users Manual: http://download.meltemus.com/qtvlm/qtVlm_documentation_en.pdf

Forum details: https://wiki.v-l-m.org/index.php/QtVlm_Virtual_Race_Mode

Many videos in YouTube in French.


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