Wind direction calibration for Southern Hemisphere

[rod]

Those of us who live in the Southern Hemisphere have to point the solar cells on our Vantage Vue's to the North, in order to catch the sun.  This means that the readout on weewx for the wind direction is in the wrong direction (an error of 180 degrees).  I have weewx connected to a Weather Envoy which does not seem to have an internal calibration function to correct this error.  

Has anyone else living in the Southern Hemisphere solved this problem?    I guess I could use the [corrections] function to put windDir = windDir + 180, but I'm not sure if this will work when winfDir is greater than 180 degrees and the sum is greater than 360 degrees.  Also, I am not sure if I also need to put windGustDir = windGustDir + 180

[Thomas Keffer]

Orient your solar collector north, then reorient your wind vane so it also points north. Instructions on how to do this are in your ISS installation manual. There can be a slight shadowing problem, but this way your console will read correctly.

-tk

On Wed, May 1, 2013 at 9:22 PM, rod <r...@lamont-tucker.com> wrote:

Those of us who live in the Southern Hemisphere have to point the solar cells on our Vantage Vue's to the North, in order to catch the sun.  This means that the readout on weewx for the wind direction is in the wrong direction (an error of 180 degrees).  I have weewx connected to a Weather Envoy which does not seem to have an internal calibration function to correct this error.  

Has anyone else living in the Southern Hemisphere solved this problem?    I guess I could use the [corrections] function to put windDir = windDir + 180, but I'm not sure if this will work when winfDir is greater than 180 degrees and the sum is greater than 360 degrees.  Also, I am not sure if I also need to put windGustDir = windGustDir + 180

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Tom Keffer

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[rod]

Thanks Tom.  Good suggestion, but I have a Vantage Vue station, not an ISS.  The shaft for the wind vane on the Vantage Vue has a flat on it so that the vane will only go on the shaft one way.  I guess I could fix this by putting another flat on the other side of shaft using a file.  But I am the kind of person who would prefer a neat software solution to a brute-force hardware solution.

What will happen if I do the following? 

windDir = windDir + 180

windGustDir = windGustDir + 180

Will there be a problem is the sum comes out to be larger than 360 degrees?

If this does cause a problem, is there a way to do the above corrections for windDir less than 180 and do the following for windDir greater than 180:

windDir = windDir - 180

windGustDir = windGustDir - 180

[Thomas Keffer]

OK, I guess that won't work!

I think adding 180 degrees will work, although it will probably make your wind direction plots look funny. You can get around this problem by using the Python ternary statement

windDir = windDir + 180.0 if windDir < 180 else windDir - 180.0

Same for windGustDir.

-tk

[rod]

Many thanks. That fixed the problem!

It might be an idea to include this statement in the User's guide.

[david murphy]

Tom,

Didn't want to start a new thread so thought this would be a good spot to post my question. Nearly 18 months of using Weewx/Raspi/Davis and all working very well (lost 1 SD card but backups meant I was back on the air in 15 minutes).

I would like to combine two weewx.sdb files, which i can do with sqlite3 dump command. My current file (weewx.sdb) which has 12 months of data (wind data fine) and my original test file (weewx.sdb.test) that has 6 months of data (with a few days missing - but I was still experimenting). I just checked my weewx.sdb.test file and see that my Anemometer at the time must have been facing south and not north..(yes i live in Australia) and thus my wind direction is out by 180 degrees. I do remember correcting the orientation when i permanently mounted my Davis station in the garden.

My question is what sql command & syntax do i use to modify my wewx.sdb.test file to correct this wind direction error before i combine my 2 files. Are there any other wind type fields that also need modifying?

Once again thanks in advance and great work by you and the rest of the weewx development team. 

[Thomas Keffer]

The SQL statements (NOT TESTED)

UPDATE archive SET windDir = windDir + 180 WHERE windDir IS NOT null;

UPDATE archive SET winddir = windDir - 360 WHERE windDir IS NOT null AND windDir >= 360;

should do it. Be sure to do a backup first!

-tk

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[david murphy]

Thanks Tom,

Will give it a whirl in the next few days and report back.

[david murphy]

Tom,

Loaded SQLite Database Browser on the PC and had a look to make sure values for windDir were modified after executing your statements and all seems

fine.

Do I need to run a similar statement on the windGustDir table ?

On Thursday, 11 September 2014 08:29:59 UTC+10, Tom Keffer wrote:

[Thomas Keffer]

Yes, you need to do something similar for windGustDir.

You will also need to delete the stats database and let weewx regenerate it.

-tk

Fat-fingered from my Android

[John D Smith]

Most of us down under just stand on our head to read the consoles ... :-) but weewx can accomodate this in the Calibrate section I am thinking.

[Andrew Milner]

I think that is what Tom meant when he said use:

windDir = windDir + 180.0 if windDir < 180 else windDir - 180.0

- to put the above statement in the calibrate section of weewx.conf

when the solar and weather vane have been aligned North rather than South.

[Thomas Keffer]

Almost. You have to consider the case where windDir=None:

windDir = (windDir+180 if windDir<180 else windDir-180) if windDir is not None else None

-tk

[Deborah Pickett]

Is there any interest in modifying the wee_config_vantage program to expose the console's own wind direction calibration value in the EEPROM?  I've recently been in that code so I'm confident that I could add read/write support for EEPROM location 0x4D-0x4E (as I did years ago for wview).

(Back story: My anemometer is on my roof connected to a retransmitter.  I only wanted to be up on the roof once so I lined it up roughly north-facing, exactly aligned to a prominent feature on the horizon.  When I got down to ground level again I checked Google Earth and measured the bearing to that feature at 349 degrees, a little west of north.  Rather than going back up and trying to rotate the anemometer shaft 11 degrees I programmed the console with an offset of 11 degrees and saved myself another trip up the ladder.  At the time I was moving the datalogger back and forth between my Vantage2 console, on which I could program the offset by pressing buttons, and my Envoy, which needed the value programmed into the EEPROM.  I wanted them both to report the same value without me having to adjust it in software.  Hence why I didn't use wview's calibration feature.)

[Thomas Keffer]

Thanks, Deboarah (aka, "The EEPROM Queen"), for the offer. That would be super! 

-tk

On Mon, Sep 22, 2014 at 4:57 PM, Deborah Pickett <debbiep...@futzle.com> wrote:

Is there any interest in modifying the wee_config_vantage program to expose the console's own wind direction calibration value in the EEPROM?  I've recently been in that code so I'm confident that I could add read/write support for EEPROM location 0x4D-0x4E (as I did years ago for wview).

(Back story: My anemometer is on my roof connected to a retransmitter.  I only wanted to be up on the roof once so I lined it up roughly north-facing, exactly aligned to a prominent feature on the horizon.  When I got down to ground level again I checked Google Earth and measured the bearing to that feature at 349 degrees, a little west of north.  Rather than going back up and trying to rotate the anemometer shaft 11 degrees I programmed the console with an offset of 11 degrees and saved myself another trip up the ladder.  At the time I was moving the datalogger back and forth between my Vantage2 console, on which I could program the offset by pressing buttons, and my Envoy, which needed the value programmed into the EEPROM.  I wanted them both to report the same value without me having to adjust it in software.  Hence why I didn't use wview's calibration feature.)

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