Math help - rolling average Wind Direction

[Melanie]

Melanie taps her fingers impatiently whilst eyeing up her whip coiled on the wall, then scribbles...

	ValueA var Word
	ValueB var Word
	ValueC var Word
	Result var Word
	If ValueA < ValueB then Swap ValueA,ValueB
	ValueC=ValueA-ValueB
	If ValueC < 180 then
		ValueC=ValueC/2
		Result=ValueB+ValueC
		else
		ValueC=360-ValueC
		ValueC=ValueC/2
		Result=ValueA+ValueC
		endif
	If Result = > 360 then Result=Result-360

There is only one 'anomaly' which could be adjusted whichever way you choose (although there is no true definitive answer). If ValueA and ValueB are exactly 180 degrees apart, you have two potential answers both of which are correct... eg ValueA is Zero and ValueB is 180, you have two answers - one being 90 and the other being 270. My little ditty above always selects the higher - 270 in this case. But what the hell do you expect for 30 seconds work?

[Darrel Taylor]

Or, here's another possibility...

Direction  VAR WORD      ; Holds the Averaged wind direction
Sample     VAR WORD       
AvgCount   CON 3         ; Number of samples to average

; Get New Sample here 0-359

If (Direction > 270) AND (Sample < 90) then 
    Sample = Sample + 360 
else
    if (Direction < 90) AND (Sample > 270) then
        Direction = Direction + 360
    endif
endif

Direction = ((Direction * (AvgCount-1) + Sample) / AvgCount) // 360

Which treats anything in the range of 0-90 as if it's 360-450. But only if the other value is on the other side of the zero point.

Then after averaging, it gets the Modulus of the result, which brings it back into the 0-359 range.

[DavidK]

If it is any help. The boys from Berkeley have an routine for averaging the anemometer direction. It is somewhat specific to their product, but the explanation may be of help. Has links to the math routines.

http://www.emesystems.com/OL2wind.htm#Wind_vane

[Melanie]

ooooh... I don't like bashing other folks work, but that is very amateurish... "Hello Mr Customer. Here's a fine wind direction guage for your airfield - oh, by the way, (at best) it doesn't work 3% of the time..."

[RussMartin]

I had to deal with aspects of this when I was working with weather and water instrumentation back in 1976. At the time, I was with the Wyoming Water Resources Research Institute. We dealt with an enormous volume of meteorological and hydrological data.

A side note for/about the "Boys from Berkeley"--there's no reason for a "dead band" on a weather vane and hasn't been for several decades. (That isn't "bashing", is it, Melanie?)

For clarity, let's agree that a vane tells us direction and an anemometer tells us that air is moving, and how fast--that there is in fact wind!

A wind direction change of exactly 180 degrees in that short a period (3 seconds) is frankly unlikely unless the vane is spinning wildly and sampled at the wrong instant, or just turning lazily, moving almost at random. Most occurrences will be in very light, almost insignificant breezes (hence little or no wind speed), so direction doesn't really obtain.

I would "trap" such a condition by looking at the absolute (i.e., unsigned) value of the difference of every successive pair of readings. If it is 180, it's a null (i.e., useless and/or anomalous); discard the earliest value and use the second against the incoming third, and so forth--truly "rolling" the values through:

If |oldest - next| = 180, then oldest = next and next = newest, then start over; else, average the data and continue. (Melanie can probably write the code in about 15 seconds!)

What is overlooked in this discussion is the fact that wind "direction" means nothing if there is no air movement, i.e., no wind speed. Much more important than a possible but unlikely 180-degree anomaly is if the vane is simply sitting there in absolutely still air. It will continue to report a direction! This would be like your television weather reporter saying, "The wind is coming from the east at zero miles per hour." Huh?

[RussMartin]

For the curious, wind data is often expressed graphically, in the form of a wind rose.

The compass is divided into 8, 16, 32, or sometimes more, equal segments. 16 is the most common.

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[wjsmarine]

Hi Folks,

Thanks to all for comments and suggestions, I'm still trying various ways to come up with one solution for all.

Russ, you make good points with your Direction without Wind speed comments. I also have a long history of Oceanographic and Meteorological experience - but as a hardware guy. My venture into writing code has only been recent by comparison so I have the problem of putting the hardware experience into the Black Art... For the record, Tracey ("Boys from Berkeley") Allen's website has been a mine of useful information for me particularly with the math side of things and would encourage anyone regardless of their qualifications to spend some time reading some of his writings.

What I didn't mention in my opening question was that I am also measuring Wind Gust, which by definition, is the highest successive wind speed in a 3 second window - hence my need to also measure the direction over 3 seconds (my opening question). You can read between the lines here to see I am also deriving Highest wind speed, Average wind speed, Wind Gust direction, Average direction and Direction deviation (Sigma Theta) - so I have a number of things to juggle and think about, all within a continuously sliding 3 second window which is moving every 1 second.

I'm making progress but it isn't pretty code! If anyone has some more to input it will be warmly welcomed otherwise I'll keep banging away to streamline things.

Kind regards to all,
Bill