[Met_help] [rt.rap.ucar.edu #56594] History for Definition of axis_ang attribute in MODE

[Randy Bullock via RT]

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  Initial Request
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Hello,

just a quick question: is the axis angle attribute giving the departure
with respect to the mathematical reference 0 deg being E or wrt to 0 deg
being N? I am particularly interested in the orientation differences so
this is critical ...

Thanks
Marion


-- 
Dr Marion Mittermaier     Manager: Model diagnostics and novel
verification 

Met Office   FitzRoy Road   Exeter   EX1 3PB   United Kingdom
Tel: +44 (0)1392 884830   Fax: +44 (0)1392 885681 
E-mail: marion.mittermaier at metoffice.gov.uk  http://www.metoffice.gov.uk


http://www.metoffice.gov.uk/research/people/marion-mittermaier


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  Complete Ticket History
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Subject: Re: [rt.rap.ucar.edu #56594] Definition of axis_ang attribute in MODE
From: Randy Bullock
Time: Thu May 24 11:30:31 2012


Hi Marion -

Regarding your question about MODE's axis angles:

The axis angle is measured with respect to
grid directions, not cardinal (east, north, west, south)
directions.  An angle of zero means the axis is aligned
with the grid "X" direction, so the axis would look horizontal
when plotted in grid coordinates.

If you're using a lambert or polar stereographic grid, then
be aware that the angle between the grid directions and the
cardinal directions varies somewhat over the grid.
But then the cardinal directions will vary across the
grid too (something that most people don't realize ...
I mean that "north" in Munich is not really the same direction
in space as "north" in London).

Finally, also be aware that the axis is unoriented, so the axis
angle takes values in a 180 degree range, rather than (for example)
an azimuth or bearing angle, that varies over a 360 degree range.
For example, we don't distinguish between an axis pointing in the
positive x direction, and one that points in the negative x direction.
Both situations are assigned an axis angle of zero.

Hope this helps.

If you have more questions, just let me know.

Randy Bullock


On Thu, May 24, 2012 at 08:37:53AM -0600, Paul Oldenburg via RT wrote:
>
> Thu May 24 08:37:53 2012: Request 56594 was acted upon.
> Transaction: Given to bullock by pgoldenb
>        Queue: met_help
>      Subject: Definition of axis_ang attribute in MODE
>        Owner: bullock
>   Requestors: marion.mittermaier at metoffice.gov.uk
>       Status: new
>  Ticket <URL:
https://rt.rap.ucar.edu/rt/Ticket/Display.html?id=56594 >
>
>
> This transaction appears to have no content

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Subject: RE: [rt.rap.ucar.edu #56594] Definition of axis_ang attribute in MODE
From: marion.mittermaier at metoffice.gov.uk
Time: Fri May 25 01:58:40 2012

Hello Randy,

Thanks for this explanation. I had hoped you would say that the
deviation was +/- 180 deg from the horizontal axis. In terms of the
angle difference thought, I notice it seems to be always positive. If
I
am interested in whether the forecast objects always have a preferred
orientation wrt the observed objects do you always take the angle
difference as F minus O? Maybe all my angle differences are indeed
positive but I just want to check whether they can be negative. Or is
the only way of knowing through visual inspection of the objects in
the
postscript files?

Marion

--
Dr Marion Mittermaier     Manager: Model diagnostics and novel
verification

Met Office   FitzRoy Road   Exeter   EX1 3PB   United Kingdom
Tel: +44 (0)1392 884830   Fax: +44 (0)1392 885681
E-mail: marion.mittermaier at metoffice.gov.uk
http://www.metoffice.gov.uk


http://www.metoffice.gov.uk/research/people/marion-mittermaier

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Subject: Re: [rt.rap.ucar.edu #56594] Definition of axis_ang attribute in MODE
From: Randy Bullock
Time: Tue May 29 09:41:52 2012


Marion -

I've been tracing through the MODE code and it seems that
the angle should indeed always be positive.  This means that
it's not something simple like F - O, or O - F.  If you think of
two lines intersecting, then (at least if they are not perpendicular)
they will make two different angles, one smaller than 90 degrees,
and one larger than 90 degrees.  The code seems to be set up to return
the smaller angle ... the one <= 90 degrees.
At least, this is what I **think** the code is doing.  It's a little
tricky.

If this calculation isn't what you need for what you're doing,
you could always take the forecast object axis angle along with
the observed object axis angle, and just do a straight-up
difference.

Randy


On Fri, May 25, 2012 at 08:58:32AM +0100, Mittermaier, Marion wrote:
> Hello Randy,
>
> Thanks for this explanation. I had hoped you would say that the
> deviation was +/- 180 deg from the horizontal axis. In terms of the
> angle difference thought, I notice it seems to be always positive.
If I
> am interested in whether the forecast objects always have a
preferred
> orientation wrt the observed objects do you always take the angle
> difference as F minus O? Maybe all my angle differences are indeed
> positive but I just want to check whether they can be negative. Or
is
> the only way of knowing through visual inspection of the objects in
the
> postscript files?
>
> Marion
>
> --
> Dr Marion Mittermaier     Manager: Model diagnostics and novel
> verification
>
> Met Office   FitzRoy Road   Exeter   EX1 3PB   United Kingdom
> Tel: +44 (0)1392 884830   Fax: +44 (0)1392 885681
> E-mail: marion.mittermaier at metoffice.gov.uk
http://www.metoffice.gov.uk
>
>
> http://www.metoffice.gov.uk/research/people/marion-mittermaier

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