Re: [ferret_users] Vector plot: force maximum?

[Steve Cousins <cousins@xxxxxxxxxxxxxx>]

Hi Mick,

Great! I think this will do most of what I want. I'm just a little unclear 
about the use of the /length option. I had been using it to scale the 
vectors but I'm not sure it is scaling the red ones down to the threshold 
the way I am using it. I'd use it the way you are (just /length as opposed 
to /length=.15) except when I do that it makes all of the vectors too 
small.

If I do:

	vector/nolab/length=.2 ulo,vlo
	vector/over/nolab/length uhi,vhi

should that scale the hi's back correctly? Is that the same as having 
them both be length=.2 :

	vector/nolab/length=.2 ulo,vlo
	vector/over/nolab/length=.2 uhi,vhi


Thanks very much.

Steve
______________________________________________________________________
 Steve Cousins, Ocean Modeling Group    Email: cousins@xxxxxxxxxxxxxx
 Marine Sciences, 452 Aubert Hall       http://rocky.umeoce.maine.edu
 Univ. of Maine, Orono, ME 04469        Phone: (207) 581-4302

On Tue, 5 May 2009, mick spillane wrote:

> Hi Steve,
>    Perhaps a better, or more easily explainable in a caption, version
> of the figure would be to do an overlay plot with two colors (or
> thicknesses) of arrow.  The first set of arrows (say black ones)  would
> be those associated with speeds below the threshold (8cm/s), the second
> set (say in red) would be those with speeds above the threshold and
> forced to have the length associated with the threshold value but the
> true direction.
>
> let spd=(u^2+v^2)^0.5
> let threshold=0.08
> ! define components of "low" speed cases to exclude speeds above threshold
> let ulo=if(spd le threshold)then u
> let vlo=if(spd le threshold)then v
> ! rescale components of "high" speed cases to have fixed length (and
> exclude low speed ones)
> let uhi=if(spd gt threshold)then u*threshold/spd
> let vhi=if(spd gt threshold)then v*threshold/spd
>
> ! then ...
> vector/nolab ulo,vlo
> vector/over/nolab/length uhi,vhi
>
> NOTE: There is one wrinkle I forgot in the first test -- you need to
> force the second (overlay) vector plot to share the same scale as the
> first. That is achieved by the "/length" option which draws the new
> vector field with the same scale length as the previous one.
>
> Attached is a graphic of a quick demo using random numbers for u,v.  The
> upper panel shows the vectors without a threshold; the lower illustrates
> what it would look like with red arrows where speed exceeds the threshold.
>
> Mick
> -------
>
> let u=0.08*randn(i+j) ; let v=0.08*randu(i+j)
> set view upper
> vector/nolab/i=1:10/j=1:8 u,v
>
> let spd=(u^2+v^2)^0.5
> let threshold=0.08
> ! define components of "low" speed cases to exclude speeds above threshold
> let ulo=if(spd le threshold)then u
> let vlo=if(spd le threshold)then v
> ! rescale components of "high" speed cases to have fixed length (and
> exclude low speed ones)
> let uhi=if(spd gt threshold)then u*threshold/spd
> let vhi=if(spd gt threshold)then v*threshold/spd
>
> set view lower
> vector/nolab/i=1:10/j=1:8/length ulo,vlo
> vector/over/nolab/i=1:10/j=1:8/length uhi,vhi
> frame/file=forsteve.gif
>
>
> Steve Cousins wrote:
> > > Long time!
> > >
> > > I have a dataset with U and V velocity components. There is a river
> > > that comes into the domain that has high values that I want to limit.
> > > What I have done so far is:
> > >
> > > let wind_mask = IF ABS( (u^2 + v^2)^.5) LT .08 THEN 1
> > > let u2 = u * wind_mask
> > > let v2 = v * wind_mask
> > >
> > > vector/xskip=2/yskip=2 u2,v2
> > >
> > > This works but it puts holes in the data. What we'd like is to have it
> > > so if a speed is greater than 8 cm/sec we scale it down to 8 cm/sec so
> > > the grid doesn't have holes in it and we can still see the direction.
> > >
> > > Thinking about this as I type (don't try this at home!) I think I need
> > > something like:
> > >
> > > let wind_mask = IF ABS( (u^2 + v^2)^.5) LT .08 THEN 1 ELSE
> > >                   (.08 /  ABS( (u^2 + v^2)^.5))
> > >
> > > ... testing ...
> > >
> > > It works! Ferret is Great! Hold on. I think it is working. It fills in
> > > the gaps. It *looks* like the scaling is correct...
> > >
> > > Is there a better way to do this?
> > >
> > > Thanks,
> > >
> > > Steve
> > > ______________________________________________________________________
> > >  Steve Cousins, Ocean Modeling Group    Email: cousins@xxxxxxxxxxxxxx
> > >  Marine Sciences, 452 Aubert Hall       http://rocky.umeoce.maine.edu
> > >  Univ. of Maine, Orono, ME 04469        Phone: (207) 581-4302