Re: Plotting constant data

On Thu, 21 Jun 2007, James Fishbaugh wrote:

> Hello,
>
> I would like to be able to display a contour plot of constant data in
> the appropriate color. However, regardless of what the constant is
> (zero in this case), the plot will always be white whereas the value
> of zero should be a light purple. Is there anyway I can get it to
> plot a solid color across the domain? Thanks.
>
> James

Hi James,

I'm afraid there's is no way to set resources and have them apply to
a constant-field contour plot. It's at the very low level of NCL where
it handles this thing, and it basically considers a constant field to
not be a contour plot.

Dave Brown said a possible workaround might be to perturb the field
with a single value that is slightly different from the constant
value, but arranging the contour levels such that this value falls
within the same level space. We're not sure if this would work though.

Another (kludgy) work-around might be to just fill in the plot area
yourself with the color you want, using gsn_polyline or
gsn_add_polyline.

Here's a simple example:

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"

begin
   ny = 20
   nx = 20

   z = new ( (/ny,nx/), "float" )
   z = 0.0

   wks = gsn_open_wks("x11","constant")

   res = True
   res_at_cnFillOn = True

   if(min(z).eq.max(z)) then
     res_at_gsnDraw = False ; Don't draw plot or
     res_at_gsnFrame = False ; advance frame yet.
     res_at_lbLabelBarOn = False ; Don't draw labelbar.

     plot = gsn_csm_contour(wks,z,res)

; Retrieve axes limits
     getvalues plot
       "trXMinF" : xmin
       "trXMaxF" : xmax
       "trYMinF" : ymin
       "trYMaxF" : ymax
     end getvalues

; Use axes limits to add a filled box around plot area.
     gsres = True
     gsres_at_gsFillColor = "LightBlue" ; Use whatever color is normally
                                        ; associated with the constant value
     dum = gsn_add_polygon(wks,plot,(/xmin,xmax,xmax,xmin,xmin/), \
                                    (/ymin,ymin,ymax,ymax,ymin/),gsres)

     draw(plot) ; Now draw plot and
     frame(wks) ; advance frame.
   else

; Not a constant field.
     plot = gsn_csm_contour(wks,z,res)
   end if
end

--Mary

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Received on Fri Jun 22 2007 - 14:03:33 MDT