[ncl-talk] Making zero value white in contour plot

[Dennis Shea]

A 'trick':  If variable 'x' is being plotted:

(a) If 'x' has an _FillValue associated with it

     x = where(x.eq.0, x at _FillValue, x)



(b) if 'x' does not have a _FillValue, just assign one. EG

    if (.not.isatt(x,"_FillValue")), then
        x at _FillValue = getVarFillValue(x)
    end if

  x = where(x.eq.0, x at _FillValue, x)

====
  res at cnMissingValFillColor   = "white"    ; maybe "background"

     plot =

On Wed, Oct 5, 2016 at 3:30 PM, Mary Haley <haley at ucar.edu> wrote:

> Jake,
>
> In NCL, the colors represent the values *between* contour levels and not
> exactly equal to a contour level. It's not possible to tell NCL that you
> want all values equal to 0 to be white, because you need a range of values
> in order to actually have an area that you can fill in a certain color.
>
> Your white value is representing all values >= -5 and < 5.  If you want
> this to more closely represent values that are equal to zero, then you need
> to provide a smaller range of values, like -0.1 to 0.1. It will depend on
> how close your values are to zero.
>
> The other possibility is to portray the 0 contour level a different way.
> For example, if you look at example coneff_1.ncl at:
>
> http://www.ncl.ucar.edu/Applications/coneff.shtml#ex1
>
> it uses a special resource "gsnContourZeroLineThicknessF" to make the zero
> contour line double thick.  Of course, in order for this to work, you
> actually have to have a 0 contour line. For example:
>
> res at cnLevelSpacingF  = 5
>
> res at cnMinLevelValF = -75
>
> res at cnMaxLevelValF = 75
>
> You may also want to visit the "symmetric" contours page at:
>
> http://www.ncl.ucar.edu/Applications/contoursym.shtml
>
> --Mary
>
>
> On Tue, Oct 4, 2016 at 7:57 PM, Jake Huff <jake.huff at stonybrook.edu>
> wrote:
>
>> Hello Everyone,
>>
>> I am currently using gsn_csm_contour_map along with cnFillColors to
>> specify which colors I want on the label bar.
>>
>> For example I am specifiying the minimum, maximum and spacing of values
>> for the contour levels.
>>
>> wks = gsn_open_wks("pdf","example")
>> gsn_define_colormap(wks,"BlWhRe")
>>
>> res = True
>>
>> res at cnLevelSpacingF  = 10
>>
>> res at cnMinLevelValF = -75
>>
>> res at cnMaxLevelValF = 75
>>
>> res at cnFillColors = (/2,9,13,18,22,28,34,40,0,55,62,70,77,84,90,95,102/)
>>
>>
>> So this makes the contour level from -5 to 5 be white by using value 0 in
>> the cnFillColors. The problem I have is that this is misleading because it
>> makes a lot of values white instead of just the zero value.
>>
>>
>> My question is how to make a contour plot with a color bar that has the
>> zero value be white and all other values be non-white.
>>
>>
>> Thanks,
>>
>> Jake
>>
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