[ncl-talk] best way to overlay two color filled contour maps?

[Mary Haley]

Xiaoming,

If you need to use transparency, or more than two colormaps per plot, then
you can't to use the workstation color map for setting the contour colors
(i.e. the gsn_define_colormap procedure).

Instead, use the cnFillPalette resource.  This powerful resource allows you
to directly associate a predefined color map (like "BlAqGrYeOrReVi200") or
an RGB/RGBA directly with a contour plot.

So, for the case where you want to reverse the "gsdtol" color map, you can
do this:

  cmap = read_colormap_file("gsdtol")
  res at cnFillPalette = cmap(::-1,:)      ; reverse the color map

But, there's already a reversed version of "gsdtol" called "gsltod", so you
could also use that directly:

res at cnFillPalette = "gsltod"

Then, for the overlaid contours, set:

res2 at cnFillPalette = "BlAqGrYeOrReVi200"

Remove all calls to "gsn_define_colormap" and "gsn_reverse_colormap".  They
are no longer needed.

There are some other things you may want to clean up in your plot:

[1] Don't use WRF_map_c. This is not an advertised function and was
deprecated a long time ago.  Instead, use:

  wrf_map_resources(fmap, res)

[2] You are setting a bunch of resource inside a do loop. This can cause
your script to run slowly, if you have a do loop with a lot of increments,
because it's needlessly executing the same code over and over.  I recommend
moving all code that doesn't need to be inside the loop, to outside the
loop.

I tried to fix your code to illustrate what you mean, but since I can't run
it, I can't guarantee that it is bug free. See the attached script.


--Mary



On Wed, Aug 31, 2016 at 10:10 PM, xiaoming Hu <yuanfangcan at hotmail.com>
wrote:

> I actually looked at those pages.
> See what I got
>
>
> using my script of http://www.caps.ou.edu/micronet/YangYi/WRF_QTT/V3.7.
> 1/wrfERA2d_Sichuan_forQTT_UCM_YSU.2014070912/plot_totalRain_
> daily_spatial.ncl
>
> Apparently it is not as nice as
>
>
> There are a few issues,
> 1. I need the two contour maps using different colormaps, one is gsdtol,
> the other is BlAqGrYeOrReVi200
> 2. In the exemplary figure, the overlay is done not through Opacity
> setting, but setting the 1st color transparent.
> If I set gsn_define_colormap(wks,"BlAqGrYeOrReVi200") , how can I make
> the fist used color transparent?
>
> Thanks
> Xiaoming
>
> ------------------------------
> CC: ncl-talk at ucar.edu; xhu at ou.edu
> From: shea at ucar.edu
> Subject: Re: [ncl-talk] best way to overlay two color filled contour maps?
> Date: Wed, 31 Aug 2016 17:11:20 -0600
> To: yuanfangcan at hotmail.com
>
>
> Did you look at the overlay page?
>
> Applications/overlay.shtml
> Applications/rgbacolor.shtml
>
> Sent from my iPhone
>
> On Aug 31, 2016, at 12:36 PM, xiaoming Hu <yuanfangcan at hotmail.com> wrote:
>
> What is the best way to overlay two color-filled contour maps (like
> below) in NCl?
>
> <overlay.png>
>
>
> Thanks
> Xiaoming
>
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