Re: colormaps/colorbars and zero point

From: Adam Phillips (asphilli AT cgd.ucar.edu)
Date: Thu Nov 03 2005 - 10:35:51 MST

  • Next message: David B.Reusch: "colormaps/colorbars and zero point"

    Hi Dave,

    There is not an automatic way to set the colormap such that the
    transition from blue->red occurs around the zero line for asymetric
    colorbars in ncl. There are three different ways you can do it, by
    setting cnLevelSelectionMode = "ManualLevels", by setting
    cnLevelSelectionMode = "ExplicitLevels", or by using gsnSpreadColors and
    gsnSpreadColorStart and gsnSpreadColorEnd. The latter option requires
    some trial and error, but shouldn't take you long... I have attached a
    script along with the resulting .gif image that shows all three ways to
    do it..

    Good luck,
    Adam

    David B.Reusch wrote:
    > I would like to be able to use asymmetric colorbars but still have the
    > zero point anchored to the switch between, for example, blue and red in
    > the BlueRed colormap. I am contouring (filled) data that has both
    > positive and negative values (e.g., anomalies or zonal wind speeds) and
    > am trying to get positives in one color (red) and negatives in the
    > other (blue) but this does not always work when using an asymmetric
    > contours range. Sometimes the zero value is two or more levels into
    > the blue (or red). Symmetric values work (i.e., the blue/red
    > transition is reliably anchored to zero) but this often results in one
    > side of zero having an awkward number of contours (e.g., the data are
    > in the range -5 to +30, so a symmetric range would be +/-30, which does
    > a bad job for the negative values).
    >
    > I've looked at the online info about cnLevels, cnFillColors and
    > colormaps and could *probably* come up with code (eventually) to do
    > this for specific cases, but was hoping someone could provide a generic
    > approach.
    >
    > I've attached an example figure that shows the problem I'd like to
    > resolve. I can also provide data/code as needed for testing.
    >
    > thanks,
    > dave
    > dbr@geosc.psu.edu
    >
    >
    > ------------------------------------------------------------------------
    >
    >
    > ------------------------------------------------------------------------
    >
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    -- 
--------------------------------------------------------------
Adam Phillips			             asphilli@ucar.edu
National Center for Atmospheric Research   tel: (303) 497-1726
ESSL/CGD/CAS                               fax: (303) 497-1333
P.O. Box 3000				
Boulder, CO 80307-3000	  http://www.cgd.ucar.edu/cas/asphilli
    example.gif

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

    begin a = addfile("slp.mon.mean.73x144.nc","r") slp = a->slp slp = rmMonAnnCycTLL(slp) finarr = slp(0,:,:) ;======================================================================== wks = gsn_open_wks("ps","example") gsn_define_colormap(wks,"BlueRed") res = True res@gsnDraw = False res@gsnFrame = False res@cnFillOn = True res@cnLinesOn = False res@cnMonoFillColor = False map = new(4,graphic) res@cnLevelSelectionMode = "ManualLevels" res@cnMinLevelValF = -3. res@cnMaxLevelValF = 7. res@cnLevelSpacingF = 1. res@cnFillColors = (/92,102,112,122,132,142,152,162,172,182,192,202/) res@gsnCenterString = "Manual" map(0) = gsn_csm_contour_map_ce(wks,finarr,res) res@cnLevelSelectionMode = "ExplicitLevels" res@cnLevels = (/-3,-2,-1,-.5,0,.5,1,2,3,4,5/) res@cnFillColors = (/82,92,102,112,122,132,142,152,162,172,182,192/) res@gsnCenterString = "Explicit" map(1) = gsn_csm_contour_map_ce(wks,finarr,res) res@gsnSpreadColors=True res@gsnSpreadColorStart = 85 res@gsnSpreadColorEnd = 192 res@gsnCenterString = "SpreadColors #1" map(2) = gsn_csm_contour_map_ce(wks,finarr,res) res@gsnSpreadColorStart = 61 res@gsnSpreadColorEnd = 232 res@gsnCenterString = "SpreadColors #2" map(3) = gsn_csm_contour_map_ce(wks,finarr,res)

    yres = True yres@gsnMaximize = True yres@gsnPaperOrientation = "portrait" gsn_panel(wks,map,(/2,2/),yres) end

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