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Setting contour levels for use with symmetric color tables

This page shows how to set contour levels to go with a symmetric, two-color color table. This kind of technique can be useful when you need to represent your negative values using one color range (say, blue for cooling), and your positive values to another color range (say, red for warming).

Some sample symmetric color tables include:

BlueRed BlRe GreenYellow
contoursym_1.ncl: If you have a symmetric color table (an equal number of colors in each range, like the BlueRed color map), and you provide a set of contour levels that go from -X to X, then you will automatically get a nice symmetric set of colors above and below 0.0, because NCL will automatically span your color map.
contoursym_2.ncl: This example is identical to the one above, except that mpFillDrawOrder is set to "PostDraw" so that you see the gray filled land on top of the filled contours.
contoursym_3.ncl: If you don't know the range of your data apriori, but you want to make sure the contour levels go from -X to X in steps of xint, then you can use symMinMaxPlt.

This procedure sets the following resources:

res@cnLevelSelectionMode = "ManualLevels"
res@cnMinLevelValF = -X
res@cnMaxLevelValF = X
res@cnLevelSpacingF = xint
contoursym_4.ncl: This script, which was contributed by Chad Herman, calculates the proper values of gsnSpreadColorStart and gsnSpreadColorEnd for a two-color color table to ensure such that negative values will be in the first range of colors, and positive values will be in the second range of colors.

You can tweak the plotting limits to see how well the code correctly calculates the gsnSpreadColorStart/End values. Note that if the color bar spacing is larger than the value where you want blue to transition to red, the transition will not occur at the intended value because it's too small to resolve.

Note: using gsnSpreadXXX resources is the older method of spanning a color map, which still works. You can also use cnFillPalette to directly associated colors with a contour plot.