How do I create a negative to positive value raster with neutral 0?

Question

I have a raster layer with a band with values from -137 to 9229. I want to create a color scheme showing how far from 0 the positive numbers are in one color, and how far from zero the negative numbers are in another color, with a neutral 0 (that, unmistakenly, doesn't lean towards the positive or negative colors, nor diverges from the color scale).

Moreover I'd like a "-55"-area to be just as far from the neutral color (0) as a "+55"-area, but in different colors. So if neutral would be #333, then -5 might be #338, and +5 would be #833, even though the array of values, as in this case, is heavily biased towards positive numbers.

Hope that explains my problem. I'd greatly appreciate any help I get! This is for my Bachelor thesis project on the creation of the metro and highway system in DC during the 50s to 80s.

EDIT: Oh, gosh. I forgot to tell you I'm using QGIS 1.7.4 on an OSX Lion. And have very limited access to Cartographica and ArcGIS full suite.

Best regards, Henrik

Answer 1

1 Band Raster Plugin will take the existing data and fit a great range of preconfigured palettes(?). This is a quick way to get a very good colour table established. However you will find it is fragile for editing. Nevertheless, I recommend you start here. You might have to be a bit exploratory.

detail is here - http://www.bc-consult.com/freebies.htm

Answer 2

You can use the "Colormap" option in the Properties dialog to do this. You can select specific values and colours for each step (i.e. -55 / +55 can be assigned # #FF00FF / #07000B).

1. Right-click on the raster layer and select Properties;
2. Under the "Style" tab select Color map: Colormap and press Apply;
3. Go to the Colormap tab and add the values you are interested in together with custom colors:
   1. Click "add entry" and enter a value;
   2. Double-click the Color column and choose a colour;
   3. Continue until you have all the colours in the ranges you need.

You'll want to select "Color interpolation: Linear" to make sure that all the intermediate values are filled in.

It might be easier to tweak the resulting colormap using a text editor! "Save style" in the Colormap tab and then open the resulting .qml file in a text editor. You'll find a section in this XML file that looks like:

<customColorRamp>
      <colorRampType>INTERPOLATED</colorRampType>
      <colorRampEntry red="255" blue="255" value="-2.300000" green="0" label="Custom color map entry"/>
      <colorRampEntry red="51" blue="51" value="0.000000" green="51" label="Custom color map entry"/>
      <colorRampEntry red="7" blue="179" value="2.300000" green="0" label="Custom color map entry"/>
    </customColorRamp>

The RGB values (separately under red="#", etc) can be entered manually, and each value="#" as well.

Have a look at the Colorbrewer site for some good ideas about sequential colour schemes.