How do I use ranges for raster colour map in QGIS

Question

I have a raster layer in GeoTIFF format, with continuous data values between -1 and +1. For the colour map, I would like...

• >0 to be green
• = 0 to be grey
• <0 to -0.25 to be pink
• <-0.25 to -0.50 to be dark pink

...etc to 1. Is this possible with QGIS? I tried putting ranges in the colour map box, but it didn't work.

If not, I'll do it in R.

Answer 1

This seems to work for me on a -1.0 - 1.0 NDVI raster

• Double click on the raster layer in QGIS table of contents/layer list
• Click "Style" tab (1st tab on the left)
• Select "Render as" -> "single band gray" and "Single band properties" -> "Color map" -> "Colormap"
• Click "Colormap" tab (2nd tab from the left)
• Click "Add entry" and add the following entries:
  • -0.4999999 (< -0.5)
  • -0.2499999 (>-0.5 <-0.25)
  • -0.0000001 (> -0.25 <0.0)
  • 0.000000 (=0)
  • 1 ( >0 <=1)

Answer 2

One way would be to classify your raster using GRASS r.reclass from the Sextante toolbox. You'll need a 'rules' text file to do this, there are some examples here. Example 2 is probably something like you'd need (with a bit of editing, of course). r.reclass groups raster values into classes following the rules set in the 'rules' text file.

Nick.