[GIS] How to make transparent the background of topographic map in QGIS 1.8

basemapqgisqgis-1rastertransparency

I have a raster topographic map, i.e. white background and black contour lines, houses, roads and so on. I want to make transparent the white background, so if I overlap the topographic map to an aerial photo, I see only contour lines, houses and other elements atop the photos.

In ArcGIS this operation is trivial. How can I do it on QGIS 1.8.0?

I explored many forums but I didn't find the answer. The solution is not the command "transparency" on the property window, because that command makes everything transparent.

Best Answer

  1. Right-Click Layer Properties and select Transparency tab. enter image description here
  2. You would have a default Row in the transparency pixel list. Delete that Row(Use Icon 3)
  3. If you just have to make the White part transparent, Create a new Row(Use Icon1). Enter 255,255,255 for Red,Green and white and Set 100 for percent transparent column.
  4. Click Apply and you are done.

Some sample results.. enter image description here

enter image description here

Additionally, if you want to make any other color on the raster transparent, You can click the Icon2, This would ask you to select a pixel on the raster using mouse. This might make QGIS go minimized for some weird reason, but you can restore it from the taskbar. Then click the color on the raster which you want to make transparent. It would add a new row for that color and make that color transparent for the raster.

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