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How do I merge black and white rasters into one file using QGIS?

Also, rasters are overlaying so I´m interested into preserving only black colour.

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    Welcome to GIS SE. At the moment you question is a little limited. A simple answer would be in QGIS you would use the 'Raster > Raster Calculator' to merge two layers together. Understand that the colour of the cells is only a representation that QGIS makes for a value. Could you update your question to include what values it is you are trying to preserve/using. EG. if black = 1 and white = 0, then you would simple add the two layers together. Check this link out learn more about the calculator: docs.qgis.org/2.2/en/docs/user_manual/working_with_raster/… – Knightshound Nov 2 '16 at 12:04
  • Thank you for you answer! Values I would like to preserve is as you described; 1= black and 0=white. I tried to add it but nothing came out as result... – QGIS user Nov 2 '16 at 13:28
  • Welcome to gis.stackexchange! Please note that a good question on this site is expected to show some degree of research on your part, i.e. what you have tried and - if applicable - code so far. For more info, you can check our faq. – underdark Nov 3 '16 at 17:52
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To add two rasters together, whilst preserving a value, as long as raster cell values are 1 or 0 is to multiply them in the raster calculator.

"raster1" * "raster2"

Multiplication works because: 1*1 = 1 and 1*0 = 0

Set the raster calculator to output the same crs as both your other layers, otherwise you could create a raster that ends up in a different location.

  • What if black should be preserved if either a or b has black pixels, or both? – user30184 Nov 2 '16 at 17:32
  • Not sure what you're fully asking. hypothetically, wouldn't a raster have all those examples/variations you suggested? this isn't my particular expertise but would you not try using a polygon to clip a particular area you wish to preserve. Or have I completely missed the point here. – Knightshound Nov 3 '16 at 8:10
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    If raster 1 has a line, let's say road, it has black pixels where the road is located. If raster 2 overlaps but does not contain that road I suppose that the resulting merged raster should have ones, not zeros because black=1? With your formula it would be white (0 * 1=0). But I do not know what is the aim of the exercise and I am just guessing. Perhaps a simple image merging by defining 0 as a nodata leads to good result. – user30184 Nov 3 '16 at 8:20
  • That's a good explanation. Using addition would certainly fix that problem, but would then also introduce a lot of new values, which would change the data. Just looked at the merge tool and your right, that would probably be best if zero was nodata. – Knightshound Nov 3 '16 at 8:39
  • Can you take max((raster1),(raster2)) with raster calculator? – user30184 Nov 3 '16 at 10:24
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So as user30184 wrote - roads needs to be preserved (0*1=0) and this is wrong solution. I managed to solve the problem in GRASS GIS. I don't think it is possible to solve it in QGIS.

Merge tool don´t work since rasters overlap (white area than covers black).

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