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I have changed the No Data of a raster Forest.asc into No data = 0 using Raster -> Conversion -> Translate option in QGIS. So, when I add the output raster layer Forest_1.asc in QGIS and check for the minimum pixel value, it is showing that Minimum = 1 and Maximum = 3. But, when I open this Forest_1.asc in TextPad and read the values, I can see that it is written that No Data Value = 0. But, I am not able to find any zero as pixel value in the ascii file. I can find only 1 and 3 as pixel value in ascii file.

How can I solve this issue ?

  • Are there no-data pixels in on the map? Why would there be? – HeikkiVesanto Feb 19 '18 at 11:24
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    @HeikkiVesanto The exact issue is that when I open the ascii file in a TextPad, I have to see the No Data values as zero. Previously, when the No data value was 3 for this raster data, I was able to see the value 5 in the matrix inside ascii file (ascii file was having values 3 and 1; where 3 represents feature is absent and 1 represents that feature is present). – Sreeraj Feb 19 '18 at 12:00
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    @HeikkiVesanto But now, I changed the No Data value to 0 using Raster -> Conversion -> Translate option in QGIS. I can see the header "NODATA_value 0" inside ascii file. But, it is showing the previous no data value 3 instead of the new no data value 0. I want to get an output which replaces 3 to 0. I hope that you have understood what is the exact issue. – Sreeraj Feb 19 '18 at 12:00
  • Welcome to gis.stackexchange! Please edit the title of your question to include enough information for future visitors to be able to find this thread when looking for the same problem. – underdark Feb 19 '18 at 20:16
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I have solved this issue by using the Reclassify values algorithm of SAGA GIS.

Processing Toolbox -> SAGA -> Raster tools -> Reclassify values 

It is possible to change a single pixel value in the matrix to a new value; and also it is possible to reclassify in a particualar range of values as well.

In this Reclassify values algorithm, I have used the option 'old value (for single value change)' = 3.0 and 'new value (for single value change)' = 0.0 . Thus I have successfully solved this issue.

It is also possible to implement this algorithm using Python programming in QGIS Python Console; as given below.

processing.runalg("saga:reclassifygridvalues","C:/Users/Sreeraj/Desktop/Forest.asc.asc",0,3,0,0,0,1,2,0,"0,0,0,0,0,0,0,0,0",0,True,0,False,0,"C:/Users/Sreeraj/Desktop/Forest_output.asc")

By using the above python code, I am changing all the pixel values = 3 into value 0.

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