[Qgis-user] QGIS Raster Calculator - cannot add two mutually exclusive areas with logical statement because nodata excludes the other's data

[Nicolas Cadieux]

Hi,
The first step is to remove the nodata value from the file.  In 
"translate" that would give the following command (that needs to be 
manipulated using the yellow pencil.)

ex: gdal_translate *-a_nodata none* -of GTiff 
C:/Users/Nicolas/Desktop/SRTMGL12delete/Que/Que_SRTMGL1_WGS84_EGM96.tif 
"C:/Users/Nicolas/Desktop/for 6tb/Que_SRTMGL1_WGS84_EGM96b.tif"

In my case, this gives me a file with new value (-32768).  I can now 
manipulate that value. Now I need to change this value (-32768 = missing 
SRTM Tiles in the sea with no land) to zero (the sea level).  This can 
be done in raster Math.
ex:
(("Que_SRTMGL1_WGS84_EGM96b at 1" = -32768)*0) + 
(("Que_SRTMGL1_WGS84_EGM96b at 1" > -32768)*"Que_SRTMGL1_WGS84_EGM96b at 1")
What is going on???

A) ("Que_SRTMGL1_WGS84_EGM96b at 1" = -32768): creates a mask with 1 and 0 
(1=satisfy condition, 0=does not satisfy condition)
B) Then you multiply this mask with a value : *0. Therefore -32768 
becomes 0  (if it was *30, then this would reclassify all -32768 to 30).
C) You then add to this map the pixels that are bigger than  -32768: 
(("Que_SRTMGL1_WGS84_EGM96b at 1" > -32768)
D) And finally, you multiply this mask by the original values: 
*"Que_SRTMGL1_WGS84_EGM96b at 1")

You can also reclassify using GRASS and SAGA in the processing plugin.

Hope this helps
Nicolas




Le 2015-06-03 10:26, robertlozar [via OSGeo.org] a écrit :
> Thank you for your reply.
>
> I realize you can change zeros to nodata. I wish to know how to do 
> opposite: change nodata to zeros in QGIS.  As I mentioned, I would 
> like to know how it is done in QGIS as I have already done it in the 
> other plugins.  Is it possible, or is this a bug in QGIS?
>
> Thanks, Bob
>
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