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I have a project that contains an 8-bit paletted layer (call it the mask layer) and a float 32 single band layer (call this the clip layer). I create a new raster by using the raster calculator, defining the extent to match the clip layer, and the value is defined as

(mask@1 >= 0) * clip@1

The resulting raster, when viewed with gdalinfo, has a NoData value of -3.4028234... e+038. Yet the input clip layer has no NoData value. Where is QGIS getting this NoData value from?

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Have you noticed that -3.4028234...e+38 (which is a base-10 representation of (2 - 2^(-23))*2^127 up to 24 bits of precision) would be represented as the 32-bit binary value 11111111111111111111111111111111B in single-precision floating point format? – whuber Feb 24 '14 at 17:38
I assumed it was the min value for the datatype, though I didn't look it up. So I guess that's sort of a "yes" :-) – Llaves Feb 24 '14 at 19:26
up vote 4 down vote accepted

This is the default NoData value for QGIS. It is the smallest possible value for a signed 32-bit float data type (I think you meant to type a minus in front of the value you quoted). You need not worry about a NoData value appearing especially if non of your data is NoData. QGIS won't suddenly allocate some data cells as NoData. It is just part of the definition of the raster - you can always change it if you want some other value, though since you don't have any NoData, it is a bit irrelevant.

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