I`m trying to add several raster files to one (like a merge). I tried using the raster calculator (in ArcGIS 10.2.2 and in QGIS 2.8.1 and QGIS 2.10). Anything I do leads to rasterfiles which have only NULL-Values.

Every raster has the same extent, same projection type and pixelsize. All rasters are in floating point and have NULL-Values. I tried many things but mainly I just tried to add them like:

Raster1 + Raster 2 + Raster 3 and so on.

I also tried the melt function in QGIS and the GRIDsum function in SAGA. It must be a problem with my data but I don´t have an idea what could be wrong. Anyone an idea what I could try?

Edit: Also I should mention, that there is a NULL value for every pixel in at least on of the rasters used.

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  • 2
    Any math involving a null results in null. Those will have to be converted to 0 or ignored. – Chris W Nov 9 '15 at 18:40
  • I understand. Thx. But I find it a bit strange, that nothing plus something euals nothing..... – Sebastian Nov 9 '15 at 23:11
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    It might help to think of it as null isn't nothing, it's I don't know. Zero is nothing. Null is unknown, no data - it could be an unknown value more or less than zero. And unknown plus anything is still unknown. – Chris W Nov 10 '15 at 0:31
  • I see. But how can I ever change that than? – Sebastian Nov 10 '15 at 0:48
  • It really depends on your data. When talking about null values in rasters, they are often called nodata cells as well as null values. We have a number of questions on dealing with null/nodata in rasters. If converting the null values to zero works for your case (there are implications for results when treating a null as zero - sometimes it can skew or alter them depending on the data), take a look at gis.stackexchange.com/questions/167606 which is just one of several questions about doing so. Some spatial analyst tools simply ignore nodata values, or can be set to do so. – Chris W Nov 10 '15 at 20:55

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