With four DEM files downloaded by Python Elevation package, I merged them in QGIS 2.8.3.

I use raster-> Miscellanaous -> Input direction, no more attribute was added.
enter image description here

The four dem data has the elevation range roughly from (-100,2000) meters.

But in the merged file, the data range was from -30000 to 2000 meters.

The data distribution are shown here:

enter image description here

  • Your files are in Geotiff format, dou you know what is the null value for your files? This can vary depending on the format.... – Gerardo Jimenez Jun 20 '16 at 13:46
  • 1
    Did you check the validity of the individual DEM layers? perhaps one of them corrupted? As @GerardoJimenez says, the negative values look suspiciously like a signed 16 bit minimum value of -32768 – HDunn Jun 20 '16 at 13:47
  • I'll recheck my data – Han Zhengzu Jun 20 '16 at 13:52

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