I was hoping someone could help explain a behaviour that I noticed while working with downloaded MODIS 17A3 .hdf files from here: ftp://ftp.ntsg.umt.edu/pub/MODIS/NTSG_Products/MOD17/MOD17A3/
The readme.txt file from the link above suggests a scale factor needs to be applied to the digital values of the NPP datasets to generate the biophysical variable. However, when I load a given .hdf file into QGIS 2.10.1 on Mac OSX Yosemite 10.10.5 the raster numbers appear to have the scale factor applied already (on the same order of magnitude as the User Guide's NPP values once the scale factor is applied: http://www.ntsg.umt.edu/sites/ntsg.umt.edu/files/modis/MOD17UsersGuide.pdf, Biophysical_pixel = scale_factor * digital_value = 0.0001 * 421 = 0.0421 kg C m-2.).
If I convert a single .hdf file to .tif, the 'scale-factored' number is conserved in the generated .tif file.
If I convert a single .hdf file to a virtual raster (.vrt) in QGIS, the 'scale-factored' number is no longer conserved and I am on the 'digital_value' order of magnitude again. I ask this because I want to combine several of these original .hdf files.
I tested in ArcMap 10 and it opens the .hdf on the 'digital_value' order of magnitude.
Does someone know why this might happen?