For example, in the file `MOD21A1D.A2003001.h12v04.061.2020084062741.hdf` I got from EarthData Search, the first five `QC` values are all 54 (hexadecimal 36). The corresponding values from OPeNDAP, available at [link] ://opendap.cr.usgs.gov/opendap/hyrax/MOD21A1D.061/h12v04.ncml.ascii?QC%5B766:1:766%5D%5B0:1:0%5D%5B0:1:4%5D , are all -0.509091 (hexadecimal 86b56fbf in 32-bit IEEE floating point). So how can I interpret the OPeNDAP version?
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Thank you for pointing out this issue. The OPeNDAP URL you are accessing is one that is configured to automatically apply scale factors and offsets to the science datasets. This issue was unknown to us. We will investigate this behavior for that OPeNDAP URL. We do have another OPeNDAP URL that is configured to not apply the scale factors and offsets. That endpoint is, in fact, returning correct data for the QC layer. The following link will provide correct, unscaled data for M*D21A1D and M*D21A1N: https://opendap.cr.usgs.gov/opendap/uns/MOD21A1D.061/h12v04.ncml.html