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The file format was documented by ESRI online. (older version here) and supports integer (8,16 or 32 bit) pixel values in signed or unsigned format, in either little- or big endian storage order.
BIL files in FEWS
In the Public Interface (PI interface) of FEWS, an implementation of the BIL format is also provided, as documented here.
The PI implementation of BIL files however includes several adaptations to specific FEWS purposes (i.e. using floating point data, extension for time series using blocks) which causes this FEWS implementation to not fully comply with the official specifications of the BIL format and changing this structure would break backward compatibility so it was decided to create a new parser, which als properly supports the BIL, BIP and BSQ format.
This new implementation is fully compliant with the ESRI technical specifications, but does NOT offer specific FEWS extensions to the format for dealing with time series or floating point pixel values, this new importer can be configured with the new "BILBIPBSQ" identifier, whereas the existing PI implementation of FEWS BIL files uses the identifier "BIL".