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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, with the exception of 1-bit images.
BIL files in FEWS
In the Public Interface (PI interface) of FEWS, an implementation of the BIL format is also provided, as documented here.
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