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The file format was documented by ESRI online. (PDF version) and supports integer (1- to 32- bit) pixel values in signed or unsigned format, in either little- or big endian storage order. The 1- and 4- bit integer pixel formats have not been implemented (yet) as they seem less useful for FEWS, but the rest of the documentation is fully implemented.
Note: It is assumed that your data is using unsigned integers, unless the keyword "pixeltype" is used and its value is"signedint".
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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