At least two items in your GRIDMASK file seem incorrect:
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the VAR-LIST attribute should list the variables as character*16 strings, i.e. “CC HB JL” … etc., while your attribute is "CC HB JL SP SY "I see now that this likely was a markdown issue, not an issue with your file. Apparently markdown removed those extra spaces following the variable names both in my comment and your copy of the file header, so probably you do see those required extra spaces when you ncdump the file on your system. -
The variables representing the region masks are expected to be float, not int by the ISAM code.
How did you create your GRIDMASK file? The header mentions M3MERGE , but M3MERGE shouldn’t have created that type of VAR-LIST attribute. Did you do any editing / file manipulation after running that tool?
The discussion in the following threads suggests that you could try a combination of ncdump and ncgen to convert the variable type to float, but I don’t know if that approach tried by that user was successful. Maybe you could also try to reach out to them to see if they can share their experience:
What’s the maskdata’s format?
How to make ISAM_REGIONS file?