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Mercator settings from .PRJ
I shouldn't really call this a bug, since .PRJ files are inevitably approximate.
The issue is that for Mercator projections, GM 12.01 (and previous versions) overlooks the True Scale Latitude parameter written in a PRJ file (for some reason) as standard_parallel_1. It always defaults to 0.
This is the case even for .PRJ files GM has written itself.