"Values in the mask are defined from north-to-south, west-to-east; the first byte in the mask defines the watermask value for the northwest corner of the tile."
This doesn't make it clear whether the values are in latitude-major or longitude-major order. Either order can be interpreted as complying with that sentence. I'm guess latitude-major order, so that the north edge is traversed first?
"Values in the mask are defined from north-to-south, west-to-east; the first byte in the mask defines the watermask value for the northwest corner of the tile."
This doesn't make it clear whether the values are in latitude-major or longitude-major order. Either order can be interpreted as complying with that sentence. I'm guess latitude-major order, so that the north edge is traversed first?