Have you opened 3,2 and checked it prior to Gimp?
It could be the tile export from World Machine was corrupted. Very occasionally it will do something like that. Looks like tile blending.
Oh, yes. A week ago when I first got them I opened them up in Paintshop Pro and absentmindedly noticed some of these "artifacts" but didn't pay them much mind not having much experience with this kind of thing. Plus since the tiles were so big I was only opening up one at a time (i.e., the Lonely Mountain tile, the x=3 y=4 tile for Moria/Isengard, etc.)
It was only in the last few days when I started trying to mosaic a couple of them together that it became readily apparent something was going on. I think you hit the nail on the head when you said, "Looks like tile blending." If you look at the bottom right corner of the map you can see where it looks exactly like that. There's a thick river that looks blended into the mountains, not eroded out of it. And now that I look closer at the y=2 tiles I can see some more evidence of what looks like more than one thing getting blended together.
And for the record, I am loading these from the original BMPs I downloaded from google. I stuck them all in a directory and copied them out to another directory I was doing things to so these are the originals. Google zipped them up before I could download them, but I don't see how that could have such drastic effect on them?
I really appreciate you taking the time to check this out!
EDIT: Kind've changing the subject...I don't know if you guys are familiar with
Ventrilo, but it's a simple voice chat program I use with some friends/family. It also has text chat which we actually use more than the voice part. Anyway, I've got a voice chat server I pay for that I'm pretty much on the entire time I'm at my computer. I started using it back when I played WoW, plus it's pretty much the only way I keep in touch with one of my younger brothers (Nazghash).
I'd love for you guys to get on sometime for a chat! You'd have to download and install the client, but it's a very simple/unobtrusive program. I'm GMT -6. If you guys are in England (like I suspect) timing the chat may be our biggest difficulty.
If you do decide to get on, my server info is: morcrist.typefrag.com, and the port is 49450. There is no password. Get on and hit the Chat button on the right to get in the text chat part. One of its main attractions is the ability to paste links when you're talking.
I leave my computer on a lot, so if I'm on but unresponsive remember that and the fact that I'm GMT -6.
Anyway,