Well, everyone's welcome to edit the map.
Faulgor, that's beautiful, fantastic work! I would add the 4 cell outline on the absolute Western edge of Skyrim's coast though, if you look closely it is there. I am excited to go to that dungeon again... or to whatever it is now. For all we know, it could have been renovated into an Orc settlement. Speculation!
Another thing I want to point out is that we should probably still add the http://img826.imageshack.us/img826/2858/skyrimmap.png to the beginning of this thread. Just because a lot of people won't bother to find the original thread, and I don't want to get stuck on any path dependency when it comes to observing the map. There is a lot there.
I would also like to point out that the icons at the bottom left on the original are not a key to the terrain types, as they match the colors of the square icons on the map rather than the terrain shading. They are a key to the landmark icons: forts, daedric ruins, dwemer ruins, caves, shrines, statues, dragon lairs, etc. What I am wondering now is what the icons could be, beyond the guesses I just made, and whether there will be more landmark types considering that 2 of the icons are penciled in so they were still working on potential landmark types at that point in the game. I am expecting more landmarks overall than Oblivion had, especially because there are so many more types. I think that they wouldn't have added more types if they wanted to decrease the number of locations.
Also, I think that for now you are right to leave The Reach as tundra, because there is no other terrain type listed, and I imagine that the terrain is probably tundra type short hills around the canyon. However, it does look like a different shade than tundra to me. It actually looks quite similar to the shade of the Volcanic Tundra but I am not so sure. Can you imagine a volcanic looking canyon? That certainly doesn't seem right to me, unless the canyon was actually created by a gigantic eruption caused by some kind of God-dragon erupting from the earth, rather than the river.