Currently the terrain generation algorithm is bugged, most likely in several ways. There was a solution for probably the most serious one at http://getsatisfaction.com/mojang/topics/terrain_generation_is_a_broken_scrambled_mess, so I've implemented it. I wouldn't really call this a real mod, because it's just switching two variables in the code, but you have to install it, like any other mod (overwriting the exisiting class file in minecraft.jar). The credit for finding the bug goes to jmlsteele, who btw has a legit patcher that fixes SMP terrain generation. I searched, but I couldn't find any other mod, that could do this fix for SSP. If there is any, this post is obviously redundant and I've just wasted a hour of my time figuring out how to mod minecraft.
I tested it on a new map. I walked around a bit before saving this image.
I deleted all the chunks using MC MapDeleter / Pine Tree Generator, then I installed this mod, and walked a bit around the same area again. The game automaticly regenerated the same chunks (the random seed for the map is still the same), but this time sand and grass became alligned.
As you can see this solution is probably not perfect. Most of the sand seems to be in the right place, but there are still a few wierd things around, that I'm just going to ignore. All I can say, that it's definetly better with this fix than without.
The fix only affects the terrain, while it's being generated, so it does nothing with your already existing save file. If you want to fix your world, that's already generated, then you can just download the Map Deleter I already mentioned. There you can manually select the bugged chunks, delete them, and after applying this fix, Minecraft should properly regenerate them as soon as you get close to the area in-game (not tested yet). You obviously lose anything you had in those chunks. Wierd things might also happen with trees growing around the edge of the deleted chunks.
The BiomeTerrainMod already uses the rx.class file, without fixing this bug. I don't use that mod, but according to some people, the resulting terrain is still affected by this bug, so I fixed the same thing there too. Install this only after BiomeTerrainSSP06b. Download BiomeTerrainMod version
Just to be safe, always back up everything! I'm not responsible for any damage you might think this mod caused. The normal one should be working OK though. I don't really care about BiomeTerrainMod, so I haven't tesed it, and probably never will. It might not even work at all.
Heh damn, I always thought it looked strange and now seeing it fixed its obvious just had the chunk coord directions flipped. Would explain why I ran into the same issue when I was rewriting the terrain generation for my own stuff :-p Just thought I had made a mistake.
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You're a lone commando tasked to destroy the Nazi Zombie Federation's dam.
oh wow. i will use this next time i make a map. but right now i have too much stuff on the map im using right now to risk anything.
+1 to you sir. hopefully notch will see this and use it.
This problem is already officially acknowledged for at least a month or so. Still, it hasn't been fixed. My guess is that the whole generation process is planned to be rewritten in the future, and that's why this hasn't been fixed. Or at least I hope there is some reasonable answer. But I grew tired of waiting, and like I said the solution was already available.
Awesome to have, or at least see... It apparently conflicts with the terrain mod, since that is designed to work off the bugged file, I suppose, and so the fixed one causes it to crash. Rather ironic, in a way...
Private Mod Note
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I thought you'd contracted a disease that gave you the same metabolism as an oak tree - Maggarg II
It heals all of your goddamn hearts! - kittensamurai
Does this work for errors related to nether travel? Basically, I travelled far away in the nether, and when I came out there's huge walls of rock. If I delete the sharp wall chunks and use this, will it blend it so it looks right?
can this be use, edited or written for smp? perty plz.
please dont respond "its for single player,derr" i know...i would like to know if it could be used to mod the server.jar files...
Derr, evidently you didn't bother reading the 4th sentence in the OP where he said, "The credit for finding the bug goes to jmlsteele, who btw has a legit patcher that fixes SMP terrain generation." You can find the link to it in the second sentence Damoklesz posted in the OP.
Damoklesz, thank you for doing this fix and bonus you attempted to do it for the BTM too.
Omg thank you so much, I had no idea how much this issue was bothering me until installing this. It is often said that there is no world as good as your first and for me this was true, every world since then has seemed off, tainted, unappealing, sometimes downright ugly. I had thought it was because they weren't the first world I had created. In reality I realize, it was because they looked wrong, chunks messed up all over the place, it destroyed the landscape entirely, it robbed the game of any and all visual appeal.
I installed your mod, intending to generate world after world looking for a good one. I got stuck on the first one I tried, it just seemed so perfect again. I have no desire to continue my other worlds now, this is my new home.
Thank you again, such a simple fix with such a profound effect.