Hi Im trying to edit the Tamriel Worldspace Height Map so the Summer Set Isles is included so you can walk or swim to them. But when i try i just get ugly height maps and errors and rips. This happens even when i try and make the isles on a seperate heightmap. Could someone please give me a very good tutorial on how to make a beautiful height map. Also can you make a height map using Blender?

Vagrant0 wrote a good basic http://cs.elderscrolls.com/constwiki/index.php/Heightmap_editor, so that's where I'd start. Be forewarned that there are no shortcuts for making a good heightmap. The key to using the heightmap editor is patience. If you zoom in and use small brushes with low settings you can edit the landscape with just as much accuracy as you can with the landscape editor. Start with the general size and shape using a large brush and then zoom in a bit and do another pass with a bit more detail and repeat. Make sure you smooth after making changes, and keep an eye on the elevation. As long as you use the documentation on the wiki you should be fine.

I also recommend you make some preliminary concept sketches so you know what you're doing before you start. It'll go a lot faster if you have a really good idea what you want.

You might also want to make the island its own worldspace instead of being part of the Tamriel one. It can be extremely hard to blend new land from a mod with the vanilla land without excessive editing along all the borders. If you're concerned about having Tamriel in the distance, you can always make sure that the world you build is several quads to the south and west of 0,0 and just setup the worldspace as a child of Tamriel. You won't be able to swim between the two, but you might be able to see some hints of the distant shoreline on a clear day.