When I open the Heightmap editor, with the worldspace Tamriel loaded into it, I have a nice view of the heightmap of the Imperial city and a small amount of land surrounding the city isle.

However, when I move the viewer to try and see more of the tamriel worldspace's heightmap, it's black, as if the only area is the Imperial City.
Does anyone know what I'm doing wrong?

Thanks.

you have to refresh the preview (under the file menu, iirc)- this will bring up a small window with all the heightmap. you can then click through the window to select 4 quad chunks to view in the main editor.