I see your problem there, but cannot think of a solution even for the first sewing/splicing step.
high detail terrain viewed from side:
. . .
. . . .
.. ..
low detail terrain viewed from side:
. . .
both terrains spliced/sewn:
.____.____.
. . . .
.. ..
(lines represent low vertex count terrain result)
How are you going to achieve that if they have a different amount of vertex?
EDIT: I just came up with an idea that might help. Since we are not working with terrains but with models that look like them... You have an advantage over terrains, you don't need to make all the rows the same.
Low detail terrain:
. . . . . .
. . . . . .
. . . . . .
. . . . . .
Mid terrain (used for between HighLOD and LowLOD for splicing):
. . . . . .
. . . . . . . .
. . . . . . . .
................
................
High detail terrain:
................
................
................
................
................
................
................
................
the vetex calculations for the middle terrain will be a pain, but it should be achievable this way.
The array (or whatever) holding the height data should be the same as any high detail terrain for all of them. on the high detail one loading is strightforward, on the low detail just skip x vertexses in the height data for each row, and also skip x rows in between. The tough part will be the middle terrain that is used to connect one LOD step to the other. The values to take and skip from the height array will probably have to be hardcoded (at least per row)
to make the coding simpler each LOD sep must be a multiple of the prevois one. and in the middle terrain each row must be a multiple of the previous rows... and you will special versions of these "middle terrains" for the corner terrains.
And if you plan on having several LOD steps, than that is just going to be h3ll to program and get working, but it should be possible (painfull, but possible)
