Hello!
Basically you´re on the right track. For large worlds I would make terraincells too.
An example: For an 81920 square quant landscape I could use 100 (10*10) 8192 square quant terrain cells. For these cells, when importing the pic for the heightmap (1024*1024 bitmap), I use these settings: 257*257 vertices, 16 quants side length. In order to get more height differences, you have to scale it up in MED. Use the blue arrow (left click and drag) and look at the statistics for exact scaling. Of course all terrain cells have to be scaled exactly the same way.
I use 16 quants side length, instead of 32 in order to avoid a too big terrain in MED, it´s more easy to do corrections, if the terrain is smaller and in WED you can scale it up twice. Then you have the desired side length of 8192 quants (256 polys * 16 * 2). I always try to use power of two numbers.
Now it took me a while to find out, how to make the terrain cells seamless.
I also use a paint programm in combination with a terrain editor.
Let´s say I have a 1024*1024 pixel bmap I could split it for example in 4 512*512 bmaps, arranged like this:
B1 B2
A1 A2
They would match seamless, but this would not result in seamless borders, when I place them in the level. The trick is to copy pixelcolumns and rows.
Copy for example the column of pixels from the right border of A1 to the left border of A2. Copy the upper column of pixels from A1 to the lowest position in B1. Now, when importing the bmaps the border vertices will match.
When you place them in WED, you´ll have to zoom in very close in order to place them correctly, and you can use the position panel of object properties for xyz position. Especially the z-position is difficult, and I had to place them manually in the xz / yz - windows. But the borders match perfectly.
A note of caution: I don´t know so far if this method works, if the terrains are placed far away from the origin in WED, because the z-values will be uneven numbers. I will test this sooner or later.
There is another possibility to create height differences: make a colored heightmap. When importing a heightmap, there is a possibility to choose an individually different weighting for the color channels. So you basically have more than just 256 different heigh values in a bitmap. It´s somewhat complicated to achieve good results however, if you´re interested, I posted about this in the Future Forum. "MED-Terrain" or someting like this.
Another note of caution: If you make corrections in MED, don´t touch the border vertices of the terrain cells.
I hope this wasn´t too cryptic, maybe there are more simple methods. And of course most of the numbers here can be changed according to your needs
Edit:
It works! I made a test, where I placed two terrain cells (A1, A2) far away from the origin:
A1 (xyz) position: 3100 / -45000 / -1000
A2 (xyz) position: 11292 / -45000 / -1374.41
Then I removed A2 from the level and wrote a little function, that creates A2 during gameplay. A2 appeared. There is only a little white line at the border, but I can live with that.
