Had another go with tiling non-flattened terrains but this time I got it to work. I was actually pretty close the last time I tried. My main problem was that I over-complicated things te last time and possibly had one setting wrong in the blend shader.
I replace "Border blending" with a blending shader that does pretty much the same thing. The terrains are stacked in width order with the widest terrain on the bottom. Each terrain (except for the bottom one) has a matching blend shader made with the image map. The top terrain uses it's own blend shader, but each terrain below it uses the inverted blend shader from the terrain above it.
The image map is inserted as a 100m square centred at 0,0,0 and a transform shader is then used to produce the variations to match each terrain to save RAM.
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Off-centred terrains will require a bit more creative masking, but the approach is basically the same and a few simple colour subtract/add nodes should do the trick. I'll have to refine my mask for some of my terrains that only have a very small overlap, but after that I should be able to re-release the wider terrains for my terrain sets.
Another issue that appeared in this demo is that the TERs may need to be adjusted to match each other just to fine tune things. The accuracy of elevations for each terrain will have a tolerance of +/- 0.5 terrain spacing. While this may not have much impact between terrains in mountainous areas it can have substantial effects on flat areas. The main flat area of course is sea level. For example, my 50m resolution Ben Nevis terrain has a seal level of -25m, wile the 200m resolution terrain in this demo has a sea level of 100m. I'll probably use TG0.9 to "burn" the required adjustments into the TER files rather than add extra adjustment nodes in TG2. The difference in scale between terrains will also exacerbate any variations.
While this method of padding terrains is limited because of the way TG2 projects the terrain onto the planet, it should be possible to provide a wide enough terrain to allow for a camera position high enough that the rest of the planet could be covered using an spherically projected image map as a displacement function. The trick then is to remap the global bump map to the right orientation which is also relatively straight forward.
Demo images are the Swiss Alps, 3 terrains (50, 100, 200m resolution). Surfacing is just an image map from 3DEM so I could see where the sea was.