JibbSmart and HeelX, thanks a lot for your useful inputs.
Thanks especially for the pictures, HeelX, they actually give some insight.
I hope that I find time in the near future to build such models to see how it works out in Acknex.

Re-thinking about the blending of the sand and the grass texture, I think that it is a combination within a multitexture shader of the alpha of the blended textures and the alpha of the blendmap.
Look at the repetition of the grass in the following pic:

A gap with a sharp shape between some blue flowers and a yellow flower is visible at the left and at the right of the pic. It is only slightly distorted.

A question to the experts:
is it difficult to change one of the multitexture shaders of 3dgs
to use the alpha channel of the textures?

I didn't find anything within the shader tutorial about alpha, and the wiki, the second source where I expected information, doesn't show up.

Nope, it isn't difficult I think.