Hi,
If the second terrain (i.e. the one you can translate, rotate and scale) doesn’t cover the first terrain entirely, the heights along its borders must be equal to zero, otherwise you get a seam.
To ensure that the heights are equal to zero along the borders of a terrain, you can use a mask whose values will be 0 along its borders.
There are several nodes that can generate such a mask: the Disk node, the Circular symmetry node or the Painted mask node.
Also if the heights along the borders are ‘almost’ zero, you can use a Apply curve node to "squash" the terrain and get exactly zero.
