Landscape Edges Welding/Joining

Hello,

I would like to ask for a help? I want to continuously expand map and since it will be quite big, I need to import it in parts. But the smooth tool only works if I import the terrain in one batch. I need to import an area of approx. 450x200km squared. And since it’s quite big, I thought it would be better if I just import it on the go, as I will work on the map, tile by tile. I’ve tried to import tiles, e.g. map_x0_y0,map_x1_y0 . I can join these two. Then I’ve imported map_x0_y1,map_x1_y1, again, those were joined nicely, but I was not able to join or smooth over the first two landscapes. Does someone know, please, how can I join the terrain edges that were imported separately?

Thank you for your help, appreciate it:)