FOSS gamedev and creative worlds

Hello,

I'm building a model of my city on Minetest using altimetric data.
I first started with a flatland, in order to remove all these mountains, and am now building the ground according to my altimetric data.

My question is :
Can this ground be 1 node thick, or should I fill the gap between the ground node and layer 0. Is there a speed difference for the server?

Thanks