Mesh - repair

Hello,
can someone help me. I cannot make these meshes solid.
I tried everything i find on internet. I need that to be ready for printing.

needforprint.3dm (908.8 KB)

Is there some way to fix these please?
Thank you

QuadRemesh will do it.

There are good resources for solving this and more. For starters:

Kyle Houchens also has some excellent resources.

you have a few very tiny meshfaces that are open. i managed to close it by using ShowEdges set to naked edges then use the zoom function of it you might have to swivel the mouse a bit to find them. the bigger part has 2 of such did not look at the small one i guess its similar, if you delete all faces that are close to it and use FillMeshHoles you get a closed part very soon without breaking up the rest of the geometry.

the functions @John_Williams mentioned are great for stuff where its not so crucial to keep the edges in tact which if that does not matter you defintely can use here but here you have some very defined terrain edges which get maybe get smoothed.