Height map increasing the details

L3DT is able to rescale heightmap, for example my 51km terrain is 1024 grid and 50m cell size, would work in-game if I bump it up to 2048 and 25m?

And if so, how can I add some bumps and lumps into the terrain, was there some erosion feature in L3DT or something?

These are good questions, here is the answers:

It will work fine in-game with those specifications.

For terrain erosion you are better to do in Wilbur. In L3DT you can do terrain height map editing manually, which would take lot of time and effort compared to Wilbur's automated tools.

In Wilbur you can select areas and do quite a few different noise and erosion things - there is a whole bunch of parameters which are basically straight copies of the Leveller GForge plugin.

More questions:

How did you choose the cell size when you ported them over, I mean how did you keep bridges as is if you bumped up the details or do they automatically stay the same, I'm not totally familiar how beefing up the details work?

Answers:

12.5m cell size is the best you can go as 6.25m wont work in Visitor 3.

When porting terrains from 50m cell size you need to keep in step or objects will get dislocated. So 50m → 25m → 12.5m.

Its usually 256×256 50m for Operation Flashpoint era terrains.

You could go to a 512×512 with 25m, or a 1024×1024 with 12.5m.

What happens ArmA 2 in-game to the terrain when you do it, does it just go smoother and objects stay in place ok or?

Whole point of using those step down multiples is that actual coordinates stay the same so missions are unaffected, all stuff is in same exact grid coordinates as before. When you re-sample it just adds more cells - it interpolates the new ones between the points of the old.

So some stuff like roads need re-smoothed. Generally looks smoother.

Special Objects

However when you use L3DT to increase the size of your heightfield, all works fine it seems but bridges or harbor objects end up in water on east end. Like when you have perfectly working bridge in 50m cell size, then you increase it to 12.5m size and the bridges east end is now in water and not land like it was previous. Who knows if this issue can be avoided using some other algorithm to increase the terrain heighfield size than L3DT's dunno. If you have no bridges or special objects, you're fine, if you have… then well, you got problems with special objects.