Hi Link.
This is truly a massive map. Recently I created a 2,500 sq KM map (50x50 km) and that is quite the endeavor. I think you’ll find that unless you plan to rent a server or have access to some serious computing hardware, you’ll either want to lower your expectations for the map size or you’ll want to use a very low resolution. Allow me to break down the process based on my own experience:
For software, World Machine is sort of the go-to program. For a landscape of this size, or anything even remotely close to, you’ll need to use a tiled landscape. A tiled landscape is simply a single, large landscape that’s broken into many smaller portions that can be loaded in and out of memory for the player. In other words, this is the methodology you’ll have to use for a large landscape. Unfortunately, unlocking the tiled-build feature of World Machine requires a professional license, which is can be pricey depending on your situation. Check out their website.
I can’t tell you how to use World Machine, you’ll have to read the documentation and watch tutorials on Youtube to learn the ropes. Once you’ve actually created your landscape though, you’ll setup the tiled-build settings. The number of tiles, the resolution of each tile, and the export settings. I recommend putting the tiled blend to 100% and leaving “flip y axis” unchecked. Because of a disagreement with UE4, this will cause your landscape to be imported rather scrambled, but it won’t be mirrored. If you’d prefer your landscape to be mirrored (backwards) but not have to go through the trouble of manually reorganizing them, then go ahead and check “flip y axis”.
Before exporting you’ll want to create something called weight maps within World Machine. Again, theres lots of documentation for this, but essentially weight maps are black and white images based on the height and angle of the terrain generally that are used to tell the engine where to place certain materials. You’ll generally want to use weight maps rather than splat maps for tiled landscapes like this. The white parts of the weight map will be where your material is applied in UE4. For example, if I have a mountainous landscape, the highest elevation of the landscape will be exported as one weight map, the lowest as another. Then in UE4 within your landscape material, you’d assign your layer to the corresponding weight map. So your snow texture would correspond with the higher elevation weight map, and maybe your dirt texture would go with the lower elevation.
So, building your landscape… Wih my 50x50 km landscape it took a bit over a week to render on my machine, with an i5 4690k and a gtx980 and 32 gb of ram. For a landscape of your size I imagine it would take months. Granted, that was at a resolution of 1 pixel per meter, relatively high-res. Whatever path you decide to go, once you build your landscape your ready to import to UE4. The tutorials posted below will cover that fairly well (the world composition one is more relevant to you). So by default, world composition will setup all of your landscape pieces and they will appear/disappear at certain distances. By default, there are no low resolution versions of your landscapes, so what this means is that if you are further away from a landscape piece than its streaming distance (the distance that the landscape pieces will load into the world for the player) you will simply see empty space. If you click on the details for the landscape tile, you can generate LODs for your landscape pieces automagically thanks to UE4.8. You’ll want several LOD’s for each piece, with varying levels of detail that make sense for you.
This is just a simpification of a rather involved process, the other tutorials I posted go into great detail, but I hope this helps. Keep in mind when creating HUGE landscapes, you’ll need to populate them with assets. Good luck
https://wiki.unrealengine/World_Machine_to_Unreal_Engine_4_-_In_Depth_Guide
https://wiki.unrealengine/World_Machine_to_UE4_using_World_Composition