If I can chime in with a few comments...
How to go about making a really efficient open ended digital world? In GTA3SA, they were able to accomplish this greatly.
Differences in engine design.
UE3 uses a default maximum world of 512k UUs which is approximately 10.5km x 10.5km. By using 1/2 default scale, you can get up to 21km x 21km. This is large enough for most any game that is of the FPS/RPG/Action style. Anything beyond this is difficult to implement with UE3, even with access to the engine source.
Since 256*256 height map is the largest terrain bitmap size Unreal can use
is it possible to stitch several terrains together to create larger terrain areas
And is it possible ... to preload these ... together to have zoneless maps.
Would it be lighter for processor/memory to use level of detail models for terrain?
there is a possibility of using US geospatial data
Not true. You can use 1024x1024 if you like. You just have to balance detail to file size and rendering speed. There are also numerous "tricks" to simulate terrain that is 8192 or higher on smaller heightmaps.
Terrain is more complex than most people think when considering the optimal density for a specific use and design. Using anything over 512x512 starts to significantly increase map file size etc.
Externally, yes, but there will be issues if you attempt to do this in the editor.
Even if you use level streaming you are limited to the maximum world size.
I assume you are referring to LOD staticmesh sets. No. Using the existing terrain system is definitely the most flexible and optimized.
The issue to keep in mind with using DEM data of any sort is that unless it is better than 1/6th arc-second it is almost useless for detailing in-game. 1/3 arc-second and larger might be fine for flight sims and google earth, but it is too low for FPS/RPG etc. games.
It should also be proper 16-bit data.
... UE3 automatically applies LOD to terrain inherent in a map when it's published ...
... it should be reasonable to stitch sections of terrain together.
LOD is user adjustable in the Terrain settings in the editor. These must be used correctly however, as there are specific ratios of MTL and MCS.
I recommend not attempting to match tiled terrains together in the editor. There are numerous reasons why specific setups work and others don't.
For specific map designs, a dual terrain setup can be used for optimization, which incorporates a high-density central play area terrain and low-density border terrain.
Otherwise stick with a single properly chosen terrain setup.
It does seem that UE3 isnt good at making big worlds since its games were built with average map size online play in mind. Theres a lot of techniques to use like the zone and streaming whatnot, so far im testing a 1024 by 1024 terrain (probably gonna split it into multiple terrain if that is a better technique rather than one big terrain), making lakes and hills.
So long as your world is between 10km x 10km to 20km x 20km you are fine.
Beyond this you would have to send players to another independant map, which will incur load times.
Level streaming is limited to the maximum world size.
I don't recommend splitting terrain, this won't achieve anything over the existing component culling and LOD optimization methods already implemented.