I'm looking into seamless maps. Not sure if its the correct term but it basically means I don't have any loading screens or map transitions.
The term "open world" is also used, but can also mean "go anywhere, do anything, at anytime".
To avoid confusion i prefer the term "seamless world".
First off, if the world is small enough,you can fit it all in RAM at once, then just load the whole shebang at program / game start, and you're done.
If its too big for that, you split it up into what's commonly referred to as "terrain chunks". A section of the the world map (usually square), which is loaded or generated as needed into a "chunk cache" - a memory pool. Chunk size varies by title. Caveman 3.0 uses 300 foot (~100 m) chunks. A chunk basically contains all the info you need for drawing and collisions for that part of the world map, plus info required to implement least-recently-used (LRU) discard of old chunks.
For example, a "terrain chunk" in Caveman 3.0 consists of:
1. a list of 25,000 renderables max (trees, rocks, plants, etc) including 4 interleaved ground meshes (one per texture tile).
2. 1 instance buffer of 22,500 verts, and 5 instance buffers of 7000 verts for drawing instanced plants,
3. a 2D collision map at a resolution of 1 foot.
The list of rendrables, the ground meshes, the instance buffers, and the collision map are all generated in real time on the fly from underlying data. The underlying data consists of a world map, with squares 5 mile across. this determines elevation, vegetation, and water. Elevation and water are used by the height map function, which is used to generate the ground meshes. vegetation type and generic "plant maps" (implemented as sparse matrices) determine the location of trees and rocks and such. Instanced plants use a generic "pattern map" to determine scale, rotation, jitter, etc based on their location n the chunk. When an object is added to a chunk, its also added to the collision map for that chunk. The entire world is randomly / procedurally generated. The same sort of thing can be done with a static hand edited world paged from disk.