The size of X-Plane's global scenery is due to two factors:
- Some aspects of the scenery are very detailed, and this takes up disk space.
- Some aspects of the scenery are computed ahead of time, not by X-Plane, and this takes up disk space.
A few data sources make the scenery pretty big:
- The scenery is generated from 90m SRTM data for the entire world.
- US scenery includes the entire US road grid.
- Coastline data is fairly detailed in the US and for most oceanic coasts.
The other source of file size is that X-Plane precomputes aspects of the scenery to reduce load on X-Plane while you fly. (We do this to improve frame rate.) In particular, the scenery uses a class of algorithms that are expensive to compute, and thus we compute them ahead of time and save the result (using up file space).
- Land class changes not on a regular grid, but along iregular polygonal boundaries following the shape of the terrain. This produces much more natural terrain transitions in the mountain, but requires more storage.
- "Auto-Gen" forests and buildings don't follow a grid - rather, they fit into the irregular spaces made by vector roads and water. This fitting process is too slow to run inside X-Plane, so every auto-gen building and forest area must be pre-computed and saved.
One interesting side effect of saving the end results of building locations ("pre-placement", what X-Plane does) rather than the formula for how to place them ("auto-gen", what FS X does) is that we can make the placement algorithm for buildings significantly smarter without any increase in file size or decrease in frame-rate. Having already eaten the cost of putting every building into the DSFs, we can make those locations better without hurting performance.
That algorithm to pre-place buildings also can have access to source data that isn't available in the final DSF, allowing for effects that might not be possible in an auto-gen system.