With Outerra the situation is a bit different, as will be the results.
OT already comes with the whole planet modeled using global Earth elevation data (90m resolution). Climate support is being added, that will lead to having Earth without the effect of civilization. Trees will be placed automatically using coverage maps and probability models.
Atop of it will come a civilization layer that will be predominantly vector based, and will allow to overlay nature with man made structures. Fields, pastures, orchards, as well as roads, city blocks (handled by generators or manually populated) and so on.
These vector data allow for continuous fractal refinement and continuous detail down to the ground level, which is what we are after.
Custom DEMs can be imported too, for example to refine a local area.