Flight Simulator's earth model is designed to be entirely populated with scenery. The scenery doesn't have to include a lot of detail everywhere, but everywhere a user can go, there should be some sort of scenery.
The concept of synthesized scenery based on “seeds” is used to define large areas where precise scenery definitions aren't yet available, but broadly defined satellite data is. For example, using seeds, you can specify a bordered area and define it as a desert. The graphics system then creates a desert within the area that you specify.
You can specify well-defined and highly detailed areas quite easily using standard graphics language systems to create object scenery. You can use textured polygons, lines, and points to model an area to reflect reality to the degree that you choose.
Because of the respective advantages of these two approaches, Flight Simulator 98 uses two separate graphics systems—a scenery synthesis system called the seed system that populates the whole world with synthesized scenery, and a graphics language-based scenery system called the object system.
These two systems operate simultaneously with very little interaction between them. It’s as though a movie is being projected with two projectors overlapping on the same screen; part of the picture is being generated by the seed system, and part by the object system.
Flight Simulator includes a seed database that populates the entire earth with scenery. You can either draw scenery over the seed scenery that is already there or use a special seed command that “punches a hole” in the seed scenery turns off any given seed area so that you can fully design an area using the object system.
Ideally, all seed synthesized scenery will be replaced with object scenery. But the area to be covered (that is, the world) is so huge that it will take more storage capacity on disk drives and a lot of design work to fully fill the world with objects. Flight Simulator 98 replaces a lot of seed scenery with object scenery, but the majority of the earth is still populated with synthesized seed scenery. For now, both seed and object systems have their place. These two graphics systems combine to form the BGL graphics language.