The bottom up approach means starting at a more localized level, such as the settlement where a story takes place. We may decide some land features are at various distances from here and any impact this causes. Later, we decide what sovereign power type exists, even if this decision changes previous work on the settlements and geography. We might decide that an adjacent sovereign power is needed and invent it, tailoring it to our current needs. We may not have decided which hemisphere this place is or the continent, which means we may not have considered climate much. This matters more with things that don’t move (places) because species and animals can be relocated and found in multiple places.
An advantage of the bottom-up way is that it forces us to focus on our immediate setting needs, like a city we’ll use. We may skimp on things we won’t need yet, saving time. Story can also inspire our inventions. For many of us, it’s also easy to envision a city that has specific features or a mood, and we may be unsure how outside factors could contribute, though hopefully this series has changed that.
One problem with this approach, for places, is that the overall picture indelibly impacts the local one. We might decide there are mountains and lush vegetation in a certain direction, then choose a hemisphere only to find out that those mountains would prevent that vegetation. Similar factors could render our work less sensible. Another disadvantage is lack of scope. When we only invent what we need and a little more, our world can seem too tightly focused and like there isn’t a broader world out there. This may not matter in a short story or one where characters don’t travel, beyond mentioning other places or peoples and things influenced by them, but for more epic uses, we need to at least hint at a broader world.