Every tile in Dwarf Fortress is described as Outside or Inside, Light or Dark, and Above Ground or Subterranean. These keywords can be observed from the menu, when the cursor is located over any single tile. Currently, only the 4 following combinations are possible:
- Outside Light Above Ground - outdoors
- Inside Light Above Ground - outdoor tiles covered by constructions
- Inside Dark Above Ground - obsidian towers built by goblins
- Inside Dark Subterranean - underground caverns
The entire fortress site starts out as Inside Dark Subterranean. After that, a ray for Outside and a ray for Light Above Ground falls straight down from the highest z-level for every (x,y) position on the local map, changing each tile's modifiers until something that blocks that particular "ray" is encountered. Due to this behavior, any of the previous three tile combinations will never be encountered in the same (x,y) position on a lower z-level than a higher numbered combination.
 Outside vs Inside
This attribute is determined by whether there is a roof of some kind over a tile. Weather only affects tiles which are Outside. Almost any kind of geometry will block the Outside ray including natural and constructed walls, floors, stairs, and fortifications, and open and closed bridges, floor grates, floor bars, and hatches. A tile which is Outside will always be Light Above Ground, but an Inside tile may be either Light Above Ground or Dark Subterranean.
 Light Above Ground vs Dark Subterranean
Plants such as grass, trees and shrubs will grow on sand or dirt where it is Light Above Ground. This attribute also helps determine which types of crops can be planted on farm plots that have been constructed on top of the tile in question. Dwarves who spend excessive amounts of time in Dark Subterranean areas may suffer from cave adaptation. When the "Soldiers Can Go Outdoors" or "Dwarves Stay Indoors" rders are in effect, civilian or all dwarves (respectively) will cancel their current job if they find themselves in an Light Above Ground area. Very few types of geometry block the Light Above Ground ray: only cliffs (natural walls), ground (natural floors), and natural stairs (those carved out of cliffs or ground). Tiles which are Dark Subterranean will always be Inside, however Light Above Ground tiles may be Inside or Outside. Once exposed to the evil, everlasting light that bathes the world, there is no way to make a tile dark again.
Additionally, Above Ground tiles within 5 tiles of the edge of the map will not allow certain buildings or constructions to be placed. It is, however, possible to build them in an underground area and subsequently channel out the roof.
- On a hot+ map, plants which have a constructed roof above them will stay green, and water which has a constructed roof above it will not evaporate.
- Make sure all key fortress areas are Dark Subterranean if you wish to use the "Dwarves stay indoors" order.
- To help dwarves avoid cave adaptation, use Outside Light Above Ground in high traffic areas, such as a meeting hall. Also, if you use a planned bedroom design for your living quarters with a single main entrance, you can use channels to make the entrance Light Above Ground to force all dwarves to pass through it when they go to their rooms to sleep.
- Although obsidian made from magma (and nigh-floating in the sky, perhaps) will not make tiles Dark Subterranean, it will prevent tiles below it from becoming Outside Light Above Ground. If you build bridges over it, it, and all the once-sky tiles below it, will stay Inside.