Last time I played Dwarf Fortress, I tried to reroute a river through a
dolomite block canal, but two tiles produced a "to near to edge" warning while trying to build the walls.
That shouldn't happen since tiles are underground and next to an edge wall, not free edge.
After testing for some time, I figured out that it should be a new bug. As far as I know, building walls within four tiles near an free edge (above ground) is forbidden, though it works undergound next to an edge wall.
Bug: If you channel natural floor four tiles next to free edge or if floor is missing, (due to river, cut tree over hall, etc.) the tile one z-level below is marked as next to free edge and walls cannot be build on it either. This can continue for multiple z-levels if you continue to channel natural floor. Due to some reason ice floors are regarded as missing natural floor and can create the same problem even if not channeled.
I guess creating new "natural floor" would work, but I don't even know whether "natural" floors can be built.
Any idea how to solve this problem?
Best Answer
According to the DF wiki, "You're not allowed to wall within five squares of the edge of the map... but this rule has more loopholes than the US federal income tax code."
Although you can channel to the second square/tile from the map edge, this doesn't solve your problem since the last (map edge) tile still blocks your outgoing water flow/canal. You can either have your dwarves dig a hole downward at that point, hoping that a cavern layer under that hole is open to the map edge so the water drains out there, or else smooth and then fortify an edge tile prior to filling your drain canal with water. Liquids such as water and magma will flow through fortifications; so you don't need to actually channel or dig the last tile.
Some tactics allowed by DF that may apply to this problem:
1) You can build floors all the way to map edge. A floor built over a ramp that is then removed, removes the ramp.
2) You can smooth un-mined stone and then fortify it even if at the map edge.
3) You can build a drawbridge at map edge. A raised drawbridge acts as a wall.
4) You can channel to the 2nd tile from the edge.