I'm afraid you misunderstood my diagram... I meant an indefinitely long diagonal corridor surrounded by rock. I'll make it clearer:
<snip>
I was afraid the tree structure wouldn't be able to efficiently represent that as one unit. A quadtree wouldn't, but if you can then that's cool.
It would from the B-line "vectorised" format. The tree structure was just the start but after merging
based upon adjacent tree-ends that are mergable (in a manner I could spend ages explaining, but won't[1]) that would end up with something not so much a classic quad-tree.
It would actually end up saying something like: "[1,1]->[4,1]:southwall; [4,1]->[14,11]:southeastwall; [14,11]->[11,11]:northwall; [11,11]->[1,1]:northeastwall", in whatever shorthand data format is required, and the ":southwall" being shorthand for what might (in reality) be something like ":{walkable,fliable}tozone<otheropenzoneID>" (when not enclosed in that direction) and ":southeast" maybe ":{diggable}tozone<undergroundzoneID>" (when that still is). Noting that this concept of mine has subtly changed since, anyway.
edit-added-footnote: [1] Not because you wouldn't understand, but because I know I'd spend ages on it, when you can probably work it out... sorry, just re-read what I said, and was afraid it looked a bit arrogant.