A colony sees a union of discs. Every structure looks a little way; a Listening Tower looks a long way. Take the Tower down and the view narrows again.
That is derived state and it should stay derived. It is cheap to compute, it is always correct, and storing it would mean a cache to invalidate every time anything is built, upgraded or destroyed.
But it produced a bad rule. If what you may build on is what you can currently see, then demolishing a Tower silently revokes permission to build on ground you surveyed a month ago — and worse, the map forgets terrain you personally went and looked at. Neither is how memory works.
So there are two facts, not one
Visible is what is being watched right now: derived, every frame, from what is standing. Explored is what this colony has ever seen: history, and history does not get smaller.
The rules use explored. Surveying ground earns you the right to build on it, permanently. The picture draws explored-but-not-visible ground dimmed, because you remember the land and nothing is watching it.
One bit per tile
61 x 61 tiles = 3,721 bits = 466 bytes
240 x 240 = ... = 7.2 KBSmall enough to keep on the colony row and not think about again. The fold happens in one place — the sync that runs when a colony is read — and it only writes when the frontier actually moved, so a player staring at a colony they have fully explored generates no writes at all.
The part that turned out to be a picture problem
The first attempt drew unexplored ground at a colour eight points away from the stage behind the plot. Eight points a channel is nothing. Unsurveyed ground and empty space became the same colour, the plot lost its outline, and the map stopped looking like a map with a frontier and started looking like a lit patch floating in a void.
Unknown ground has to be darker than the space around it. That is what draws the plot’s silhouette, and the silhouette is the thing that says there is more of this, you just have not been there. It also has to keep its grid: a tiled dark area reads as terrain you have not surveyed; an untiled one reads as nothing being there.
There is a follow-on that only appeared once the terrain palette got its colour back. With saturated ground, dimming remembered land by darkening it makes it look like night. It has to lose its chroma first and its brightness second — memory desaturates. Live ground then becomes the only coloured part of the map, and colour is a far stronger edge than brightness.