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It is possible to do land reclamation by placing a ship containing engineers on an ocean or lake tile, and then ordering those engineers to transform the tile into swamp.

However, I have noticed that not all water tiles can be transformed into swamp. Sometimes the transform to swamp button for the engineers is blanked out. From experience, only some ocean/lake tiles next to a land tile can be transformed; it is not possible to transform an ocean tile in the middle of a vast ocean into land.

What are the exact conditions under which an ocean or lake tile can be transformed into swamp?

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Terrain transformation requires the following two conditions to be met:

  1. The tile is surrounded by at least N land tiles.

The number N is determined by the current game's ruleset, specifically the option ocean_reclaim_requirement = 30%. By default, since the game defines that each tile borders 8 other tiles, this is equivalent to needing 3 adjacent land tiles. In order to transform an arbitrary tile in the middle of the ocean, you would need to transform at the minimum a 3-tile wide bridge of tiles towards it, for example in a pattern like this:

Technique to polderize an ocean

  1. The tile needs to not be in the permafrost region of the map. These are any tiles that are frozen at the start of the game. So snow or ice tiles.

There's another map parameter thaw_requirement that sets this. By default the value is 101, which does not allow thawing or freezing tiles via terraforming.

Also see the relevant wiki page, or browse the code over at github: the relevant section is the function terrain_surroundings_allow_change in /common/map.c, and the function can_reclaim_ocean.

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  • What software did you use to create the figure?
    – Flux
    Mar 8, 2021 at 19:02
  • @Flux It's just a screenshot of a spreadsheet, using 'no borders' and centered text.
    – aphid
    Mar 9, 2021 at 9:00
  • Regarding "the tile needs to not be in the permafrost region of the map": are you sure about this? I used to transform the entire polar region into plains and grassland.
    – Flux
    Mar 9, 2021 at 16:30
  • @Flux I'm quite sure. The default value for thaw_requirement (percentage of non-freezing tiles around a frozen tile that's needed to be able to thaw it) is 101% (never). I suppose you could make your own modded terrain.ruleset file (see freeciv.fandom.com/wiki/Editing_Terrain). I don't think you can easily do this for a game that's already in progress though.
    – aphid
    Mar 11, 2021 at 9:44

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