Here's a summary of how CivIV BTS 3.19 alters the start sites for all civs. (This post is an update of the information found in this thread: http://forums.civfanatics.com/showthread.php?t=252429, without which creating it would have been much harder.) Note that the code for each of these steps can be overridden in Python, and evidently is sometimes. I.e., I have seen peaks in the start site BFC on the Great Plains map.
In all cases, the "BFC" refers to the big fat cross of the city minus the start tile itself; the start tile itself is not altered by many of the effects. Where the BFC is mentioned but the start tile can be altered, I note that explicitly. When tiles are altered in steps 6-9, each tile in the BFC is checked once, with the order of the tiles being randomized.
CivIV BTS 3.19 Startsite Normalization Procedure
1) Add Rivers: If the start site does not have fresh water, checks to see if a lake can be added in the BFC (using the same code as step 3). If it can add a lake, it tries to add a river in a way that might or might not provide fresh water. (This river always starts in the SE corner of the start tile, but its direction can be any of the four.) If it sees it cannot add a lake, it tries to add a river in a smarter way instead, that always gives fresh water to the start site if it succeeds.
2) Remove Peaks: Replaces peaks with hills. This is done for all tiles in the 7x7 square area centered on the start site.
3) Add Lakes: If the startsite doesn't have fresh water yet, adds a lake somewhere in the BFC. A lake can be added to a non-river-adjacent land tile if all tiles adjacent to it are not water. The search order is the inner ring first, starting with the tile 1N of the start site (this is where most lakes will go). If a lake cannot be sited in the inner ring, the outer ring will also be attempted, starting with the tile 2N of the start site.
4) Remove Bad Features: Removes bad features (jungle and sea-ice) within 3 tiles of the start site. The chance to remove the feature is 50% for most tiles, but just 25% for resource tiles. In addition, removes all coastal sea-ice within 4 tiles of start site.
5) Remove Bad Terrain: Replaces sub-par terrain tiles with grassland. A "sub-par" tile for this purpose is any tile with natural yield of less than 2 food+hammers (that is, snow, desert, unforested tundra, although some of these terrain types will be acceptable with a resource on them). This is done for all tiles in the BFC including the start site. It is also done for all of the coastal land tiles, and 50% of non-coast land tiles, in the 3rd ring.
6) Add Food Bonuses: Tries to make the BFC have at least one (land) food resource or two seafood resources. In addition, if the site has no other resource tiles with 2+ food, and no floodplains/oases, the site will get one more food resource (of either kind). Resources added will depend on the tile type of the randomly-chosen BFC tile that the program gets to first, that can accept a resource:
7) Add Good Terrain: Tries to make sure you have at least four "good" tiles in the BFC, "good" being any tile with a natural (unimproved) yield of 2+ food and 1+ hammers. It will replace up to four non-resource flat land tiles with grassland forests, to try to make that true.
8) Add Extras: Tries to improve the BFC until it has at least 80% of the best start-site's value. (Not sure yet what this "value" is exactly, but presumably it is at least semi-reasonable.) It does this in three steps, checking after each change, and stops when/if it reaches 80% value, or when it runs out of stuff to do:
a. Add a sprinkling of extra forest to some of the BFC tiles that are forestable. Tile must be non-resource land tiles, without a current feature. Although all tiles are checked once, there is a random chance that any tile will be skipped over that increases with the number of successfully-added forests. Hence, even in a low-value site with all 20 BFC squares able to take a forest, this step still won't forest more than 2 or 3 tiles on average.
b. Add certain kinds of food or production resources to BFC tiles, until the site has four or more resources, with seafood counting as 2/3 of a resource. The resource added to a tile will depend on the starting tech and terrain; in a normal ancient start it is this:
c. Add a forest to every BFC tile that can have one.
9) Add Hills: Tries to make sure the start site has enough hills. If there are less than 3 hills in the BFC including startsite, it will add hills until there are three. Hills can be added only on land, not already a hill, not floodplains or oasis, and if a resource tile, the resource must be allowed on hills.
In all cases, the "BFC" refers to the big fat cross of the city minus the start tile itself; the start tile itself is not altered by many of the effects. Where the BFC is mentioned but the start tile can be altered, I note that explicitly. When tiles are altered in steps 6-9, each tile in the BFC is checked once, with the order of the tiles being randomized.
CivIV BTS 3.19 Startsite Normalization Procedure
1) Add Rivers: If the start site does not have fresh water, checks to see if a lake can be added in the BFC (using the same code as step 3). If it can add a lake, it tries to add a river in a way that might or might not provide fresh water. (This river always starts in the SE corner of the start tile, but its direction can be any of the four.) If it sees it cannot add a lake, it tries to add a river in a smarter way instead, that always gives fresh water to the start site if it succeeds.
2) Remove Peaks: Replaces peaks with hills. This is done for all tiles in the 7x7 square area centered on the start site.
3) Add Lakes: If the startsite doesn't have fresh water yet, adds a lake somewhere in the BFC. A lake can be added to a non-river-adjacent land tile if all tiles adjacent to it are not water. The search order is the inner ring first, starting with the tile 1N of the start site (this is where most lakes will go). If a lake cannot be sited in the inner ring, the outer ring will also be attempted, starting with the tile 2N of the start site.
4) Remove Bad Features: Removes bad features (jungle and sea-ice) within 3 tiles of the start site. The chance to remove the feature is 50% for most tiles, but just 25% for resource tiles. In addition, removes all coastal sea-ice within 4 tiles of start site.
5) Remove Bad Terrain: Replaces sub-par terrain tiles with grassland. A "sub-par" tile for this purpose is any tile with natural yield of less than 2 food+hammers (that is, snow, desert, unforested tundra, although some of these terrain types will be acceptable with a resource on them). This is done for all tiles in the BFC including the start site. It is also done for all of the coastal land tiles, and 50% of non-coast land tiles, in the 3rd ring.
6) Add Food Bonuses: Tries to make the BFC have at least one (land) food resource or two seafood resources. In addition, if the site has no other resource tiles with 2+ food, and no floodplains/oases, the site will get one more food resource (of either kind). Resources added will depend on the tile type of the randomly-chosen BFC tile that the program gets to first, that can accept a resource:
forested tundra: deer
unforested flat plains: corn
unforested flat grassland: corn
unforested grassland hills: pigs
jungle: pigs
coast: clams
ocean: fish
Other tiles either won't exist at this stage in the procedure (i.e. unforested tundra, peaks), or won't accept any food resource (non-tundra forests reject all resources).unforested flat plains: corn
unforested flat grassland: corn
unforested grassland hills: pigs
jungle: pigs
coast: clams
ocean: fish
7) Add Good Terrain: Tries to make sure you have at least four "good" tiles in the BFC, "good" being any tile with a natural (unimproved) yield of 2+ food and 1+ hammers. It will replace up to four non-resource flat land tiles with grassland forests, to try to make that true.
8) Add Extras: Tries to improve the BFC until it has at least 80% of the best start-site's value. (Not sure yet what this "value" is exactly, but presumably it is at least semi-reasonable.) It does this in three steps, checking after each change, and stops when/if it reaches 80% value, or when it runs out of stuff to do:
a. Add a sprinkling of extra forest to some of the BFC tiles that are forestable. Tile must be non-resource land tiles, without a current feature. Although all tiles are checked once, there is a random chance that any tile will be skipped over that increases with the number of successfully-added forests. Hence, even in a low-value site with all 20 BFC squares able to take a forest, this step still won't forest more than 2 or 3 tiles on average.
b. Add certain kinds of food or production resources to BFC tiles, until the site has four or more resources, with seafood counting as 2/3 of a resource. The resource added to a tile will depend on the starting tech and terrain; in a normal ancient start it is this:
forested tundra: deer
unforested plains (flat or hill): marble
unforested flat grassland: corn
unforested grassland hills: pigs
jungle: pigs
coast: clams
ocean: fish
If the resource could not be placed because of forest, and the site is at least half water, the forest gets removed, and then a resource is added as per above.unforested plains (flat or hill): marble
unforested flat grassland: corn
unforested grassland hills: pigs
jungle: pigs
coast: clams
ocean: fish
c. Add a forest to every BFC tile that can have one.
9) Add Hills: Tries to make sure the start site has enough hills. If there are less than 3 hills in the BFC including startsite, it will add hills until there are three. Hills can be added only on land, not already a hill, not floodplains or oasis, and if a resource tile, the resource must be allowed on hills.
