Terra Cognita – Organic Map Discovery

In vanilla EU5, the fog of war is mostly static. Terra Cognita changes that – countries gradually learn about the world through their neighbours, trade partners, allies, and enemies. The map fills in naturally over time, the way geographic knowledge actually spread historically. No more spamming steal maps.

Each month, the game picks one method of spreading map knowledge to try (like border sharing or trade). Border spread and trade are equally the most common, followed by subject relationships, and finally alliances and maritime contact. There is a chance to fail and do nothing.

How many areas a country can discover each year is governed by a new Cartography Limit. Every country starts with a base limit of 1 discovery per year. Two advances raise this cap:

• Universal Cosmographies (Age of Discovery, requires Exploration Fleets) – +1 limit, +10% Cartography Speed
• The Longitude Problem (Age of Absolutism, requires Cartographic Mapping) – +1 limit, +20% Cartography Speed

This gives a maximum of 3 discoveries per year. War discoveries are free and do not count against this limit.

Cartography Speed is a separate bonus that makes discoveries more likely each month. It comes from advances like Universal Cosmographies and The Longitude Problem, from being in the Age of Discovery, and from pushing your society toward Innovation. Several countries have their own unique advances that grant additional Cartography Speed: the Ottomans, Muscovy, the Dutch, France, England, Italian cultures, Portugal, and Castile all have country-specific advances.

Countries at war and colonial nations always get a chance to discover through war or colonial exploration before the normal monthly pick happens. If that doesn’t result in a discovery, the normal pick still runs — so these countries effectively get two shots per month.

Across all diplomatic channels discoveries are always based on what the partner knows – that is it will try to discover an area on the map that they know and you don’t. This means exploration is still required to uncover areas unknown by anyone. Additionally, for diplomatic channels (except for trade), areas outside your subcontinent/continent/uninhabited are increasingly harder to discover.

All channels (other than a small chance for trade), discoveries always border what you already know.

Your country slowly picks up geographic knowledge from its land neighbours. Discovery spreads outward from what you already know — you won’t suddenly learn about a faraway region, but you will gradually fill in the map around your borders. Up to 5 of your largest neighbours are checked each month.

If you’re an island nation with few or no land borders, border spread automatically falls back to maritime spread, so you’re not left out.

• Requires at least -25 opinion with the neighbour
• Does not work with countries you’re at war with
• Higher opinion improves chances

Coastal nations can learn about foreign lands across shared sea zones. The game picks up to two random sea zones where you have a maritime presence, then looks for a foreign port on the other side. This is how island nations like England learn from mainland Europe. A second sea zone is only checked if the first attempt didn’t discover anything.

Maritime spread only works with countries you don’t already share a land border with (those are handled by border spread).

• Requires ports and at least -25 opinion
• Does not work with land neighbours or countries you’re at war with
• Higher opinion improves chances

If your country owns a market center, you gain map knowledge from merchants trading in your markets. The more merchant power a trading partner has in your market, the better the chance of learning their geography. Up to 20 merchants across one of your markets are checked each month.

Most of the time, trade discovery spreads outward from what you already know, but occasionally knowledge can jump to completely disconnected areas. Trade spread has no distance penalties, so it’s equally effective regardless of how far away the source is. This channel grows stronger as the game progresses — in the mid to late game it becomes the most powerful channel, and discoveries made through trade then propagate to neighbours through other channels.

• Requires owning a market center
• Requires at least -100 opinion (only truly hostile countries refuse to share)
• Does not work with countries you’re at war with
• Higher foreign merchant power improves chances

Overlords and their subjects share map knowledge with each other. Discovery spreads outward from what you already know, similar to border spread.

Overlords learn from their subjects faster than subjects learn from their overlords. Areas on a different continent from your capital are harder to come by.

• Requires at least 50 opinion
• Higher opinion improves chances

Friendly countries share map knowledge with you, as long as they aren’t your subjects or overlords (those have their own channel). Discovery spreads outward from your known map, similar to border spread. Distant areas are harder to discover. Up to 5 allies are checked each month.

• Requires at least 100 opinion and friendly relations
• Does not work with subjects, overlords, or countries you’re at war with
• Higher opinion improves chances

When you’re at war, you learn about the land you’re occupying. If you control enemy territory in an area you haven’t fully discovered yet, there’s a good chance your scouts, spies, and soldiers will map it out.

War spread is always checked first before the normal monthly pick. If it doesn’t result in a discovery, you still get the normal pick — so being at war gives you an extra chance to discover each month. War discoveries do not count against your Cartography Limit.

• Requires being at war and occupying enemy territory in an undiscovered area
• Opinion doesn’t matter — you learn by fighting

Colonial subjects gradually explore the land around their colonies. Settlers, hunters, and interactions with native populations slowly reveal adjacent uncharted territory. Colony exploration cannot discover areas in Africa.

Like war spread, colony exploration is always checked first. If it doesn’t discover anything, the normal monthly pick still runs.

• Only available to colonial subjects
• Does not discover areas in Africa

Portugal and Castile have a unique advance available in the Age of Renaissance — The Master Chart (Portugal) and The Seville Archives (Castile). These grant a Cartography Speed bonus but also protect their geographic discoveries from leaking to other countries through peaceful channels (border, maritime, trade, and alliance spread). Subject/overlord relationships and wartime discoveries are not affected.

This protection is very strong at first, but erodes over time through three stages:

• Stage 1 — 10 years after Iberian colonisation of the Americas begins (rumours spread)
• Stage 2 — 5 years after a Western European country embraces the Printing Press (maps are printed)
• Stage 3 — 2 years after a Western European country embraces Global Trade (the secret is out)