D20 Mechanic: Exploration

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This page is an overview and detailed explanation of the exploration system used in T20, a unique subsystem designed by yours truly after many years of thought and design.


It goes something like this:

Hexes are six miles across, and have one primary terrain type. We'll call this a wheel. And yes, the distance across a hex varies (from face to face, vertex to vertex, vertex to face), but the point here is abstraction, so we don't care. Hexes are a wheel across, done.
Hexes are members of zones, which define potential creature encounters within its constituent hexes.
Hexes have a very important number, Travel Points (TPs). When you enter a hex, it begins at full TP, and as you make exploration checks against the hex, you lower its TPs if you manage to hit the hex.
Hexes have predetermined TP thresholds at which you can leave the hex and move to another.
Hexes have points of interest, which have TP thresholds you must reach before you can find the point of interest. Note that meeting the TP threshold does not mean you automatically find the PoI, just that you might.
Once you have discovered a point of interest, if you return to the hex in the future, you can automatically go there once you reduce the hex's TPs to that PoI's threshold.
Each adventuring turn at the hex level takes a compass, which is about 60 minutes (and is exactly equivalent to five hands); 24 compasses are in a cycle (which is a normal day; and also means that 4 compasses equal a watch).
When traveling in a hex, you may encounter obstacles. These are exploration challenges - cliffs, rivers, and the like. If you defeat the challenge, you continue. If you don't, the hex (probably) regains TPs as you are forced to find another way around (but see the next point).
Some obstacles may interact with the hex in other ways. For instance, a deep chasm that bisects a hex may indicate that, once encountered, the party cannot reduce the hex's TPs until it is defeated.

...and there you have it. That's the core mechanic of how this all goes down.


So this all came to me in a vision, so bear with me. We need to get some thoughts down on paper.

I think the TP threshold to leave a hex is probably going to be half its max TPs. That gives us room for putting things in "out of the way" places in a hex - if you're just moving on through, you're not going to find everything, but you will see a reasonable amount of the hex so that's probably a fair number.

Translating Old Concepts

Remember veer from Journey? That's gone. We're not doing that. Same thing with nodes and subzones and just all that crazy stuff. It was too damn much.

I'm not sure how we'll handle getting lost or turned around... thinking on it, because we've got a lot more mechanics and moving parts, I don't think it actually matters if the players know that they're lost. They can't say "oh, we just turn around" - to leave a hex, you have to reduce its TPs, no questions asked. That represents the idea of figuring out that you're lost, regaining your bearings, and all that jazz.

Notions on Hex Design

So hex design winds up being somewhat related to monster design. Because of that, we need some guidelines.

The big thing to keep in mind here is abstraction, abstraction, abstraction. If at any time you find yourself saying something like, "well, it depends where $FEATURE is in the hex," you are doing it wrong. We want exploration to be fun and engaging, but in order for it to be so there has to be a limit on the information the DM needs to know.

So the idea of a chasm bisecting a hex, preventing you from lowering its TPs at some arbitrary point unless you beat it? Yeah, that's a thing. But now that we've mentioned that...

Problem Case

Hex 0213 has a town on the south side and is bisected east-west by a deep chasm.

If you enter the hex from the S, SE, or SW faces, you'd expect to be able to find the town.

If you enter the hex from the N, NE, or NW faces, you'd expect to not be able to find the town.

How do we resolve this?

Quick answer: hexes are abstract, fool. You can totally divide a hex into conceptually smaller units without having to do all that much more work on the map. Just note that a hex is bisected, and done. This means we do need to track from what direction a party enters a hex, but honestly that does not seem like that much additional overhead to handle this particular conundrum.


One thing I do want to implement, because it seems important, is a hit point system to go along with this whole deal. Hit points recover at an astonishingly fast rate, which is fine, but in that design paradigm, we need another resource that depletes slower, or is at least harder to recover.

Example: Going to Goblin Wall

Those of you familiar with Journey will recognize this scenario.

As always, we have a band of intrepid adventurers heading out into the wilds in search of a dungeon known as the Goblin Wall. They have a vague idea of where it is - some forested region northeast of their hometown - but beyond that have no information. There is a road that leads north out of town, but at some point they will need to head into the wilds to find their destination.

They leave town. They are now in the town's hex, of which the town only occupies a small part. The hex is a level 1 plains hex. In order to leave the hex, they need to reduce its TPs to at least half of its current value. The presence of a road means they can't get lost, but they do have to actually travel.

They spend the first compass of their adventuring day reducing the TPs of the plains hex. They do so, and manage to hit the threshold necessary to leave. They enter the next hex north, a level 2 plains hex, and still have a road to follow.

It takes three compasses for them to reduce their current hex's TPs to the point where they can leave. During the second compass, they get a random encounter: a friendly farmer says hello and shares some local gossip. They're not anywhere near dangerous territory yet.

The next hex is a level 2 plains hex bisected NW/SE by a river. The bridge across the river was washed away recently, but when they reach the bridge's TP threshold after two compasses they find some carpenters from town who are working on repairing it. In the meantime, they have to ford the river to continue, which is a hazard encounter.


Once the river is dealt with, they continue, spending another compass to reach the traversal threshold. At this point they have spent seven compasses, so they decide to keep going. The next hex is a level 3 hills hex, and is near the zone representing the goblins of Goblin Wall. Here they may begin encountering goblins from there.

One compass into the hills hex, they get a monster encounter. The party's point rolls for the encounter type, and they succeed by 3: a preemptive attack, so in the transition to tactical, the goblins will place first, then the players.

Once combat is done, they decide to try to track the goblins back to their lair. The tracks head northeast out of the hex, off the road.

Two compasses later, they have hit the traversal threshold, and head NE, into a forest hex. At this point, they have spent ten compasses.

One compass into the forest, they have another monster encounter. Point rolls for encounter type, and fails by 7: they are ambushed by a group of goblin scouts. In transition to tactical, the players will place first, then the goblins, who will also get a surprise round.

After the battle, the party decides that they need to rest (they're low on healing surges).