Hexcrawl: Finding Features

A common element of Old School Role-Playing (OSR) is the hexcrawl, where characters explore a map split up into discrete six mile hexes, each with some number of features. The idea is that this format creates an easy way for a DM to run a "sandbox" where the characters travel from hex to hex, encountering whatever is there. But how do you determine whether the characters encounter anything? A six-mile-wide hex is quite large, encompassing around 31 square miles. As anyone who has been hiking even once can tell you, that's a lot of area. That encompasses entire national parks, entire towns or cities in some cases. The chance of your characters encountering a particular ancient temple somewhere in this area is likely quite minuscule.

Many DMs simply ignore this problem. Much of the point of the hexcrawl is so that characters can explore and encounter things with regularity. As such, upon travelling through the new hex they have a 100% chance of encountering the hex's feature, because the DM wants them to. There is nothing wrong with this approach. But some may desire a bit more verisimilitude.

The first thing that must be considered is the size of the feature. Larger things are obviously easier to spot. Also, a ruined castle on the side of a mountain will be much more visible than one on the ground. 

Terrain must also be considered. On an open plain in Kansas, buildings if any height stick out like a sore thumb. In the density of the Amazon rainforest one would have more difficulty finding a building. So visibility is a factor as well.

Let's consider the two questions we want to answer: 

1 - What is the chance, while passing through a hex, that characters come across a particular feature?

2 - How long would it take a party travelling together to search a hex with reasonably completeness?

The answers to these two questions lie intertwined in the answer to "how much area can the party see at once?" This depends mostly on the terrain, but also upon the size of the object being spotted. Various densities of forest can have drastically different levels of visibility. And this can vary even within a few minutes' walk. So our attempt to create workable rules for this must compromise simplifications with realism. 

Other potential cases, such as whether or not the structure is excessively large and atop a hill, or right by a road or trail, will be ignored here. Some features would be found automatically. While it is possible a roadside structure would be missed, such a probability would be assigned separately. Also, we will be ignoring the characters perceptive ability. There are a lot of issues with the modern roleplaying game's use of "spot checks" and "perception checks" which will be explored further in another article. 

Let's consider a simple example: an area of hills, open terrain, and few trees with average visibility of 1000 feet. A group of characters will thus be able to see 3,140,000 square feet at a time, or 0.11263 square miles. This is out of around 31.77 square miles per 6 mile hex. So at a single time a group can see around 0.354% of a 6 mile hex, with such visibility. 

This is just stationary visibility, though, and next to useless for what we want to know. Now let's say the group crosses the hex. In doing so they would carve out a "path" of visibility 2000 feet wide and roughly 6 miles long. Extreme accuracy really isn't important here so let's round to 5,000 feet to a mile, giving us 2.4 square miles of area seen by the characters as they cross the hex. This means, assuming a straight line crossing of the hex at opposite midpoints, and perfect spotting, a 7.55% chance of spotting a particular feature in that hex. For lower visibilities, like 500 feet, the chance become lower, dropping to 3.77%. Not very likely indeed!

The biggest issue here, however, is our assumption of a straight path. Moth paths taken, especially varying by terrain, will not be straight, and thus will cover slightly more area. It is possible to assume that twice as much ground is covered by a meandering path, giving us a convenience 15% probability which is close to 1-in-6. Close enough that it has become my personal rule that a 1-in-6 chance for hexcrawl features is my base. In hills, it can raise to 2-in-6. In plains, to 3-in-6. For structures atop hills, it can becomes 4 or even 5-in-6. As stated before, the size of the structure must also be taken into account. The chance of finding a feature depends on so many factors that the mathematical calculation is useful only to give us a vague baseline for reckoning, which can then be worked into a simple and modifiable game mechanic. 

The question "is all this worth it?" must also be considered again. The added bookkeeping required to run this method at the table might not justify the benefits. This does allow extra mileage to be gained from a single area, and the characters never really know what is out there, if a nearby hex contains some still-undiscovered dungeon. But one must track which features have been found, perhaps with checkmarks by each feature on the map key. Perhaps rolling once for each feature to see what is encountered can work. 

As such, here is my proposed method for handling feature discovery in a hexcrawl:

- Arrange the features in each hex in descending order of likelihood to be encountered.
- Start with the most likely, and roll to see if the characters spot it.
- If they do, that is what they find at first, and they may either explore it further, or continue on.
- If they do not encounter the first feature, or ignore it, proceed to the second, and repeat.

This means the most obvious features are discovered first, as one would expect, and attract the most attention. But continuing to explore yields more and more obscure features. I would suggest no more than three major features per hex, and often an average of less than one. However, this is my personal opinion, and depends entirely on the density of your world, the size of your map, and the amount of content you are willing to produce. I might also suggest mentioning the possibility for secondary and tertiary features to your players, or even giving a "freebie" discovery, perhaps on their way back to town. Or you could simply let them come across it naturally. It depends heavily on how transparent you are with your players about the presences of hexes as a game mechanic.

As for searching an entire hex, for 500 feet of visibility (a reasonable amount to spot small features that might be hard to find, hence the searching), a 1,000 foot diameter path that is 3 miles long would allow 1.788% of a 6-mile hex to be searched per hour. This would mean it would take almost 56 hours to thoroughly search the hex, or an entire week straight of 8 hour days. However, this would mean a hex was entirely searched for any significantly-sized buildings visible within 500 feet. With hirelings, this process could likely be quickened, or its completeness enhanced.

So there you have it. For a quick summary, it might be a good idea to have a 1-in-6 chance of any feature visible within 1000 feet to be spotted. On open hills, this can be raised to 2-in-6. On open plains, 3-in-6 or even 4-in-6. And large structures on elevated ground would likely have a 5-in-6 chance of being spotted, at least for a party that is paying attention. Something that can never be taken for granted, to be fair....