Trail Bubble

The bubble you see is an estimate, built from years of historical Appalachian Trail data — when hikers tend to start, and how fast the pack tends to move. That means the map is never empty: there is a believable bubble from day one, even before anyone is sharing.

When hikers do share where they are, the estimate gets sharper in that stretch of trail. The label in the corner of the map is honest about which you are looking at: “grounded in N real hikers” where people are actively sharing, and “modeled estimate” everywhere else. The more people share, the closer the bubble gets to the real thing.

Trail Bubble is designed around a single principle: the model only needs to know how far along the trail you are. It does not need — and will never request or store — your latitude and longitude.

Here is exactly what happens when you tap Share my location:

1. Your browser reads a GPS fix. This happens entirely on your device.
2. A local calculation finds the point on the trail centerline nearest to your GPS fix and records the cumulative distance — your trail-mile — to the nearest hundredth of a mile.
3. The raw GPS coordinates are discarded immediately. They are never written to memory, state, or storage of any kind.
4. The only value transmitted to the server is that single trail-mile number, along with the trail name, your hiking direction, a timestamp, and your anonymous device identifier.

There is no table of GPS coordinates on any server. There is nothing to leak in a data breach because coordinates were never stored. The guarantee is structural, not just a policy: the server literally cannot receive your latitude or longitude because the client never sends it.

Your position is also never shown as an individual dot. Every reading is folded into an aggregate density curve that represents the whole class. You appear as part of the crowd, not as a trackable point.

When you share a position, the server records the following five fields and nothing else:

• TrailThe trail you are hiking, e.g. "Appalachian Trail".
• DirectionNOBO (northbound) or SOBO (southbound).
• Trail-mileA decimal number, e.g. 652.4. No coordinates.
• TimestampThe date and time you shared.
• Anonymous device IDA randomly generated string created on your device. It has no connection to your name, email address, phone number, or any account.

There are no accounts. There is no sign-up. There is no way for Trail Bubble to know who you are, because the app was deliberately designed to never ask.

The one exception is the contact form: if you choose to message us, we store the email address you provide so we can reply. That email is used only to respond to you and is deleted on request. It is never linked to your shared trail-mile readings.

Sharing is entirely opt-in. You can browse the bubble indefinitely without sharing anything. If you do choose to share:

• Pause or stop sharing at any time using the Stop sharing button in the Share sheet.
• Delete all your shared data using the Delete my shared data button in the Share sheet. This permanently removes every position record associated with your anonymous device ID from the server. The action cannot be undone.

You can find both controls by tapping the Share button in the top-right corner of the viewer.

Trail Bubble v1 is a web app. Web browsers do not support continuous background location tracking, and Trail Bubble does not attempt to use any workaround to simulate it. Sharing is a deliberate foreground action: you open the app when you have a cell signal, tap Share my location, and the app reads your position once. Nothing runs in the background. Nothing drains your battery while the app is closed.

In practice, most AT hikers check in when they reach a town, a road crossing, or a high point with signal — exactly the rhythm the app is designed for. Even one or two check-ins per day per hiker is enough to sharpen the model meaningfully.