---
title: "GAPTCHA"
description: "GAPTCHA: Find, scan, win"
url: https://www.joekarlsson.com/work/gaptcha/
---

# GAPTCHA

Find, scan, win

2017 · with Sean Nakamura, Tony Gaskell, Brandon Baris and Joelle Torneros

[See it live →](https://gaptcha.joekarlsson.com/) · [View on GitHub →](https://github.com/JoeKarlsson/GAPTCHA)

GAPTCHA (Geo-Augmented Place That Checks Authenticity) is an augmented reality scavenger hunt. We printed square markers and left them around for people to find. Scan one with the web app and a green 3D arrow shows up over your camera view, pointing at a hidden checkpoint. An arrow gives you a direction but not a distance, so the idea is to collect bearings from a few markers and walk to where they cross. When you think you're standing on it, you tap Did I Win.

Near the original spot in Honolulu you hunt the hackathon checkpoint. Anywhere else, it hides a demo checkpoint 100 to 300 meters from you, and a live feed shows simulated hunters playing alongside you.

The photos below are from that weekend.

## Watch it

This demo plays itself. To hunt for real, open gaptcha.joekarlsson.com on your phone with a printed marker. [Open it full screen →](https://gaptcha.joekarlsson.com/)

[Embedded: GAPTCHA, playable in the browser](https://gaptcha.joekarlsson.com/?embed)

## Screenshots

![The GAPTCHA homepage: "GAPTCHA, Find. Scan. Win." and a Scan a Marker button over a green-tinted brick wall, next to an iPhone showing a GAPTCHA marker](https://www.joekarlsson.com/images/projects/gaptcha-home.webp)

The homepage

![Testing GAPTCHA at a hackathon table: Brandon Baris, grinning in a red shirt, holds out a phone showing the camera view while a teammate behind him holds up a phone with a GAPTCHA marker and another phone running the AR view, with laptops and other teams all around the cafeteria](https://www.joekarlsson.com/images/projects/gaptcha-hackathon-testing.webp)

Testing the AR at the hackathon

![A finger pointing at a printed GAPTCHA marker on a concrete floor: a black square with a black triangle in the corner and gaptcha.herokuapp.com printed underneath](https://www.joekarlsson.com/images/projects/gaptcha-find.webp)

Find a marker

![Two people jumping in the air with confetti falling in a school cafeteria full of hackathon tables, an AT&T "Mobilizing Your World" banner behind them](https://www.joekarlsson.com/images/projects/gaptcha-win.webp)

Win

## How it's made

Node.js · Express · A-Frame · AR.js · Leaflet · AT&T M2X

We built it at AT&T's hackathon in Honolulu in March 2017. My part was the server, the direction math and the homepage: an Express route that took your phone's position and worked out the bearing to the checkpoint with the Vincenty formula, and a live map from AT&T M2X, which also held the checkpoint so it could move during the game. Heroku and M2X are long gone, so in November 2026 I moved the checkpoint logic into the browser.

AR.js finds the printed marker in the camera, and the browser works out the compass bearing from your GPS position to the checkpoint and turns the arrow toward it. Everything runs in your browser now: the checkpoint, your tries and your last 50 scans live in local storage, and Did I Win checks whether you're within 20 meters.

_Diagram: How a GAPTCHA hunt works: you scan a printed marker facing north, your phone's location and the checkpoint give a compass bearing, a green 3D arrow on the marker turns to it, and after scanning a few markers you walk to where the arrows cross and tap Did I Win, which wins within 20 meters_

[See it live →](https://gaptcha.joekarlsson.com/) · [Get the code on GitHub →](https://github.com/JoeKarlsson/GAPTCHA)

[← All works](https://www.joekarlsson.com/work/)