A Colorado Trail relay

The Colorado Trail runs about 490 miles from Denver to Durango. In July 2025, my wife Kelly and our friend Bex did a unique relay of the trail; Kelly ran the wilderness sections, where bikes are not allowed, and Bex mountain biked the rest. They carried a Garmin InReach with them, and those of us watching the adventure used the (kinda jankey but serves its purpose) share.garmin.com site to follow them when they were out on the trail.

I wanted a way to show their progress through the state¹, and help folks get a better picture of how the relay worked in a day-to-day sense (who was on the trail, how far they were going, etc). So I built a site to track their progress, https://www.gadom.ski/ct-relay/, and few of the bits of work to build the site seemed interesting enough to share.

Source data

The first thing I needed was the Colorado Trail itself, and that proved trickier than expected. There's plenty of folks' personal GPS tracks out there on the internet, but I wanted to use an "authoritative" source — you can't just trust anything you find on the internet, you know what I mean? After wasting some time on portals² like https://trails.colorado.gov/, I found my way to Colorado Parks & Wildlife's (CPW') ArcGis portal, and specifically their COTREX Trails dataset. Can't get much more authoritative than that!

The GeoJSON feature collection weighs in at 229MB with almost 100k features, and (surprise surprise) there's no single "Colorado Trail" feature. However, there are many (229) segments in that dataset that include a Colorado Trail Segment N name. A little bit of SQL (via DuckDB and its excellent spatial extension), a bit of manual cleanup with QGIS, and I had myself a single line string of the Colorado Trail³.

Tracking data

Now I needed to overlay their progress on that line. There's no pretty way to say it — I (lightly, legally⁴) reverse-engineered Garmin's API⁵. share.garmin.com uses a WebSocket API to ship data for the current viewport to your browser. To get Kelly and Bex's locations, all I had to do was open up their live tracker, zoom to the day's points, open up Developer Tools, and grab the latest UpdateUser message from the WebSocket connection. Easy enough, right?⁶

Putting it together

The rest was straightforward — if we were living in a different timeline where I actually know how to write frontend code. The rest was pretty hard but a learned a lot 🤣.

• The site: https://www.gadom.ski/ct-relay/
• The code: https://github.com/gadomski/ct-relay/
• Just the Colorado Trail stuff (w/o the relay): https://github.com/gadomski/colorado-trail.