Keystone is one of those mountains I know like the back of my hand.

It’s safe to say the proverbial back of my hand just got way more detailed thanks to FATMAP, a recently-launched mobile app that pairs ski maps with satellite imagery. This marriage of old-school, hand-drawn trail maps and modern-day tech creates a true-to-life 3D rendering of every trail, bowl and lift line at Keystone — plus nearly a dozen other resorts in Colorado and 27 total across the globe (think high-profile areas like Jackson Hole and Chamonix in France, one of three original test sites). The app was built by two GIS pros from the U.K., Misha Gopaul and Dave Cowell, and their professional attention to detail shows.

Here’s how it works at Keystone, the first Summit County resort to get the FATMAP treatment: The development team took everything you find folded into a bulky paper map — trail names, locations, difficulty ratings and nearby chairlifts — and layered a 3D skin overtop to create a fully-interactive, fully-mobile diorama of the resort, all 3,148 acres and 131 trails worth.

The concept is tantalizing, especially when you consider its birthplace in the Swiss Alps, where the majority of terrain is craggy, jagged and above treeline. Couloirs and chutes are the name of the game in the founders’ European stomping grounds, and so an incredibly tactile and accurate map was what they needed. When they headed into the backcountry for ski mountaineering races and training, accuracy became even more important for pre-trip planning. Take those in-field benefits and add a mouth-watering price tag of $0 — yes, the basic FATMAP app is free for iPhone or Android — and this app is a must for any ski quiver, right alongside powder skis and an avalanche beacon (plus the know-how to use all three).

But does the 3D satellite mapping concept translate to Colorado ski areas like Keystone, where terrain is split about 60-40 between tree-lined runs and hike-to bowls? Is it user-friendly? Or, like U.S. FATMAP manager Matt Doyle told me, is it still tailored for expert skiers more interested in the backcountry than resort skiing? And, maybe more importantly, how useful is it for someone like me, someone who’s been skiing Keystone for two decades?

Field test

I took FATMAP for a test drive on a quiet Wednesday morning right before the end of January. I’d loaded the app to my Samsung Galaxy S6 the day before and downloaded the Keystone map. Plus one for FATMAP: Every map is loaded directly to your device, so the app itself only connects with 4G or whatever for updates and in-app features like GPS, run tracking and trail ratings. Aside from that, simply browsing the maps isn’t a major drain on your battery.

The map navigation is intuitive for anyone who’s used Google Maps or similar apps. You zoom in and out by pinching the screen, and the 3D maps are worth zooming in just to see the detail. It doesn’t show every last tree and pathway in the glades — satellite images are only so sharp, even when you upgrade to the high-res premium version — but that’s why you’re standing on the mountain, ready to drop.

Here’s something embarrassing: I still don’t know the names of every run (or even most of the runs) at Keystone. Like the streets in my childhood neighborhood, I know exactly where to go and about five different ways to get there, but I can’t always remember the names. It makes giving directions a pain, and it’s honestly a side effect of growing up on that mountain. Take me to Vail, and I’ll rattle off all of the proper nouns.

That said, the biggest benefit of FATMAP for a longtime Keystone rider is the depth of information. The main menu links to a bunch of features (run pitch, run tracking, etc.) highlighted by two main buttons: lifts and trails. From here, users can filter trails by difficulty — green, blue, black and expert or out-of-bounds — and browse lifts by where they are and what kind of terrain they access.

The benefits for newcomers are endless. If you’re the sort who likes to write down every run of the day before leaving the coffee shop, the complete and detailed menu of trails and lifts is an encyclopedia like no other. If you’re the sort who likes to see every run before it makes the itinerary, the 3D map with clickable trails and lifts is a godsend. Every run includes a detailed description, location, pitch, vertical feet and total distance in miles, plus a GPS option to show where it is related to your current location and a fly-by mode to slowly cruise down the 3D rendering. Pretty cool.

And that’s the rub. FATMAP is perfect for trip planning, but it’s not always the best for on-mountain browsing. Even with the download feature it drained my battery from full to 30 percent in just 2.5 hours, and the GPS tracking didn’t work as soon as I dropped to Labonte’s Cabin between Dercum and North Peak.

Still, I was impressed by just how intuitive and intelligent FATMAP is. I look forward to springtime, when the company will launch full maps for Copper Mountain and Breck. The back of my hand just got a whole lot smarter.