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Slope Angle Shading in GaiaGPS and Caltopo

Slope angles are useful for estimating level of effort
Slope angles are useful for estimating level of effort

One of the hallmarks of good hiking and backpacking navigation is picking trails or routes that reduce the level of effort required to climb mountains or descend into valleys. The steepness or slope angle of a trail is usually the best indicator of the level of effort required to climb or descend it. Steep slopes are harder and require more energy to climb than more gradual ones, even though the elevation gained is the same over the long run. You can also make educated guesses about what terrain conditions will be like on very steep slopes, since they’re often boulder choked or sheer slabs that make for slow hiking. In winter, slope angle is also a good indicator of potential avalanche terrain that is best avoided.

While you can quickly assess the steepness of a slope on a topographic map by recognizing tightly bunched contour lines, computing the actual slope angle is difficult to do on the fly, because you need to do some basic trigonometry to compute it. It also requires very accurate map measuring tools, which you’re not normally going to carry with you.

That’s where digitally computed slope shading comes in. Slope shading is a visualization tool provided in good navigation apps and planning tools like GaiaGPS (available for iPhone and Android) and Caltopo.com, which is a FREE web-based planning tool, best used with a desktop or laptop computer. It can colorize a map and help you evaluate route difficulty or danger, so you can compare alternative approaches or descents. Both GaiaGPS and Caltopo use the same color scheme for depicting slope angle (in fact GaiaGPS licenses many maps and data from Caltopo), where darker colors represent increasingly higher slope angles.

Caltopo Slope Angle Scale
Caltopo Slope Angle Scale

For example, hikers and backpackers will find that a 27-29 degree slope angle makes for tolerable, but steep hiking. Slope angles higher than that are much more strenuous and will really slow you down. Slope angles in the 35-45 degree range will be more like climbing than hiking, requiring the use of your hands as well as feet to ascend. For backcountry skiers and winter hikers, slope angles between 30 and 45 degrees are the most likely to avalanche when other factors are present such a slabs or weak layers.

Here’s a quick example to illustrate how you’d use the slope angle feature to evaluate the effort levels to climb a mountain using several different trails.

The three easiest trail to climb Mt Washington are the Jewell Trail from the west, the Nelson Crag Trail from the east, and the Crawford Path from the south. I’ve shown them here on a Google Terrain map created in Caltopo. I’ll show an example using GaiaGPS below.

The 3 easiest trails to climb Mt Washington
The 3 easiest trails to climb Mt Washington

Here are those same trails with slope angle shading turned on.

The three easiest trails with slope angle shading. Note how they avoid orange and purples slopes.
The three easiest trails with slope angle shading. Note how they avoid orange and purples slopes.

Here are the three hardest trails up Mt Washington, the Great Gulf Trail from the north, the Huntington Ravine Trail from the west, and the Tuckerman Ravine Trail to the south of it, with slope angle shading turned on.

The steepest and most difficult trails up Mt Washington.
The steepest and most difficult trails up Mt Washington.

As you can see, the difficult trails all run through dark red and purple colored zones which indicate 35-50 degree slope angles. These trails are very strenuous to hike and climb up and run across boulder fields or up steep rock slabs. They’re all avalanche zones in winter and full of avalanche debris each spring.

Planning routes at home on Caltopo with your computer or laptop is one thing, but GaiaGPS lets you use the same slope angle shading and make the same kind of route decisions in real-time on a phone in the palm of your hand. Here’s the same area around Mt Washington displayed with USGS maps as a background (so you can see where the trails are) in GaiaGPS.

Slope angle shading on Mt Washington in GaiaGPS
Slope angle shading on Mt Washington in GaiaGPS

Slope angle shading in GaiaGPS (and all map layering) is considered a premium feature, so you need to buy a premium license to use it ($32 per year or $128 for 5 years). It’s totally worth it if you hike or ski in mountainous terrain and need to make real-time course corrections based on the conditions you find on the ground around you.

Wrap Up

This a quick intro to slope angle shading and how you can use it for navigation. Personally, I use it ALL the time for planning routes at home and making course corrections in the field when I hike off-trail. If you’d like to learn more about map layers like slope angle shading and digital navigation tools, see my post An Introduction to Map Layers for Backcountry Navigation. 

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5 comments

  1. Thank you for this tutorial. Since I’m out for the summer I finally have time to try to use avenzia, and caltopo, and get to import this on to my phone. I have always used paper maps and am very handy with a compass, but can see the advantage of using my phone in conjunction, with paper. I have a lot of questions still……

    • I still encourage compass use in conjunction with apps for the simple reason that you can walk with a compass in your hand, glancing at it to check that you’re on the right bearing. You can’t do that with a cell phone and not fall flat on your face!

      It also helps to have multiple navigation instruments and check them against one another to catch discrepancies. GPS apps are still not as reliable as a compass/paper map and it is prudent to crosscheck.

  2. I need to get Caltopo and GaiaGPS to slope map the contents of my freezer–it likes to avalanche on me! I got buried this morning while trying to navigate to the frozen strawberries and cherries to add to my breakfast concoction. If not for the quick response of Grandma’s rescue party, it could have been a dangerous situation!

  3. Zachary G Robbins

    I use it all of the time to pinpoint probable locations of undocumented waterfalls off-trail, and possible rocky viewpoints on the sides of mountains during bushwhacks and trail hikes. I wish more of my hiker friends had familiarity with CalTopo and its usefulness!

  4. Bill in Roswell, GA

    Indeed, that is some cool stuff!

    I didn’t know DEM shading had moved into the free public domain. DEM shading was awesome in ArcGis, but that is expensive to pay for unless you’re doing it for a living.

    Thanks so much for sharing that Philip!

    Cheers,
    Bill in Roswell

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