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