What makes a trail network difficult?

Have you ever gone hiking and when you get to the trailhead, you see the various trails you can take? All these routes vary in difficulty from easy, intermediate, expert and so on. If you are like me, I tend to challenge myself and pick the expert trail, even though I am not an “expert” hiker. My short gasp of confidence is usually quickly overcome by exhaustion and regret as I hike up a treacherous trail, with steep slopes, rough terrain, and meandering bends/curves. After stopping 5 minutes into my hike, I often wonder, what constitutes an easy versus an expert trail? Is there some kind of metric or is it a combination of things? In addition, how do conservation authorities, city municipalities, parks measure and/or other trail network institutions determine trail difficulty?

After consulting the internet, I found that many factors can influence a trails’ difficulty, but the two main factors are terrain roughness and how steep the accents and descents on a trail network are. While that does seem pretty straight forward, it doesn’t answer my second question, how do these variables get measured? Consulting the internet once again, I found that assessing trail network routes is usually accomplished by a third party company, such as a surveying company. These companies are hired to assess and map out trail routes for conservation authorities, municipalities and parks. This task is also something that has to get completed every 1 – 2 years as trails requiring updating. This seems like a mundane tasks that cost time and money, especially if a trail network has not changed much over time.

Thinking back to the two variables that influence a trails difficulty: Roughness and Slope, I am reminded of the field of Geomorphometry. Both of these variables are examples of topographic attributes that are easily extracted from digital elevation models (DEM) with the appropriate software. With increasing availability of geospatial data, especially data collected over conservation areas and parks, assessing and mapping trail networks may not require a third party company.

Here at Whitebox Geospatial, we have an innovation that solves this problem – the new AssessRoutes tool, which is part of the General Toolset Extension for WhiteboxTools. This tool applies various topographic attributes to assess variability in slope, elevation, roughness and visibility along a line vector, such as a trail network but the application does not stop there. There is no reason why it also couldn’t be applied to a road, river, or any other route (i.e. any other vector line).

Prior to running, a user must specify the input line vector, the input DEM and the output line vector. The tool works by breaking the vector lines, or for the purpose of this blog, a trail network into equal segments. Once that is complete, the AssessRoutes tool then preforms five operations to extract topographic information as a measure of trail difficulty.

Trail Network Difficulty

Average Slope

How steep the ascents and descents are on trail network is a large factor in determining its trail difficulty. The AssessRoutes tool calculates average slope for each segment on the trail network. How steep the ascents and descents of a trail is one of the two most important factors that influence trail difficulty. For example, if a trail network is broken up into 6 segments and each segment results in an average slope of 1 – 3°, that could classify as an easy or beginner type of trail. The image below shows average slope for a trail network near Grand Junction, Colorado, USA. There are two networks in this image, the vector that extends through the mountain is a trail network and the network that follows the river bed is a stream network. This shows the robustness of the AssessRoutes tool in application all vector networks. Green segments depict small slopes (i.e. 0 – 4°) while orange depict moderate slopes (5 – 8°) and red shows steeper slopes (8 – 21°).

Minimum and Maximum Elevation

What better way is there to get a rough idea of how steep the ascents and descents on a trail network than to measure the minimum and maximum elevation values? The AssessRoutes tool will calculate the minimum and maximum elevation values. Conservation authorities and parks can use this information to report the peaks and troughs of a trail network or to show expected elevation range on a trail. The image on the left shows Minimum Elevation, which ranges from 1902 m ASL in green to 2542 m ASL in red. The image on the right shows Maximum Elevation, which ranges from 1905 m ASL in green to 2546 m ASL in red.

Relief

Speaking of elevation range, the AssessRoutes tool will also output relief of each segment on the trail network. Overall, relief is another metric to gauge how steep the ascents and descents are on a trail and factor in to determining a trail’s difficulty. For myself and likely a lot of other casual hikers, knowing how high I have to hike is an important factor in deciding between a easy or expert trail. Knowing this prior to hiking a trail would be extremely useful for conservation authorities and parks to use for determining trail difficulty but also to advertise at the trailhead. The image below is the AssessRoutes Relief output, relief ranges for 0-3 m in green, to 4 – 11 m in orange and 11 – 29 m in red.

Sinuosity

Seldom are trail networks straight lines, but the opinion from some hikers could be they aren’t interesting in hiking trails that meander too much. Some hikers may prefer to hike trails that are straight lines. While it is not always the case that a straight trail is easier or shorter than one that meanders more, the sinuosity of a trail network can help to determine its difficulty. For those that group straight trail networks with easier trails in terms of difficulty, knowing that sinuosity was used as a metric to calculate trail difficulty could be a helpful. The AssessRoutes tool calculated sinuosity as the ratio of the along-surface (i.e. 3D) path length, divided by the 3D distance between the start and end points of the segment. A line segment with a larger sinuosity value will have more bends compared to a segment with a smaller sinuosity value. The image below shows the sinuosity out from the AssessRoutes tool. Low sinuosity values are shown in green and range from 0 to 1.06, while medium sinuosity values are shown in orange and range from 1.06 to 1.1 and high sinuosity values are shown in red and range from 1.2 to 6.5.

Change in Slope

In determining trail difficulty, the other major factor was terrain roughness. In graduate school, my entire thesis was built around terrain roughness so I can understand a hiker frustrations when they show up to a trail head and expected the trail to be a dirt path but instead it is pebbles. Not only is this frustrating, especially if you wore the wrong pair of shoes, but it is much easier to walk on a dirt path than it is to walk on a path of small pebbles/stones. The AssessRoutes tool calculates the number of changes in slope direction as measure of terrain roughness. The image below shows the AssessRoutes tool roughness metric output, the higher the number of breaks in slope a line segment contains the rougher the surface. Green segments show low roughness values that range from 0 to 17, while medium roughness values are shown in orange and range from 30-55 and high roughness values are shown in red and from from 55 – 82.

Maximum Visibility

Lastly the AssessRoutes tool, calculates maximum visibility. This metric is based on the Yokoyama et al. (2002) Openness index, which calculates the average horizon angle in the eight cardinal directions to a maximum search distance. This information can help determine where you may encounter beautiful vistas and scenic views along the route. The image below shows the AssessRoutes Maximum Visibility output. Line segments shown in green have low visibility values, meaning they are less exposed than those segments shown in red which more exposed.

For this application, slope and roughness are the two most important factors in determining trail difficulty. These topographic attributes and so many more are at the core of the field of Geomorphometry. Geomorphometry is in our DNA here at Whitebox Geospatial Inc., and you won’t find a duo more passionate about the field than John and I. The AssessRoutes tool is a novel innovation created by Whitebox Geospatial Inc. This tool provides six metrics to help determine trail network difficulty. Successful application of the AssessRoutes tool does come down to the quality of your DEM and a vector trail network data sets. Check out the AssessRoutes tool in our General Toolset Extension. Let us know if you use this tool for any interesting applications.