The Harmony of Difficulty Rating Systems
When preparing to embark on a hike, I want to know if a trail suitable for me (my skill level and the time/energy I have for the hike). I would like to review a map and have trails clearly identified based on their level of difficulty and level of required exertion. There are numerous classification systems which grade trails for hiking and mountain biking including the Swiss Alpine Club Rating scale, German Single-track Scale, International Mountain Mike Association scale, Yosemite Decimal System, Australian Walking Track Grading System, the Sierra Club scale, the Appalachian Mountain Club, and Shenandoah National Park and Pigeon Forge National Park, and the US Forest Service and others. These rating systems variously take into account surface condition and obstacles, steepness, trail visibility, required gear, required wayfarer skills, risk/exposure, length, and elevation change.
Several of the established scales focus on exertion level based on length/elevation with different formulas but don’t take into account technical difficulty (National Park Service and Appalachian Mountain Club, Sierra Club). Several focus terrain and skill required with criteria for peak and/or average incline, surface material, smoothness, visibility, required gear, and risk/exposure (SAC, MTB, YDS, and USFS). While others combine these factors (Australian System, and Adventure Nerds). Others have proposed modifications/alternative scales that have not been widely adopted, but attempt to merge and isolate key factors.
OSM has well established tags for ways based on the Swiss Alpine Club Rating scale for hiking and the German Single-track Scale for mountain biking, but these are not always used. There are other tags that are related to the factors which contribute to the difficulty assessment (surface, smoothness, visibility, incline), again with variable adoption. The stamina and time required for a hike are a combination of elevation change and distance which are also OSM tags recommended for routes, but stamina and time will also be influenced by the technical difficulty of the ways.
Personally, I think it is useful to segregate the technical difficulty of a path from the exertion level required to complete it. I think it would be interesting to render a map with ways based only on technical difficulty, and routes based on a combination of difficulty and exertion required to complete.
I have taken a pass at correlating all of the various scales and harmonizing their criteria and factors, and then identifying the values for the relevant tags in OSM. The scales are not entirely consistent, so this table represents a number of choices I have made. It relies most strongly on SAC hiking scale, but pulls in descriptions from the other scales at similar levels. I intend to use this table along with an algorithm to render trail difficulty based on a combination of whatever tags are available, first sac_scale if it is used, but then inferring the difficulty from the other relevant factors. Each of the factors are independent, (a well marked and visible scale may have very challenging obstacles), but the overall difficulty could be some combination of all the factors which contribute. Such an algorithm could also flag paths which are missing critical information about difficulty or have confusing/conflicting metadata potentially in need of validation (e.g. surface=paved & sac_scale=mountain_hiking, or smoothness=horrible & mtb:scale=0). Also, these presume some fair weather condition. Adverse weather (snow, rain, wind, heat) could significantly increase the difficulty and change the gear required versus the trail under fair conditions.
Difficulty Levels, OSM Tags, and Existing Scales
|Class - Color - Title||Description||surface||smoothness||sac_scale||trail_visibility||Incline (peak rise/run)||width||MTB : YDS : NFS : AWTGS : IMBA|
|Class 0 - Grey - Accessible||Experience, Gear, and Risk: No hiking experience required. Suitable for wheelchair users who have someone to assist them. No risk.
Surface and Visibility: Hardened or surfaced and clear.
Obstacles: Flat even surface with no steps or steep sections. No obstacles present (<1”).
Hands: Hands not required.
|Asphalt, Chipseal, Fine Gravel, Paved, Concrete, paving_stones, Compacted, metal, wood||Excellent, Good, Intermediate||N/A||Excellent||< 8%||>24”||MTB 0 : YDS NA : NFS C5 : AWTGS G1 : IMBA 0|
|Class 1 - Green - Easy||Experience, Gear, and Risk: Hiking on a well groomed trail. Athletic shoes sufficient. Map not needed. No Risk.
Surface and Visibility: Surface hardened or compacted. Vegetation cleared outside trailway.
Obstacles: May have a gentle hill section or sections and occasional steps or small obstacles such as roots and stones (<2”)
Hands: Hands not required.
|Sett, unhewn_cobblestone, cobblestone, stepping_stones, unpaved, gravel, pebblestone, grass, grass_paver, sand, woodchips (potentially unpaved, rock, ground, dirt, earth, mud)||bad||hiking (T1)||Excellent||< 40%||18” - 24”||MTB 1 : YDS Class 1 : NFS Class 4 & 3 : AWTGS G2 :IMBA 1|
|Class 2 - Blue - Moderate||Experience, Gear, and Risk: Hiking on a trail. Hiking boots recommended. Limited potential for falling.
Surface and Visibility: Continuous established footpath. Vegetation may encroach trail. Partially steep terrain.
Obstacles: Obstacles common (<8”). Steps and flat stairs. Short steep hill sections.
Hands: Hands not required.
|unpaved, rock, ground, dirt, earth, mud,||very_bad, horrible,||mountain_hiking (T2)||good||<70%||12” - 18”||MTB 2 : YDS Class 1: NFS Class 2 : AWTGS G3 : IMBA 2|
|Class 3 - Red - Difficult||Experience, Gear, and Risk: Simple scrambling. Average orientation/navigation skills. Sturdy hiking boots recommended. Some potential for falling encountered.
Surface and Visibility: Usually marked, but trail not always visible, and signage not always visible.
Obstacles: Obstacles common including large boulders, logs, and large steps (<15”). exposed passages can be protected with cables, Steep grades
Hands: Hands required to maintain balance.
|Same as Class 2||very_horrible||demanding_mountain_hiking (T3)||intermediate||<100%||1-12”||MTB 3 & 4 : YDS Class 2: NFS Class 1 : AWTGS G4 : IMBA 3|
|Class 4 - Black - Very Difficult||Experience & Gear: Specialized hiking skills and gear, including navigation and emergency first aid. Weather exposed portions, rock ledges, fields, and slopes. Potential danger is encountered and fall protection may be recommended.
Surface and Visibility: Tracks are likely to be very rough and unmarked.
Obstacles: Characterized by blocky terrain with counter climbs, scree fields and landslides, high steps and fallen trees and extremely steep grades.
Hands: Hands required to advance.
|Same as Class 2||impassable||alpine_hiking (T4)||bad||>100%||0||MTB 5 : YDS Class 2: NFS Class 1 : AWTGS G5 : IMBA 4|
|Class 5 - Orange - Expert||Experience, Gear, and Risk: Scrambling with increased exposure. A rope could be carried. Good terrain assessment and orientation skills required. Falls could be fatal.
Surface and Visibility: Occasionally pathless.
Obstacles: Basic/easy climbing sections.
Hands: Handholds necessary to bear weight.
|rock||impassible||demanding_alpine_hiking (T5)||Horrible||>> 100%||0||MTB 6 : YDS Class 3 : NFS N/A : AWTGS N/A : IMBA N/A|
|Class 6 - White - Climbing||Experience, Gear, and Risk: Basic to advanced climbing.
Surface and Visibility:
|rock||impassible||difficult_alpine_hiking (T6)||N/A||MTB 6 : YDS Class 4-5 : NFS N/A : IMBA N/A|
References on Existing Scales
Australian Walking Track Grading System :https://www.nationalparks.nsw.gov.au/safety/bushwalking-safety/australian-walking-track-grading-system
USFS - Trail Classes : https://www.fs.usda.gov/managing-land/trails/trail-management-tools/trail-fundamentals
German Single Track Scale : http://www.singletrail-skala.de/
Appalachian Mountain Club: https://www.outdoors.org/adventures/trip-rating/
Yosemite Decimal System : https://en.wikipedia.org/wiki/Yosemite_Decimal_System
Phoenix Parks Trail Ratings : https://www.maricopacountyparks.net/park-locator/maricopa-trail/trail-ratings-etiquette/
Reflections on Rendering
I have also started on a MapCSS style for JOSM based on this table for my own use and exploration, available on github here https://github.com/DynamicsGeek/TrailRenderingWithOSM.git. I also intend to expand the algorithm to take into account access indications as being identified by the trails working group.
'To be developed: Swiss guidelines for SAC rendering are ...., but many other platforms (ski resorts, hiking/biking trail apps, german trails) for difficulty rending use variations on a white-green-blue-red-black rendering scheme. This is more familiar and intuitive to me personally.