OpenStreetMap Trail Cartography and SAC Hiking Scale Conventions AI-pedia

OpenStreetMap (OSM) is a free, editable geographic database maintained by a global community of volunteer mappers. Unlike traditional topographic mapping agencies, which apply centralized national standards, OpenStreetMap relies on an open data model of nodes, ways, and relations paired with arbitrary key-value tags. In backcountry navigation and outdoor recreation, OpenStreetMap has become the foundational data source for consumer navigation tools, mobile mapping applications (such as OsmAnd, Komoot, and AllTrails), and specialized rendering engines (such as OpenTopoMap and Waymarked Trails).

Because the physical conditions of unpaved trails vary from flat, crushed-gravel urban walking paths to severe high-alpine routes requiring technical scrambling, mappers require standardized descriptive keys to communicate difficulty, terrain hazards, and visibility. The primary international framework used within OpenStreetMap to indicate technical foot travel difficulty is the Swiss Alpine Club (SAC) hiking scale, stored under the database tag sac_scale=*. This scale operates alongside path condition markers, predominantly trail_visibility=*, surface=*, and incline=*.

Translating these multidimensional database attributes into legible, intuitive cartographic styles poses substantial visual design and safety challenges. Different map styles (or renderers) use line thickness, color palettes, dash intervals, and supplementary iconography to convey whether a route is a well-beaten family walking track or a dangerous, exposed traverse. Misinterpreting these conventions can lead hikers into hazardous terrain beyond their physical capabilities, making trail cartography in OpenStreetMap a subject of ongoing technical refinement, ethical debate, and search-and-rescue concern.

The OpenStreetMap Trail Data Model

In OpenStreetMap, linear geographic features are represented as ways, which are ordered lists of geographic points called nodes. The functional and legal classification of a way is defined by tags. For pedestrian paths and backcountry trails, three primary tags form the root classification:

• highway=footway: Used primarily for designated urban or suburban pedestrian paths, constructed sidewalks, and paved walkways within parks. Footways typically imply a maintained surface and gentle gradient.
• highway=path: A general, multi-use non-motorized path. This tag is the global standard for backcountry hiking trails, singletrack mountain bike paths, and unpaved rural tracks. It does not carry an inherent implication of width, surface, or difficulty.
• highway=track: An agricultural, forestry, or access road broad enough for four-wheel-drive vehicles or tractors, often used by hikers. Tracks are further categorized by physical firmness using the tracktype=* tag (graded grade1 for solid asphalt or concrete through grade5 for soft mud or sand).

Because highway=path is deliberately broad, it requires secondary physical attributes to describe the reality on the ground. A smooth dirt path in a municipal forest and a knife-edge rock traverse at 3,000 meters elevation can both be tagged highway=path. To resolve this ambiguity, mappers apply secondary tags that detail physical composition and difficulty:

• surface=*: Describes the physical ground material (for example, ground, dirt, gravel, scree, or rock).
• smoothness=*: Evaluates physical tractability for different wheeled transport modes, but also indicates surface regularity for pedestrians (for example, good, intermediate, bad, or very_horrible).
• incline=*: Specifies the slope angle or percentage of the path.
• width=*: Quantifies the average physical width of the usable tread in meters.
• sac_scale=*: Quantifies the technical difficulty of the walking or mountaineering route.
• trail_visibility=*: Quantifies how readily visible the path's tread is on the ground.

The Swiss Alpine Club (SAC) Hiking Scale

The Swiss Alpine Club (German: Schweizer Alpen-Club, SAC; French: Club Alpin Suisse, CAS) established its hiking scale (SAC-Wanderskala) in 2002. It was designed to standardize route descriptions across the Alps, replacing subjective terminology such as "easy" or "difficult" with objective physical criteria based on terrain type, necessary equipment, fall risk, and navigational demands. In 2008, the OpenStreetMap community formally adopted the SAC scale through a standard tagging proposal, establishing sac_scale=* as the global key for pedestrian technical difficulty.

The scale consists of six sequential difficulty tiers, designated T1 through T6. In Switzerland, these tiers correlate directly with official trail markings maintained by cantonal trail associations:

• T1: Hiking (Wandern). Marked with solid yellow trail signs and indicators. The trail is well-cleared, horizontal or gently inclined, with no risk of falling. Flat footwear is sufficient.
• T2: Mountain Hiking (Bergwandern). Marked with white-red-white painted horizontal stripes. The trail is continuous; terrain can be steep. Some fall hazard exists. Hikers require surefootedness and sturdy trekking boots with profiled soles.
• T3: Demanding Mountain Hiking (Anspruchsvolles Bergwandern). Marked with white-red-white stripes. The trail is not always visible; exposed sections are often secured with fixed cables, chains, or metal steps. Hikers encounter steep drops, loose rock, scree, and talus. The hands are occasionally needed for balance. Sturdy mountaineering boots are required.
• T4: Alpine Hiking (Alpinwandern). Marked with white-blue-white stripes. The path is often faint or absent. Route-finding involves crossing boulder fields, steep grassy slopes, or stable snow fields. Terrain contains significant fall risks and requires simple rock scrambling (grades I and II on the International Climbing and Mountaineering Federation / UIAA scale). Terrain requires alpine boots, route-finding ability, and basic orientation gear.
• T5: Demanding Alpine Hiking (Anspruchsvolles Alpinwandern). Marked with white-blue-white stripes. Mostly pathless terrain across glaciers, firn (compacted perennial snow), steep scree slopes, or exposed rock faces. Involves sustained scrambling up to UIAA grade II. Ice axes, crampons, and safety ropes may be required. Extensive alpine experience is mandatory.
• T6: Difficult Alpine Hiking (Schwieriges Alpinwandern). Rarely marked or cleared. Pathless terrain across precarious rock, rotten ridges, steep scree, or glaciated terrain with severe fall hazards. Involves rock climbing up to UIAA grade II+. Requires mastery of alpine safety equipment and rope handling. It is the boundary tier between alpine hiking and technical mountaineering.

Tagging Values in OpenStreetMap

In OpenStreetMap, the T-scale values are translated into explicit English strings rather than numerical identifiers. This deliberate design practice avoids confusion with other numerical systems, such as the French alpine scale or the Yosemite Decimal System.

SAC Tier	OSM Tag Value (sac_scale=*)	Official Alpine Marker	Terrain Profile	Fall Hazard Level	Hand Use Requirement
T1	hiking	Yellow	Wide, continuous, gentle gradient	Virtually non-existent	None
T2	mountain_hiking	White-Red-White	Continuous path, moderate to steep	Low to moderate	None under normal conditions
T3	demanding_mountain_hiking	White-Red-White	Intermittent tread, exposed drops, scree	Substantial on margins	Hands needed for balance
T4	alpine_hiking	White-Blue-White	Pathless, talus, simple rock scrambling	Severe; fatal fall hazard	Hands needed for propulsion (UIAA I)
T5	demanding_alpine_hiking	White-Blue-White	Exposed rock ridges, glaciers, steep snow	Extreme; continuous fall risk	Sustained scrambling (UIAA II)
T6	difficult_alpine_hiking	Unmarked / Rare Blue	Rotten rock, high exposure, vertical drops	Lethal on slip or misstep	Climbing up to UIAA II+

Separation from Technical Climbing Scales

The SAC hiking scale terminates where technical rock climbing begins. When a route requires dedicated climbing harnesses, dynamic belay systems, and technical pitches exceeding UIAA grade II, the route is classified as technical mountaineering or rock climbing. In OpenStreetMap, technical rock climbs are not tagged with sac_scale=*. Instead, they are tagged with:

• sport_climbing=* or climbing:grade:uiaa=*: Specific climbing grades referencing established rating systems.
• via_ferrata_scale=*: Used for protected climbing routes (klettersteig) equipped with permanently installed steel cables, rungs, and ladders, using scales such as the Hüsler scale (K1 to K6) or the Schall scale (A to F).

Trail Visibility and Path Demarcation

Physical difficulty does not directly dictate whether a trail is visually obvious on the landscape. A steep, technically demanding T3 path through an alpine pass may be marked by bright paint flashes every ten meters, while a flat, completely safe T1 walking route across an open, untracked grassland meadow may feature no visible ground tread whatsoever. To prevent the conflation of navigational clarity with physical hazard, OpenStreetMap maintains the independent tag trail_visibility=*.

The visibility scale has six defined values:

• trail_visibility=excellent: The path is unambiguous, deeply worn into the earth, paved, or bounded by continuous physical borders. Navigational aids are completely unnecessary to stay on the route.
• trail_visibility=good: The tread is continuous and readily identifiable by eye along almost its entire length. Markers (such as paint blazes or wooden posts) are present at decision points and intersections.
• trail_visibility=intermediate: The path is mostly visible, but sections are covered by leaf litter, loose scree, or grass. A hiker must periodically glance ahead to locate cairns (piles of stones used as landmarks), blazes, or the next cleared tread.
• trail_visibility=bad: The ground tread is faint, heavily overgrown, eroded by seasonal water runoff, or obscured by boulder fields. The route cannot be tracked continuously by physical indentations in the dirt alone; navigation requires looking between distant markers, map reading, or checking satellite navigation.
• trail_visibility=horrible: There is no continuous path on the ground. The route consists of occasional stone cairns, faint scratches on rock surfaces, or worn vegetation separated by dozens of meters of untracked terrain. Extensive navigational orientation is required.
• trail_visibility=no: There is no physical evidence of a path on the earth. The route traverses solid bedrock, open scree slopes, sand dunes, or dense bush where progress is made purely by compass bearings, GPS waypoints, or topographic ridge lines.

A path marked with trail_visibility=bad or trail_visibility=horrible informs both map renderers and route-planning algorithms that travel speeds will be drastically reduced and that users risk losing the trail if visibility drops due to fog, rainfall, or nightfall.

Cartographic Rendering Conventions

OpenStreetMap does not produce a single, monolithic map. The database consists solely of vector geometries and text tags. A renderer is a software suite that parses these tags and applies styling rules to draw a visual raster or vector map. Because different renderers serve different user groups, cartographic treatment of trail difficulty and visibility varies significantly.

OpenTopoMap

OpenTopoMap is one of the most prominent open-source topographic map renderers utilizing OpenStreetMap data. Modeled visually after official European national mapping products (such as the Swiss Federal Office of Topography, swisstopo, and the German Bundesamt für Kartographie und Geodäsie), it uses a strict visual hierarchy to communicate technical terrain attributes.

In OpenTopoMap, sac_scale=* fundamentally alters line color, width, and dash patterns:

• T1 (hiking): Rendered as a simple, continuous thin red line or a fine, tightly spaced dashed red line, matching standard low-elevation walking trails.
• T2 (mountain_hiking): Rendered as a distinct dashed red line with uniform stroke and gap dimensions.
• T3 (demanding_mountain_hiking): Rendered with wider dash intervals, alternating dash-and-dot sequences, or distinct symbol overlays indicating steep sections.
• T4, T5, and T6 (alpine_hiking and above): Rendered using dashed or dotted blue lines. In traditional alpine cartography, blue lines are reserved for high-alpine, glaciated, or technical routes, visually alerting users that the way is an alpine route rather than an ordinary forest path.

OpenTopoMap also evaluates trail_visibility=*. When a path has a visibility rating of bad, horrible, or no, the standard linear dashes are replaced with interrupted dashes, widely spaced stippling, or faint, dotted lines. This interrupted styling serves as an explicit visual warning: the physical tread is broken, washouts may have erased the route, or the hiker must navigate without a visible trail underfoot.

OSM Standard Style (Carto)

The default map style displayed on the OpenStreetMap homepage (known historically as "Standard" or "Carto") is an all-purpose general map. Because it is designed to show everything from city bus stops to maritime boundaries, it deprioritizes alpine-specific distinctions.

In the standard OSM Carto style, all linear paths tagged highway=path are generally rendered as simple brown or reddish-brown dashed lines. For many years, the standard style did not differentiate between a paved municipal walking trail and a T5 alpine ridge traverse. While recent updates have introduced subtle styling variations for access and footway distinctions, standard Carto remains unsuitable for backcountry alpine navigation, as it lacks clear visual warnings for high sac_scale values or poor trail_visibility.

Specialized Outdoor Renderers

Other specialized rendering engines parse trail tags with high fidelity:

• Waymarked Trails: An open-source map service that overlays hiking, cycling, and mountain biking routes over a topographic base. It renders official hiking networks (defined by OSM type=route relations) and highlights the underlying sac_scale difficulty using distinct color bands and line weights.
• OsmAnd: A fully offline mobile mapping application that allows users to enable custom rendering styles. OsmAnd includes dedicated "Topographical" and "Off-road" visual profiles that allow mappers and hikers to color-code trails explicitly by their sac_scale grade (for example, displaying T1 as green, T2 as yellow, T3 as orange, and T4 through T6 as progressive shades of red and purple).
• Komoot and Outdooractive: Commercial navigation platforms that consume OpenStreetMap data. These platforms translate sac_scale, surface, and incline into proprietary difficulty classifications (such as "Easy", "Moderate", and "Difficult"), visually alerting users to steep drops or scrambling sections during automated route generation.

Technical Implementation: Cascading Style Sheets for Maps

In rendering frameworks like Mapnik, CartoCSS, or MapLibre GL, stylesheet rules interrogate the database using conditional selectors. The following simplified CartoCSS example illustrates how a renderer translates sac_scale and trail_visibility into distinct visual line strokes:

/* Standard mountain path */
#trails[highway = 'path'] {
  line-color: #c24e00;
  line-width: 1.5;
  line-dasharray: 5, 3;

  /* Demanding mountain hiking: altered dash pattern */
  [sac_scale = 'demanding_mountain_hiking'] {
    line-color: #a01010;
    line-width: 2.0;
    line-dasharray: 8, 4, 2, 4;
  }

  /* High alpine routes: shift to alpine blue */
  [sac_scale = 'alpine_hiking'],
  [sac_scale = 'demanding_alpine_hiking'],
  [sac_scale = 'difficult_alpine_hiking'] {
    line-color: #0044aa;
    line-width: 2.0;
    line-dasharray: 4, 4;
  }

  /* Interrupted, widely spaced dashes for poor visibility */
  [trail_visibility = 'bad'],
  [trail_visibility = 'horrible'] {
    line-dasharray: 2, 6;
    line-opacity: 0.7;
  }

  /* Pathless route */
  [trail_visibility = 'no'] {
    line-dasharray: 1, 8;
    line-opacity: 0.5;
  }
}

In this ruleset, a route classified as sac_scale=demanding_mountain_hiking with trail_visibility=bad transitions from a continuous or closely dashed brown line into an interrupted, wide-interval dark red pattern (line-dasharray: 2, 6). To the trained eye, this pattern clearly indicates that physical progress will be rough, exposed, and technically discontinuous.

International Variations and Competing Classification Systems

Although OpenStreetMap settled on the Swiss Alpine Club scale as its primary global tagging standard, mountaineering clubs and geographic agencies worldwide operate under different, historically entrenched rating systems. Mappers working in non-Alpine regions often face difficulties mapping local terrain directly to the SAC framework.

Region / Organization	Scale Name	Primary Metrics Evaluated	Key Differences from SAC Scale
Italy (Club Alpino Italiano - CAI)	CAI Scale (T, E, EE, EEA)	Technical difficulty, exposure, via ferrata equipment	Compresses T2 and T3 into "EE" (Escursionisti Esperti); uses "EEA" specifically for routes requiring via ferrata safety lanyards.
North America	Yosemite Decimal System (YDS)	Physical steepness, rock climbing hold requirements (Classes 1–5)	Class 1 is hiking; Class 2 is rough off-trail cross-country; Class 3 is scrambling with handholds; Class 4 is simple roped climbing. YDS does not measure route visibility or exposure as directly as SAC.
United Kingdom	Scrambling Grades (1 to 3)	Handhold necessity, technical difficulty of rock steps, exposure	Applied only to rocky ridges and gullies; does not cover standard walking terrain (which is left ungraded). Grade 1 aligns roughly with SAC T4.
France (FFRandonnée)	Cotation Fédérale (Effort, Technicité, Risque)	Three distinct indices: physical effort, technical difficulty, and fall risk	Decouples fall risk from technical footwork into independent 1-to-5 numeric ratings.

Because the SAC scale bundles three interrelated variables—technical footwork, exposure to vertical drops, and terrain steepness—into a single linear tier, cross-mapping can be lossy. For example, a route along a wide, flat, knife-edge ridge with vertical 500-meter drops on both sides features zero technical difficulty (the footing is flat like a T1 path), but severe, fatal exposure (characteristic of T4 or T5). Under SAC guidelines, severe exposure automatically elevates the grade to at least T3 or T4, regardless of how flat the tread is underfoot.

To preserve local nuances without breaking global consistency, OpenStreetMap supports auxiliary regional keys alongside sac_scale=*. Examples include cai_scale=* for the Italian Club Alpino Italiano ratings and yds_class=* for the Yosemite Decimal System.

Safety, Ethics, and Open-Source Governance

The direct ingestion of OpenStreetMap data by commercial consumer applications has made trail cartography a matter of serious mountain safety and liability. When an informal, unmaintained, or extreme route is added to OpenStreetMap, it often propagates within days to millions of mobile phones via third-party application updates.

The "AllTrails Effect" and Search-and-Rescue Incidents

Mountain rescue organizations in Europe, North America, and Australasia have noted an increase in rescue operations involving ill-prepared hikers navigating via mobile phones. A common factor in these incidents is visual homogenization: if an application's internal map style fails to visually distinguish between a maintained family walking trail and an unmaintained, exposed T3 or T4 scramble, users assume the route is safe and straightforward.

A path marked with sac_scale=demanding_mountain_hiking indicates terrain where an unexpected thunderstorm, a slip on wet scree, or vertigo can immobilize a hiker lacking alpine footwear and route-finding skills. When mappers omit the sac_scale=* tag—or when consumer applications render all highway=path ways using the same solid or simple dashed line—the map effectively conceals severe natural hazards.

Armchair Mapping vs. Ground Truth

OpenStreetMap operates under the strict principle of ground truth: map data should reflect verifiable reality as observed on the ground, rather than hypothetical constructs. In urban areas, mappers often trace roads, buildings, and water bodies using high-resolution aerial satellite photography ("armchair mapping").

In mountain cartography, armchair mapping presents major risks:

• Aerial satellite imagery cannot convey gradient, loose gravel, scree stability, cliff edges, or vertical drops obscured by vegetation or shadows.
• A faint line visible from space may be an active game trail (an animal track), an unstable rockfall chute, or a historic timber-dragging track that terminates abruptly at a sheer cliff.
• Assigning difficulty grades like sac_scale=* or assessing trail_visibility=* requires local field surveys on foot, ideally by mappers familiar with international alpine grading standards.

Consequently, the OpenStreetMap community strongly discourages assigning sac_scale=* based solely on aerial photography or automated topographic elevation profiles. If a mapper has not personally traversed the route or consulted official surveys, the trail should either remain ungraded or carry an explicit explanatory note via note=* or description=*.

Contested Ways: Informal Paths, Landowner Disputes, and Deletions

A recurring governance dispute within OpenStreetMap centers on informal, abandoned, or hazardous paths. The tagging community categorizes informal paths using informal=yes, indicating a user-created, unmaintained route (often referred to as a social trail, herd path, or desire path).

These trails frequently prompt conflicts between different groups:

• Conservation authorities and land managers frequently request the complete removal of informal or environmentally sensitive paths from OpenStreetMap, arguing that showing them drives damaging foot traffic through protected wildlife habitats, accelerates alpine soil erosion, or crosses private property without permission.
• Search and rescue personnel sometimes advocate for retaining mapped trails—even if unmaintained or hazardous—provided they are marked with appropriate danger tags (such as sac_scale=alpine_hiking, trail_visibility=horrible, or access=no). If an injured or lost hiker calls for rescue and provides coordinates, rescue teams need access to the exact geographic paths visible to the hiker on the ground.
• OpenStreetMap data purists assert that OSM is a factual record of the physical world. If a path physically exists on the ground and can be walked, it should be mapped in the database, with legal access restrictions recorded accurately using access=no or access=private.

The accepted consensus within the community is that physically visible paths should remain in the database, but must be tagged exhaustively to deter accidental use. This includes pairing highway=path with informal=yes, trail_visibility=bad, and explicit access restrictions. If a path has fallen into disuse, has completely grown over, or has been intentionally destroyed by park rangers, it is reclassified using lifecycle prefix tags such as abandoned:highway=path or demolished:highway=path, which prevents it from being rendered by standard cartographic engines while preserving its historical record in the database.

Best Practices for Field Mapping a Mountain Trail

Accurate field mapping of backcountry paths requires systematic data collection during or immediately following a hike. When surveying a trail for integration into OpenStreetMap, mappers follow a standardized workflow to capture the physical reality of the terrain accurately.

Field Surveying Protocol

1. Record Continuous GPS Traces: Mappers record a high-frequency GNSS (GPS, GLONASS, Galileo) track log. In steep canyons, under dense forest canopies, or alongside vertical rock walls, GPS multipath errors (satellite signals bouncing off rock faces) can displace waypoints by tens of meters. Collecting multiple logs over several traverses or averaging points helps identify the true centerline of the trail.
2. Mark Waypoints at Transitions: A trail rarely maintains uniform difficulty or visibility along its entire length. Mappers create waypoints whenever the character of the path changes—for instance, where a maintained dirt path (T2) ends and crosses into an exposed boulder field requiring hand balance (T3), or where a visible tread disappears into open bedrock (trail_visibility=no).
3. Document Physical Infrastructure: Mappers photograph and log trail blazes, cairns, signposts (recording exact text and painted color codes), fixed metal ropes, ladders, and warning signs (such as French sentier escarpé or German Nur für Geübte markers).

Applying Tags in OpenStreetMap Editors

When entering field data into editors such as JOSM (Java OpenStreetMap Editor) or iD, ways are segmented (split into distinct linear sections) at every point where physical attributes change. If a three-kilometer hike is a gentle walk for the first two kilometers but becomes an exposed scramble for the final kilometer, the way must be split into two separate ways with distinct tagging.

/* Example Tagging: Lower Approach Section */
highway = path
surface = ground
smoothness = intermediate
sac_scale = mountain_hiking
trail_visibility = good
name = Hochgratweg

/* Example Tagging: Upper Summit Ridge Section */
highway = path
surface = bare_rock
smoothness = very_horrible
sac_scale = demanding_mountain_hiking
trail_visibility = intermediate
assisted_trail = yes
handrail = wire
hazard = steep_fall
name = Hochgratweg

In this example, the addition of assisted_trail=yes and handrail=wire indicates that fixed equipment is present to aid balance, while hazard=steep_fall explicitly documents the presence of vertical drop-offs. By splitting ways and providing granular, unambiguous tags, mappers provide downstream rendering software and routing engines with the data needed to generate safe, readable maps.

See also

• OpenStreetMap
• Swiss Alpine Club
• Topographic map
• Grade (climbing)
• Yosemite Decimal System
• Trail blaze
• Desire path
• Search and rescue