Reference

Hiking & Camping Glossary

Plain-English definitions for 66 hiking, camping, and trail-data terms. Skim the table of contents or use Cmd-F to find a specific term.

Trail Difficulty & Classification

SAC Scale #
The Swiss Alpine Club hiking classification — six tiers from T1 (Hiking, no special skills) through T6 (Difficult Alpine Hiking, technical climbing required). T2 corresponds to "moderate" US trails, T3 to "hard," T4+ to alpine routes requiring ice-axe and route-finding skills. The most widely-used difficulty scale in OpenStreetMap.
Trail Visibility #
How easy the trail is to follow visually. "Excellent" = an obvious unmistakable path; "good" = clearly defined but occasional ambiguity; "intermediate" = visible most places but easy to lose; "bad/horrible" = navigation by terrain features only, expect to get lost without a map and compass.
Via Ferrata #
Italian for "iron path." A protected climbing route with permanently-fixed steel cables, ladders, and rungs. Originated in the Alps during WWI; now a hiking-to-climbing bridge format. Requires a climbing harness and a dedicated via ferrata kit with shock-absorbing lanyards.
Elevation Gain #
The total cumulative vertical climb across a route, measured in feet. A 10-mile flat trail and a 10-mile mountain trail with 3,000 ft of climbing have wildly different effort levels — elevation gain captures the difference. Cumulative gain counts every climb, even if you descend in between.
YDS Class Rating #
The Yosemite Decimal System rates terrain technicality: Class 1 = trail walking, Class 2 = simple scrambling, Class 3 = exposed scrambling with handholds, Class 4 = simple climbing where ropes are recommended, Class 5 = technical roped climbing (subdivided 5.0-5.15). Most hiking stays at Class 1-2.
Switchback #
A zigzag trail pattern that reduces grade by traversing horizontally before turning back. Cuts the steepness of a direct climb in half or more. Cutting switchbacks (taking the direct line between bends) causes severe erosion and is one of the most damaging hiker behaviors.
Exposure #
How serious the consequences of a fall would be on a section of trail. A trail with high exposure has steep dropoffs where a slip would cause significant injury or death. Different from "difficulty" — a flat trail along a cliff edge can have high exposure with low technical difficulty.

Route Types

Out-and-Back #
You hike to a destination then return the same way. Stated distance is typically the round-trip total. Great for beginners because you can turn around at any point. Same scenery in reverse, but distinct views.
Loop #
Trail returns to the starting point without retracing — fresh scenery throughout. Stated distance is the loop length. No shuttle needed.
Point-to-Point #
Trail starts and ends at different trailheads. Requires a vehicle shuttle, a two-car drop, or planning to hike back. Long-distance trails like the AT and PCT are all point-to-point on the macro scale.
Lollipop Loop #
An out-and-back that culminates in a loop — the route shape resembles a lollipop on the map. Combines the trail-walking economy of an out-and-back stem with the variety of a loop.
Thru-Hike #
Walking a long-distance trail end-to-end in a single trip, typically over weeks or months. Famous thru-hikes: Appalachian Trail (2,194 mi), Pacific Crest Trail (2,650 mi), Continental Divide Trail (3,028 mi). Successful thru-hikers are called "thru-hikers."
Section Hike #
Walking a long-distance trail in pieces over multiple trips rather than end-to-end. Most people who eventually complete the AT do so as section hikers across years or decades.
Spur Trail #
A short side trail branching off a main route, usually leading to an overlook, water source, or campsite. Stated distances may or may not include spur mileage; check before planning.

Surface & Terrain

Paved #
Asphalt or concrete surface. Smooth and accessible to strollers, wheelchairs, and road bikes. Common on urban greenways and accessible park loops. Stays hot in summer.
Gravel / Fine Gravel #
Crushed-stone trail surface. Stable underfoot but less comfortable than paved; OK for trail runners, hiking shoes, and most strollers. Drains well after rain.
Dirt / Ground / Single-Track #
Natural earth surface. The most common hiking trail type. Can become muddy or slippery after rain; can develop deep ruts in heavily used areas. Sturdy hiking shoes or boots recommended.
Boardwalk #
A raised wooden or composite walkway, typically built across wetlands, dunes, or fragile vegetation. Common in coastal and swamp parks (Everglades, Big Cypress, Cape Cod). Becomes slippery when wet.
Scree #
A slope of loose rock fragments, typically the size of golf balls to fists. Tiring to climb (one step up, half a step back), risky to descend, and dangerous in groups (loose rocks tumble onto hikers below).
Talus #
Larger rock fragments than scree — fist-sized to boulder-sized. Stable enough to walk on individual rocks but requires careful foot placement. Common above treeline.
Tread #
The walking surface of a trail — what your foot actually contacts. "Good tread" = solid, well-graded path. Distinct from the route corridor (the broader cleared/marked area).

Camping

Campsite #
A single numbered site within a campground — typically with a fire ring, picnic table, and tent pad or RV parking spot. Campsites are individually bookable via Recreation.gov for most federal campgrounds.
Campground #
A managed facility containing multiple campsites, typically with shared restrooms, drinking water, and trash service. Federal campgrounds are operated by NPS, USFS, BLM, USACE, or BOR.
Dispersed Camping #
Camping outside of designated campgrounds on public land — typically on national forests and BLM lands. Free, no reservation needed, but no amenities (no water, no toilet, no fire ring). Follow Leave No Trace and check fire restrictions before going.
Boondocking #
RV-specific term for dispersed camping without hookups — no electric, water, or sewer connection. Requires self-contained systems and water tanks. Common on BLM lands in the West.
Hookups (RV) #
Utility connections at an RV site: electric (typically 30A or 50A), water, and sewer. "Full hookups" = all three. "Partial hookups" = usually electric + water but not sewer. "Dry" / "no hookups" = none.
Pull-Through Site #
An RV site where you can drive in one end and exit the other — no backing up required. Significantly easier for larger rigs and inexperienced drivers. Premium sites usually book first.
FCFS (First-Come, First-Served) #
A campsite that cannot be reserved in advance — assigned on arrival. Most popular FCFS campgrounds fill by Friday morning in peak season. Arrive early and have a backup plan.
Recreation Area #
A federal land area (BLM, USFS, NPS, USACE, BOR) that may contain multiple campgrounds, trails, and facilities. The umbrella container in the Recreation.gov hierarchy — drilling into a "RecArea" surfaces its facilities.
Bear Canister #
A hard-sided rotational-lock food container required in bear country. Required equipment in much of the High Sierra, Adirondacks, Olympics, and some areas of the Smokies. Tools to keep food out of bears (and bears out of campsites).
Cathole #
A 6-8 inch deep hole dug at least 200 feet from water, trails, and camps to bury human waste in the backcountry. The standard Leave No Trace disposal method. Some high-altitude or fragile areas require packing waste out instead.

Terrain Features

Fourteener (14er) #
A mountain summit at or above 14,000 feet elevation. Colorado has 58 fourteeners, California has 12 (mostly in the Sierra), Washington has 2 (Rainier and Adams), Alaska has 21+. "Bagging fourteeners" is a major hiking subculture in Colorado.
Col / Saddle / Pass #
A low point in a ridgeline between two higher peaks. Cols are the natural crossing points for trails and the most efficient route between drainages. Trails frequently traverse colss because climbing a ridge over its low point requires less elevation gain than going around.
Arête #
A sharp, narrow ridge separating two glacially-carved valleys. French for "fishbone." Arête traverses are some of the most exposed hiking terrain — the Knife Edge on Katahdin, the Maroon Bells traverse, the Grand Teton ridges.
Cirque #
A bowl-shaped depression at the head of a glaciated valley, typically with steep walls on three sides. Often holds an alpine lake (a "tarn"). Cirques are textbook glacial features and frequent hiking destinations.
Tarn #
A small alpine lake, typically in a cirque or above treeline. Tarns are often the most photographed features in alpine regions. Cold, clear, and frequently still frozen into July at high elevations.
Tree Line (Timberline) #
The elevation above which trees cannot grow due to short growing seasons, wind exposure, and cold. Varies by latitude: ~12,000 ft in Colorado, ~7,500 ft in Washington Cascades, ~4,000 ft in New Hampshire. Trails above tree line are weather-exposed and lightning-vulnerable.
Slot Canyon #
A narrow canyon (often 10 feet wide or less) cut deep by water erosion. Famous examples: Antelope Canyon, Buckskin Gulch, the Subway. Spectacular when dry; lethal during flash floods. Never enter when monsoon storms are forecast anywhere upstream.
Fall Line #
The direct downhill direction at any point on terrain — the path water (or a falling rock) would naturally take. Trails that follow the fall line erode badly; well-designed trails cross it at an angle.
Bushwhack #
Off-trail travel through unmaintained terrain. Slow, exhausting, and route-finding intensive. Some routes (the Sierra High Route, parts of Wind River traverses) require deliberate bushwhacking; most lost-hiker cases involve accidental bushwhacking.

Public Lands & Agencies

NPS (National Park Service) #
The US Interior Department agency managing 423 national park units. Includes the 63 marquee "National Parks" plus monuments, historic sites, seashores, and parkways. Generally the most regulated and protected federal lands — dogs typically not allowed on trails.
USFS (US Forest Service) #
The Department of Agriculture agency managing 154 national forests and grasslands — 193 million acres. Allows multiple-use including dispersed camping, hunting, dogs, and motorized recreation in designated areas. The largest hiking land base in the US.
BLM (Bureau of Land Management) #
The Interior Department agency managing 245 million acres, mostly in the West (Nevada is ~67% BLM-managed). Allows dispersed camping, hunting, OHV use. The most permissive federal land manager. Trail infrastructure is generally lower than NPS/USFS.
USACE (US Army Corps of Engineers) #
The Department of Defense agency managing many large reservoirs and surrounding recreation areas — lakes, dam sites, and waterway corridors. Operates over 400 campgrounds, often with the best lakeside camping in the country.
BOR (Bureau of Reclamation) #
The Interior Department agency managing federal water-storage infrastructure in the western US — major reservoirs like Lake Mead, Lake Powell, and Lake Shasta. Recreation is a secondary mission; camping and hiking are available around most reservoirs.
Wilderness Area #
Land designated under the 1964 Wilderness Act, managed for the highest possible level of natural preservation. No motors, no mechanized travel (including mountain bikes), no permanent structures. The strictest protection class on federal land.
National Monument #
A protected area designated by Presidential proclamation under the Antiquities Act (rather than Congressional law like National Parks). Examples: Bears Ears, Grand Staircase-Escalante, Devils Tower. Often less developed than National Parks but with similar protections.
Recreation Fee Area #
A federal location requiring a day-use or parking fee under the Federal Lands Recreation Enhancement Act. Some have America the Beautiful Pass acceptance, some don't. Always check before driving out.
America the Beautiful Pass #
The $80/year federal lands pass covering entry fees at NPS, USFS, BLM, BOR, USACE, and USFWS sites. The single best deal in federal recreation if you visit more than 3-4 fee areas per year. Free for active military, veterans, 4th graders, and seniors (with discount).

Safety & Skills

10 Essentials #
The Mountaineers' standard safety kit for any hike: navigation, sun protection, insulation (extra clothing), illumination, first aid, fire, repair kit, nutrition, hydration, and emergency shelter. Originated in the 1930s; still the universal baseline.
Leave No Trace #
Seven principles for minimizing user impact: plan ahead, travel on durable surfaces, dispose of waste properly, leave what you find, minimize fire impact, respect wildlife, be considerate of others. Developed by the Center for Outdoor Ethics; taught universally in US outdoor education.
UV Index #
A 0-11+ scale measuring sun strength. UV exposure scales with elevation (~4% stronger per 1,000 ft of gain). Snow reflection can boost effective UV by 80%. UV 6+ requires SPF 50+, sunglasses, and covered shoulders.
AQI (Air Quality Index) #
A 0-500 scale for air pollution. 0-50 good, 51-100 moderate, 101-150 unhealthy for sensitive groups, 151+ unhealthy for all. Western wildfire smoke commonly pushes summer AQI past 150 for weeks at a time.
Naismith's Rule #
A 19th-century time-estimate for hikes: 1 hour per 3 miles of flat distance + 1 hour per 2,000 ft of elevation gain. Surprisingly accurate for fit hikers on good trail; add 50% for beginners or rough terrain.
Hypothermia #
A body-core temperature drop below 95°F (35°C). The leading cause of preventable backcountry deaths in the US. Risk factors: cold + wet + tired + wind. Most cases happen in 40-50°F weather, not subzero. Prevention: layers, dry clothing, fuel (food).
Heat Exhaustion #
Early-stage heat illness from dehydration and electrolyte depletion. Symptoms: heavy sweating, weakness, nausea, headache. Untreated, progresses to heatstroke (medical emergency). Cool down immediately and replace fluids with electrolytes, not plain water.
Altitude Sickness (AMS) #
Acute Mountain Sickness — symptoms appearing above 8,000 feet for unacclimatized visitors: headache, nausea, fatigue, sleep disturbance. Severe forms (HACE, HAPE) are life-threatening. Treatment: descend. Prevention: acclimatize gradually, hydrate, consider acetazolamide.
Lyme Disease #
A bacterial infection transmitted by deer ticks, endemic across the Northeast, Mid-Atlantic, upper Midwest, and parts of the Pacific Northwest. Early symptom: a bullseye rash 3-30 days after exposure. Treatable with antibiotics if caught early; chronic if untreated.
SAR (Search and Rescue) #
Volunteer and county-sheriff teams that locate and recover lost or injured backcountry users. Call 911 to activate; they want as much daylight as possible to find you. Most calls resolve within 24 hours when hikers stay put and signal correctly.

Data & Mapping

OpenStreetMap (OSM) #
A free, crowdsourced world map — the Wikipedia of geographic data. Volunteer contributors tag roads, trails, parks, and points of interest. The OutsideAtlas trail layer comes from OSM. Anyone can edit and improve it.
Overpass API #
A read-only query interface for OpenStreetMap data. Lets developers fetch features matching specific tag combinations within a bounding box. OutsideAtlas uses Overpass to pull trail records by US state.
RIDB (Recreation Information Database) #
Recreation.gov's public API for federal recreation site data — campgrounds, facilities, activities, media, reservations. Free to query with a developer key. The OutsideAtlas park layer comes from RIDB.
TIGER (Census Geography) #
Topologically Integrated Geographic Encoding and Referencing — the US Census Bureau's dataset of administrative boundaries (counties, states, places, ZIP codes). OutsideAtlas uses TIGER county polygons to reverse-geocode trail coordinates.
GeoJSON #
A JSON-based format for encoding geographic data. The standard interchange format for trail routes, park boundaries, and points of interest. Most modern mapping tools read and write GeoJSON natively.
GPX (GPS Exchange Format) #
An XML-based file format for GPS tracks and waypoints. The standard format for sharing trail routes between devices and apps (Garmin, AllTrails, Gaia GPS, CalTopo). OutsideAtlas doesn't yet offer GPX downloads but plans to.
PADUS (Protected Areas Database) #
The US Geological Survey's comprehensive inventory of US protected lands — federal, state, tribal, and private. Provides parcel-level ownership data showing exactly which acre is BLM, USFS, state park, or private. The data behind apps like onX and Gaia's land-ownership layer.