A new trail in a protected area looks like a line chosen for views. It is more accurately what remains after a long series of exclusions has removed every unsuitable option.
Water crossings come first
Streams dictate alignment more than any other feature, because crossings are expensive, require structures and concentrate erosion and disturbance at fixed points.
Planners minimise crossings, and place the unavoidable ones on stable banks with bedrock where possible, which often means moving the route a long way from the obvious line.
Wetlands are usually avoided entirely rather than bridged, since a boardwalk changes drainage and requires permanent maintenance in a difficult location.
Soil and gradient set what is buildable
A sustainable trail sits well below the gradient of the slope it crosses, so water leaves the tread rather than running down it. That rule alone excludes most direct routes.
Soil type decides whether a surface holds. Fine silts and volcanic ash erode readily, while gravel and rock hold traffic well, so alignments follow the durable ground.
Aspect matters too. North-facing slopes in temperate regions hold snow and moisture longer, which shortens the usable season and raises maintenance on the same gradient.
Wildlife constraints remove whole areas
Nesting sites, denning areas, calving grounds and migration corridors are mapped and avoided, and the avoidance distance can extend well beyond the feature itself.
Some exclusions are seasonal, which produces trails that exist year-round but close during breeding periods rather than being rerouted permanently.
Habitat fragmentation is assessed as well. A trail that splits a contiguous block is worse than one following an existing edge, even when the second is longer.
Cultural and legal layers narrow it further
Archaeological sites, burial grounds and places of continuing significance to Indigenous communities are consulted on and, in many cases, excluded from public routing.
Land tenure adds constraints where a park boundary meets private holdings, grazing leases or water rights, each of which can block an otherwise ideal alignment.
Designated wilderness classifications may also prohibit machinery, which changes what can be built and therefore which terrain is feasible at all.
Visitor management shapes the final line
Planners route trails to concentrate use on hardened ground and to keep people away from fragile areas, which sometimes means deliberately not going to the best viewpoint.
Loop design, gradient at the start and distance to the first point of interest all influence how far the average visitor penetrates, which controls where impact falls.
A trail that ignores these considerations produces its own informal network within a few seasons, and closing an established desire line is far harder than routing around it initially.