Evacuation maps are conditional on the spatial inputs and modeling choices that produce them. Record those choices and inspect their effects before using a map for planning.
Coordinate reference system
Use a projected CRS in meters. Buffers, grid resolution, route distance, and walking-speed conversion are not meaningful in longitude and latitude degrees.
Hazard, terrain, and road inputs
- Define the hazard zone for the question being asked; for tsunami modeling, distinguish the land-only origin zone from the escape boundary.
- Confirm the digital elevation model sign convention before using land and water thresholds. Do not assume every source uses positive values for land.
- Review road attributes and remove unsuitable features such as piers or routes that are not usable by pedestrians.
Buffers and origin density
road_buffer_m controls how far movement can extend from
the road centerline. escape_buffer_m provides local access
around candidate escape points. grid_resolution controls
the sampling density of origins; finer grids increase runtime and should
be justified by the terrain and road data resolution.
Conductance and least-cost paths
make_conductance_surface() uses a slope-based
conductance surface from leastcostpath. The following
arguments affect route geometry rather than just the final time
conversion:
run_evacpath(
...,
lcp_cost_function = "tobler",
lcp_neighbours = 16,
lcp_crit_slope = 12,
lcp_max_slope = NULL
)walking_speed_mps converts modeled route distance into
time. It should be set to the population and planning scenario of
interest.
The compact packaged run below shows the distinction: slower walking changes time conversion, while a different routing neighbourhood can also change modeled distance and route geometry.
| Scenario | LCP neighbours | Walking speed | Median min. | Maximum min. |
|---|---|---|---|---|
| baseline | 16 | 1.22 | 7.27 | 22.84 |
| slow_walkers | 16 | 0.75 | 11.83 | 37.15 |
| conservative_routing | 8 | 1.22 | 7.48 | 23.96 |