A ruler button in the top-left control stack, matching NPS's own
Area/Distance ruler. Click to add vertices; distance sums haversine
segment lengths, area uses the spherical-excess polygon formula
(same one turf.js's area() uses) rather than a flat shoelace, since a
flat approximation drifts at park-boundary scale. SI units only
(m/km, m²/km²) — no imperial toggle.
