The P.1 marker (Nawarat Bridge, 18.7875) sat inside zone 2 near its top
edge, but the bridge IS Chang Khlan's northern boundary - the layer was
~0.0095 deg too far north (the district-centroid correction in c4e0fb6
overshot). Zone 2's north edge now lands at the bridge.
The previous affine fit slid along the mostly north-south river (an
ill-conditioned direction) leaving the zones ~1 km south-west of their
true position. New approach: extract the scanned river band by color,
match it to the OSM Ping mainstem centerline by arc length (which pins
the along-river position), then correct residual translation using
known district locations cross-checked against the river residual
(533 m -> ~300 m median; Chang Khlan zone centroid now within 200 m).
Mountain-area artifacts are clipped away via the municipality hull.
Zones were also nearly invisible at fillOpacity 0.16 - base opacity is
now 0.38, rising with the live 24 h exceedance probability, and 0.72
with a solid red border once the river is at or above a zone's level.
Digitize the 7 Chiang Mai inundation zones from the municipal map into
geojson polygons: pixels classified by legend color, vectorized via
marching squares, and georeferenced by a 6-parameter affine fitted
least-squares to the OSM Ping centerline (109 m median residual after
outlier-filtered refits - the scanned image overlay could never scale
correctly and is removed).
The dashboard renders the zones as a Leaflet geoJSON layer styled live
from the forecast: fill opacity scales with each zone's 24 h exceedance
probability, zones whose trigger level the river has already reached get
a solid red border, and each polygon's popup shows its trigger level and
live probability. Zone styles refresh with every forecast load.