Written one day after the record, on evidence, and it narrows D-258's scope.
The Rationale claimed hydrology 'was not derivable at all under the old
model'. Not true, and not true when written: layer1.rs already solves drainage
AND settled-equilibrium hydrology once per body, folds the filled surface into
TerrainAnalysis, and every rung bilinearly samples it. The code's own comment
names it — 'a coarse continuous primitive computed once, sampled fresh at
every rung, never re-solved', mechanism B, D-255(f). The pattern rung 0.5 was
invented to provide already existed. I inferred the claim from Region having
no rivers without reading the layer-1 pipeline.
What actually made Global flat was a stale sentinel zeroing its extent, giving
a 2x1 canvas. Once sized correctly Global reads as a world with no hydrology
work at all. Rivers there measured negligible: 375 courses present, 458 of
518,400 pixels changed versus courses-off, because at ~39.7 km/gridunit most
courses are shorter than one gridunit.
Survives: reliefmap-as-plurality, composition-on-descent, the conservation
invariant, and the lake-shore amendment — none depend on hydrology moving.
Weakened: the stored expanded layer and its D-227 carve-out, since the
compute-once-sample-everywhere mechanism it argued for is already shipped.
T-1211 and T-1212 flagged needs-refinement with the reasoning attached; the
measurement as scoped would have priced work that is not required.
The amendment also records why this survived review: the sizing fix's tests
called resolve_canvas_extent directly rather than the serve path, and the
capture goldens could not have caught it either since they never supplied a
body radius. Two verification layers, both green, neither looking at the thing.
Co-Authored-By: Claude <noreply@anthropic.com>
jpmschweitzerandClaude
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