s57_layer: why is the Little Harbour bridge marked LETHAL on the costmap? (overhead vs restricted vs elevation)

[rolker]

Summary

During the BizzyBoat 2026-06-03 deployment (rolker/unh_echoboats_project11#211), the operator observed the Little Harbour bridge appear impassible on the costmap from a chart feature (the s57_layer) — before camera segmentation also marked it — so the boat could not autonomously transit under the span. Determine which s57_layer lethal path is responsible and whether it's intended for an overhead-passable bridge.

Current behavior (code)

src/s57_layer.cpp get_cost_from_grid (~L498–525) marks a cell LETHAL_OBSTACLE via three paths — there is no blanket "bridge ⇒ lethal" rule:

1. restricted layer present → lethal.
2. overhead clearance < overhead_clearance_ → lethal. (overhead_clearance_ default 10.0 m in s57_layer.h; configured 2.0 m in seafloor nav2_params.base.yaml:124,175.)
3. elevation > 0.0 (LNDARE/COALNE/CAUSWY/… rasterized as elevation 1.0) → lethal; or charted depth < minimum_depth_.

So the bridge cells must be triggering one of these.

Investigate

• At the bridge location (approach along ~lat 43.056, linger ~43.0559, −70.7224; pull exact from the #211 bag) inspect the S57 restricted / overhead / elevation grids to find which path marks the span lethal.
• If overhead < 2.0 m: is the bridge's charted vertical clearance (VERCLR) genuinely < 2.0 m, or is the overhead grid value wrong/missing? Note BizzyBoat air draft ≈ 1.32 m — a span charted between ~1.32 m and 2.0 m would be physically passable but marked lethal, i.e. the 2.0 m threshold may be conservative.
• If elevation > 0 (abutments/causeway as land): is the navigable channel between abutments wide enough to stay free, or does costmap inflation close it?
• Decide whether a deliberately-passable overhead span should be navigable for this USV, and if so how the layer should express it (clearance vs air draft).

Context

• Operationally deferred — BizzyBoat is avoiding bridges for now; this is a chart-layer investigation, not a blocker.
• Related: rolker/unh_marine_navigation#63 (avoider weaving along chart inflation); S57 land features lose LETHAL marking when tide_offset > minimum_depth #21 (closed — elevation/tide-offset lethal path).

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Authored-By: Claude Code Agent
Model: Claude Opus 4.8 (1M context)