[PLAN] Chart prior importer: contour → ellipsoidal Chart tiles (A2 of unh_marine_autonomy#163)

[rolker]

Closes #44
Closes rolker/unh_marine_autonomy#163

Plan: Chart prior importer — contour depth-below-surface → ellipsoidal Chart tiles (A2)

Issue

#44 (A2 of, and closes, rolker/unh_marine_autonomy#163)

Context

A1 (merged) added SourceLayer::Chart + the chart_writable flag to
marine_bathymetry_store. A2 is the importer that turns the existing raw contour
prior into ellipsoidal Chart tiles the store can load. cube_bathymetry
(sensors_ws) is the right home: it can link both marine_bathymetry_store
(core_ws) and mru_transform's datum_config (platforms_ws) — keeping the store
core datum-agnostic (ADR-0002 §D7, convert at import). The package already uses
GDAL + GGGS (bag_to_geotiff.cpp is the model) but does not yet depend on the
store or mru_transform.

Raw input (~/data/bathymetry/massabesic/store/chart/<date>/, 142 level-11
GeoTIFFs): band1 = depth below lake surface, negative-down, m; band2 = const
3.0 uncertainty; band3 = timestamp. Conversion: ellipsoidal = chart_datum_z + depth_below_surface, with chart_datum_z from resolve_datum(lat,lon,entries, nullopt) (= 52.3 m inside the massabesic polygon).

Approach (single PR, cube_bathymetry; + one sibling mru_transform change — see OQ1)

1. New executable import_chart_prior (src/import_chart_prior.cpp), CLI:
   --contour-dir <raw tiles> --store <out dir> --datum-config <yaml> [--level 11].
2. Pure conversion core (src/chart_import.{hpp,cpp} or in-lib, ROS-free): given
   a BathymetryTile (raw, depth-below-surface) + a resolved chart_datum_z,
   produce the ellipsoidal tile — so the arithmetic is unit-testable without GDAL.
3. Importer flow: load_datum_config(path); for each *.tif in the contour dir
   BathymetryTile raw = loadTile(path, level) (reads by path — epoch subdir is
   fine); per data cell compute lat/lon from its CellIndex, resolve_datum(...)
   → chart_datum_z (warn+skip cells outside any polygon), set
   {chart_datum_z + raw.depth, raw.uncertainty, raw.timestamp} into a
   BathymetryStore(level, /*chart_writable=*/true) via set(Chart, …); then
   save(store, out_dir). Resolve the datum once per tile (constant within the
   massabesic polygon) — not per cell.
4. Deps: add marine_bathymetry_store + mru_transform to package.xml /
   CMakeLists.txt (find_package + link); new add_executable.
5. massabesic datum entry (chart_datum_z: 52.3, polygon ring around the
   lake) — see OQ1 for where it lands.

Files to Change

File	Change
cube_bathymetry/src/import_chart_prior.cpp	new importer executable
cube_bathymetry/src/chart_import.{hpp,cpp}	ROS-free per-tile conversion core
cube_bathymetry/CMakeLists.txt / package.xml	+ marine_bathymetry_store, mru_transform; new exe
cube_bathymetry/test/test_chart_import.cpp	conversion-core gtests
(sibling) mru_transform/config/datum_polygons.massabesic.yaml	massabesic entry (OQ1)

Principles Self-Check

Principle	Consideration
Reuse over duplication	Reuses loadTile/save (store) + resolve_datum (mru_transform); no new tile or datum code.
Layering	cube (sensors_ws) legitimately links core + platforms below it; store core stays datum-agnostic.
Do it right	Pure conversion core with gtests; single source of truth for 52.3 (datum config, not hardcoded).

ADR Compliance

ADR	Triggered	How addressed
ADR-0002 §D7 (convert at import)	Yes	Datum offset applied here; store stores ellipsoidal.
ADR-0002 §D3 / §D4	Yes	Writes the read-only Chart layer via A1's chart_writable; lake = constant offset (off-VDatum).

Consequences

If we change...	Also update...	Included?
New importer writes the Chart store	sim harness (sim#76) builds both stores from it; costmap (autonomy#164) consumes it	Downstream, separate issues
Add massabesic datum entry	the deployment's chart_datum_path should point at the same file (single source)	OQ1

Open Questions

1. Where does datum_polygons.massabesic.yaml live + who ships it? Recommend
   mru_transform/config/ (where resolve_datum + the example live, and where the
   runtime chart_datum_node datum_config_path would point — single source of
   truth) as a small sibling PR. The importer just takes --datum-config <path>.
2. Massabesic polygon ring coordinates — need a real (even coarse) lake outline
   for the ring; a generous bounding polygon around 42.99 N / -71.39 W suffices for
   a constant offset. Source the vertices before implementing the config.

Estimated Scope

One cube_bathymetry PR (importer + tests) + one small mru_transform PR (datum
config). Depends on A1 (merged); independent of the open #148 Phase-2 importer.

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