Table of Contents

S-98 Interoperability — Design Note (PR-L0)

Status: Design / Research — no production code in this PR. This document is the contract that PR-L1 (plumbing) and PR-L2 (rule evaluation) implement against. Open questions are collected in §8; every claim derived from the spec cites a section number, and every claim that could not be verified is tagged **TBD**.

Update (issue #398 — headless multi-layer composite). The S-98 ordering + suppression engine has since been extracted off Mapsui and is now the single, renderer-neutral source of truth. It lives in EncDotNet.S100.Datasets.Pipelines.Interoperability and operates on SubLayerStackItem / StackPayload values (not Mapsui ILayers): suppression filters encoding-neutral DrawingInstructions via their VectorFeatureTag rather than Mapsui IFeatures. Two consumers share it: the Mapsui viewer (DatasetLoaderService sorts + suppresses items, then maps the ruled items back to prebuilt ILayers — validated byte-identical by the VisualRegression baselines) and a new headless HeadlessCompositor that lowers each ordered vector / coverage sub-layer into a Skia CompositeLayer and paints all datasets against one shared viewport with no Mapsui dependency. The EncDotNet.S100 facade exposes it via the IReadOnlyList<S100Layer> overload on PngS100DatasetRenderer. The rule set (R-101-102-A/B, R-101-124-A, R-104-A, R-111-A) and the S-101-under-S-102 interleave (S-98 Annex A §A-6.9.1) are unchanged; only their host moved. Pre-projected point-glyph coverage sub-layers (S-104 / S-111 fixed-station variants) are not yet supported in the headless composite path.

0. Scope of this note

The viewer currently composes a multi-product display by stacking each loaded dataset's Mapsui layers in dataset-list order (see DatasetLoaderService.FlattenLayerOrder). That is "Level 0" in S-98 terms (S-98 Annex A §4.4.1) — pure overlays, no cross-product reasoning. This note describes the minimum redesign needed to:

  1. Make the cross-dataset stacking order explicit and spec-driven (display planes, drawing priorities).
  2. Carry per-feature plane / priority metadata out of each processor so pick and any future inter-product rule can consume it.
  3. Land a first vertical slice of inter-product rules covering the two combinations users will see most often (S-101 + S-102, S-101 + S-124).
  4. Define the rule-table shape that PR-L1 plumbs and PR-L2 populates from a bundled Interoperability Catalogue.

Everything else (Level 3 hybridization, Level 4 spatial operations, predefined-combination UI, alerts) is explicitly out of scope and parked in §8.

1. Spec reference

1.1 Edition used

  • S-98 "IHO S-100 ECDIS and Interoperability Specification", Edition 2.0.0, October 2025 (IHO CL 41/2025).
  • The publication is a four-document set:
    • S-98 Main Document (general ECDIS + portrayal framing).
    • S-98 Annex A — Interoperability (Interoperability Catalogue model and ECDIS interop levels).
    • S-98 Part A (Level 1 Application Schema details).
    • S-98 Part B (Level 2 Application Schema details).
  • Downloaded as a ZIP from https://iho.int/uploads/user/pubs/standards/S-98/S-98%20Ed%202.0.0.zip during PR-L0 research. The PDFs are not yet committed to docs/specs/. A follow-up PR-L0a should drop them there (and in particular Annex A, which carries the bulk of the normative rules referenced below).
  • Section numbers in this note are written in the form S-98 Annex A §X.Y or S-98 Main §X.Y. Where a section number appears with no PDF qualifier, it is from S-98 Annex A.

1.2 Spec gaps and verification status

  • Source available: Main, Annex A, Part A, Part B — every section cited below was read directly out of the PDF.
  • Source referenced but not loaded: S-100 Part 16 (the abstract interoperability model + XML schema) — Annex A §4.2.1 makes it the normative anchor for the Interoperability Catalogue XML schema. We have S-100 Ed 5.2.1 in docs/specs/. Anything in this note that says "per Part 16 schema" is asserted on the strength of Annex A's UML diagrams; the actual XSD has not been validated against during PR-L0 — see §8 TBD-1.
  • Source not available: S-100 Part 16 5.0.0 XSD release notes; the IHO-published Data Product Interoperability Validation Checks (Edition 1.0.0, February 2025) — listed on the IHO page but not retrieved.

Anywhere in this note tagged **TBD — verify against S-98 §X.Y** the reviewer should consult the PDF before PR-L1 finalises the relevant API.

1.3 Other normative references touched by this design

  • S-100 Ed 5.2.1 (docs/specs/S-100 Ed 5.2.1_FINAL.pdf):
    • Part 9 §11.6 — display planes (the per-product, two-plane UnderRadar / OverRadar concept this codebase already implements).
    • Part 9 §10 — drawing instructions and their drawingPriority, which we already parse for S-101 and the GML/XSLT products.
    • Part 16 — the IC abstract specification (UML + XSD).
  • IMO MSC.530(106)/Rev.1 — the informative nine-tier "priority layers" list in S-98 Main §9.2.1 derives from §Appendix 2 of that Performance Standard. We adopt it as the default plane ordering for products that S-98 v2.0.0 does not cover.
  • S-52 Edition 6.1.1 (June 2015) — the legacy S-57 presentation library, normatively reused for colour set-aside and viewing group concepts (S-98 Main §10.3, §10.5).

1.4 Products in scope of S-98 Edition 2.0.0

S-98 Annex A §1.2 / Table 2-1:

Spec Edition cited by S-98
S-101 ENC Ed 1.1.0, April 2023
S-102 Bathymetric Surface Ed 2.2.0, April 2023
S-104 Water Level Ed 1.1.0, March 2023
S-111 Surface Currents Ed 1.2.0, March 2023
S-129 UKC Management Ed 1.0.0, June 2019

Out of S-98 scope but supported in this codebase (S-122, S-124, S-125, S-127, S-128, S-131, S-201, S-411, S-421). Annex A §4.1.1 says such "other similar products may also be covered on a case-by-case basis" but the closed dictionary urn:mrn:iho:prod:s98:1:1:0:products enumerates only the five above. Our design therefore treats them as Level 0 overlays with default-plane assignment derived from MSC.530(106)/Rev.1 priority layers — see §2.3.

2. Display Plane model

2.1 What S-100 / S-98 actually defines

Two independent stacking concepts collaborate:

  1. Display Plane (S-100 Part 9 §11.6; S-98 Main §7.2.9): "Display planes are used to split the output of the portrayal functions into mutually exclusive lists. An example of this is the separation of chart information drawn under a radar image and chart information drawn over a radar image."

    • S-100 ships only two canonical planes — UnderRadar and OverRadar — and our DisplayPlane enum (src/EncDotNet.S100.Core/Pipelines/Vector/VectorPipeline.cs:315) enumerates exactly those.
    • S-98 widens this: an Interoperability Catalogue may declare additional planes via S100_IC_DisplayPlane elements (Annex A §A-3.2.1.1, Part A). Per Part A §A-3.2.1.1 "A feature type may be referenced in more than one S100_IC_DisplayPlane, but the entries in different display planes must be distinguished by different attribute-value combinations or spatial primitives so that the actual instances of features are partitioned unambiguously between different display planes."
    • S-98 Main §9.2.2 anchors the radar pivot: "the radar image should be written over Portrayal Catalogue display planes with a negative order attribute; and below display planes with a positive order attribute." Our existing PC bundles (e.g. content/S111/pc/portrayal_catalogue.xml) declare <displayPlane id="OverRadar" order="1"> and <displayPlane id="UnderRadar" order="-1"> — consistent.

  2. Drawing Priority (S-100 Part 9; S-98 Main §7.2.10): "Display priorities control the order in which the output of the portrayal functions is processed by the rendering engine within a display plane. Priorities with smaller numerical values will be processed first. … If the display priority is equal among features, curve features have to be drawn on top of surface features whereas point features are drawn on top of both."

The within-plane sort order we implement (VectorPipeline.SortByPriority) is exactly that: DisplayPlane → DrawingPriority → Type (area → line → point → text). That code is correct for intra-dataset rendering and needs no change. What's missing is the cross-dataset extension.

2.2 The nine "priority layers" (informative)

S-98 Main §9.2.1 lifts the MSC.530(106)/Rev.1 "priority of information" list:

1. ECDIS visual alerts/indications (caution, overscale, …)
2. Official data: points/curves and surfaces + official updates
3. Notices to Mariners, manual input and Navigational Warnings
4. Official-caution (ENC and other cautions)
5. Official colour-fill area data
6. Official on-demand data (water levels, currents, UKC, …)
7. Radar and AIS information
8. Mariner's data: points/lines and areas
9. Mariner's colour-fill area data

The footnote (S-98 Main §9.2.1, footnote 1) makes this informative — the actual partitioning is what the Portrayal Catalogues and the IC implement. But it gives us a reasonable default plane mapping for products S-98 v2.0.0 doesn't yet cover.

2.3 Our extended plane enum

We add a third dimension to our intra-dataset enum without breaking it, by introducing an outer enum S98DisplayPlane that the cross-dataset layer-stack builder consumes; the per-instruction DisplayPlane enum stays as-is for intra-product portrayal.

Proposed values (default ordering, lowest = drawn first / farthest back):

Value Numeric Source-of-truth Maps to which products by default
BaseChartUnder 0 MSC.530 layer 5 + S-98 Main §9.2.1 layer 5 S-101 area / colour-fill features; S-57 fallback
Bathymetry 10 S-98 Annex A §8.4.1 / A-6.9.1 ("gridded bathymetry replaces depth area and depth contours") S-102
OnDemandSurface 20 MSC.530 layer 6, S-98 Main §9.2.1 S-104, S-111 (the colour-band coverage)
BaseChartOver 30 MSC.530 layer 2 + S-98 §9.2.1 layer 2 S-101 curves/points/text
OtherChartOverlays 40 MSC.530 layer 6 (catch-all) S-122 (MPAs), S-125 / S-201 (AtoN), S-127, S-128, S-131, S-411, S-421, S-129 UKC areas
CautionsAndWarnings 50 MSC.530 layer 3-4 + S-98 Main §9.2.1 layers 3-4 S-124 navigational warnings
DynamicArrows 60 S-111 PC OverRadar overlay (displayPlane id="OverRadar") S-111 arrow sublayer; S-104 trend glyphs (PR-J)
MarinerOverlay 70 MSC.530 layers 8-9 Mariner's own annotations (future)
EcdisAlerts 80 MSC.530 layer 1, S-98 Main §9.2.1 layer 1 ECDIS overscale, caution, AIO (future)

Notes:

  • The order is informative, and reviewers should treat each numeric gap (10) as expansion room for IC-declared planes (S100_IC_DisplayPlane). The actual numbers are not normative — S-98 IC entries supply their own ordering and override these defaults.
  • The intra-product DisplayPlane (UnderRadar / OverRadar) still applies within a plane. So an S-111 dataset emits two sub-layers; the colour-band one lands on OnDemandSurface, the arrow overlay on DynamicArrows. The processor is responsible for this mapping (see §4.2).
  • Where the IC carries a S100_IC_DisplayPlane with a non-default order, the IC overrides the defaults above. The defaults are the "Level 0 fallback" path required by Annex A §4.4.1.

2.4 Within-plane priority — already exists

S-101 emits <drawingPriority> integers per S-100 Part 9 §10. Our Part9DisplayListReader extracts them (DrawingInstruction.DrawingPriority). Coverage products do not emit drawing instructions and therefore don't carry an explicit priority. For coverage layers, we adopt: a single integer per sub-layer, defaulted at the processor (S-102 → 0, S-104 colour band → 0, S-111 colour band → 0, S-111 arrow → 10). The IC may override these via S100_IC_DrawingInstruction.drawingPriority.

For other GML products (S-124, S-125, …) the XSLT pipelines already emit <drawingPriority> per drawing instruction via their per-rule with-params (see e.g. S111/pc/Rules/SurfaceCurrent.xsl). So intra-product priority is already first-class for every vector product.

3. Inter-product rules in v1

S-98 Annex A is structured around four progressively more powerful Levels (§4.4):

Level Operations Status in S-98 v2.0.0
0 — Overlays None; pass-through. Fully specified (Annex A §4.4.1).
1 — Interleaving Cross-product plane / drawing-priority reassignment. Fully specified (Part A).
2 — Type selectivity + class replacement Suppress all instances of a feature type in product A when product B is loaded; substitute drawing instructions; predefined combinations. Fully specified (Part B).
3 — Feature hybridization Per-instance suppression and attribute merging. Annex A §4.4.4 notes "Support for this level is not fully elaborated in this version of the Interoperability Catalogue Specification and it should therefore not be implemented in Interoperability Catalogues created from this Specification".
4 — Spatial operations Spatial queries + geometry combination. Annex A §4.4.5 notes "Support for this level is not fully elaborated in this version of the Interoperability Catalogue Annex".

Our v1 commits to Levels 0 and 1, with a single Level 2 rule piloted for the most-visible S-101 / S-102 case. Levels 3 and 4 are deferred (§3.5) and gated on the IHO finishing the spec.

3.1 Rule R-101-102-A: S-102 supersedes S-101 depth-area shading

  • S-98 cite: Annex A §A-6.9.1 "High definition gridded bathymetry replaces (overwrites) depth area and depth contours, but soundings, aids to navigation, and obstructions are over the high definition bathymetry (interoperability Level 1)."
  • Level: Level 1 (plane reassignment); a Level 2 variant "suppress all S-101 DepthArea and DepthContour features when an S-102 dataset is loaded covering the same area" is the natural follow-up — Annex A §8.1.1 "duplicated features, same model" covers it.
  • Semantics: when an S-101 dataset and an S-102 dataset are both loaded and their coverage overlaps, the S-102 coverage layer is drawn on plane Bathymetry (between BaseChartUnder and BaseChartOver). S-101 line/point/symbol layers (the BaseChartOver plane) remain on top of S-102. At Level 1 we do not suppress the underlying S-101 depth areas — they merely end up obscured under the (typically opaque) S-102 colour shade.
  • Codebase mapping:
    • S-101 processor declares its layers as BaseChartUnder (areas/fills) and BaseChartOver (lines, points, text) — today these collapse into a single Mapsui layer; PR-L1 must split them along the DrawingPriority's area-or-not boundary, or carry the split through DrawingInstruction.TypeSortOrder. See §4.1.
    • S-102 processor declares its single coverage layer as Bathymetry.
    • LayerStackBuilder orders by (plane, withinPlanePriority, datasetOrder) — the S-102 layer lands between the two S-101 layers.
  • Level 2 extension (R-101-102-B): when both products are loaded, suppress the S-101 features whose code is in {DepthArea, DepthContour} and that lie inside an active S-102 dataset's coverage extent. Implemented as S100_IC_SuppressedFeatureLayer inside an S100_IC_PredefinedCombination keyed on the product set {S-101, S-102} (Annex A §8.4.1 "skin-of-the-earth feature replacement" + Part B §B-3.1.2).
    • Only inside the S-102 extent (2026-10-03). The rule first suppressed every S-101 depth feature as soon as any S-102 was visible. A small S-102 tile then emptied the depth shading from the whole harbour cell around it, and mid-channel views showed only the basemap. The surface replaces depth areas and contours only where it is displayed (Part A §A-6.9.1), so the S-102 processor now reports its WGS-84 grid extent (CoveragePortrayalResult.CoverageExtent). A depth feature is suppressed only when all its vertices lie inside an active S-102 extent. A feature that straddles the edge is kept: its fill paints under the opaque surface anyway. An active S-102 that reports no extent is still taken to cover everything.
    • The viewer keeps the suppressed layer tiled. The Mapsui projector rebuilds a suppressed sub-layer from its prebuilt layer. It now binds the prebuilt layer's VectorScene minus the dropped features to the new layer (S100VectorTileRenderer.TryBindFilteredScene). Before this, the filtered layer fell back to its pick-target features: it was not tiled and it lost its pattern fills.

3.2 Rule R-101-124-A: S-124 warnings render above all ENC base data

  • S-98 cite: Annex A §4.4.1 (Level 0) + MSC.530(106)/Rev.1 priority layer 3 (via S-98 Main §9.2.1) — "Notices to Mariners, manual input and Navigational Warnings" sit above "Official-data: Points/Curves and Surfaces". TBD — verify against S-98 §X.Y for an explicit Annex A rule (S-124 is not in Annex A Table 1-1, so this rule is currently a viewer-side derivation from MSC.530, not an IC clause).
  • Level: Level 0 only — S-124 is outside the IC's product list (Annex A §4.1.1 / closed dictionary urn:mrn:iho:prod:s98:1:1:0:products), so it stays an overlay. We treat the rule as a default-plane assignment, not an IC rule.
  • Semantics: S-124 features render on plane CautionsAndWarnings (50). They appear above every base-chart and on-demand-coverage plane, and below mariner data + ECDIS alerts.
  • Codebase mapping:
    • S124DatasetProcessor.Render declares its single emitted layer's plane as CautionsAndWarnings.
    • No IC involvement. The rule is the default plane assignment.

3.3 Rule R-104-A / R-111-A: Water level & current overlays sit above bathymetry, below ENC line work

  • S-98 cite: Annex A §A-6.9.1 "Gridded data will generally go over ENC and obscure ENC features, either all (interoperability Level 0) or specific features (interoperability Level 1) depending on interoperability Level chosen…" — and S-98 Main §9.2.1 layer 6 "Official on demand data (for example, water levels, surface currents, under keel clearance)".
  • Level: Level 1.
  • Semantics:
    • S-104 colour band → plane OnDemandSurface (20).
    • S-111 colour band → plane OnDemandSurface (20). S-111 arrows → plane DynamicArrows (60).
    • S-101 line/point work on BaseChartOver (30) sits above the on-demand surface but below the dynamic arrows. TBD-2 resolved (2026-10-03): keep this stacking. See §8 TBD-2 for the spec reading.
    • The same holds for S-57 cells translated to S-101 / S-401: they split into BaseChartUnder fills and BaseChartOver line work just like native S-101 (§4.2.1), so the band lands between them.
  • Citation correction: the "Gridded data will generally go over ENC…" clause is §A-6.9.1 of S-98 Part A (Level 1 Application Schema), and the same text appears in Part B §B-6.9.1. It is not in Annex A, whose appendix stops at A-6.3. Older Annex A §A-6.9.1 citations in code comments mean Part A §A-6.9.1.
  • Codebase mapping: S-104 and S-111 processors already split into multiple sub-layers (MapsuiDatasetResult.Layers + LayerNames); the processor sets a per-sub-layer plane.

3.4 Summary table for v1

Rule Level Products Operation S-98 cite
R-101-102-A 1 S-101, S-102 S-102 coverage on plane Bathymetry, between S-101 areas and S-101 line work Annex A §A-6.9.1
R-101-102-B 2 S-101, S-102 Suppress S-101 DepthArea, DepthContour when S-102 covers same area Annex A §8.4.1 + Part B §B-3.1.2
R-101-124-A 0 S-101, S-124 S-124 warnings on plane CautionsAndWarnings S-98 Main §9.2.1 + MSC.530(106)/Rev.1 §Appendix 2 (S-124 not in IC)
R-104-A 1 S-101, S-104 S-104 colour band on OnDemandSurface, below S-101 line work Annex A §A-6.9.1 + S-98 Main §9.2.1 layer 6
R-111-A 1 S-101, S-111 S-111 colour band on OnDemandSurface; arrows on DynamicArrows Annex A §A-6.9.1 + existing PC displayPlane id="OverRadar"

That's five proposed v1 rules spanning four product pairs. Three of them (R-101-102-A, R-104-A, R-111-A) are pure default plane assignments and need no IC payload at all; PR-L1 ships them. R-101-102-B and the dynamic IC-driven overrides need PR-L2.

3.5 Deferred — explicit non-goals for v1

Rule family Why deferred
Predefined Combinations UI (S100_IC_PredefinedCombination) Annex A §11 / Part B §B-3.1.2 — large UX surface; needs a layer-controls panel (gated on PR-L3).
Hybrid Feature / Portrayal Catalogues Annex A §4.4.4 — "should not be implemented in Interoperability Catalogues created from this Specification".
Spatial operations (Level 4) Annex A §4.4.5 — "Support for this level is not fully elaborated".
Skin-of-the-earth adjusting (geometry edits) Annex A §8.4.2 — Level 3/4 only.
Safety-contour synthesis from S-102 + S-104 Annex A §A-6.9.1 NOTE — "OEMs are permitted to add additional safety contour functions" — explicitly optional; significant new pipeline.
Pre-defined Combination switching UX Annex A §15.4. Tied to layer-controls UI PR-L3.
Alerts triggered by interop result Annex A §7.3 (Main) + Annex A §11.7. Needs alert engine.
Colour set-aside arbitration S-98 Main §10.3 + Part 16 — needs palette-aware rule input.

4. Codebase impact map

4.1 src/EncDotNet.S100.Core/

Additions (new types — no behaviour yet, PR-L1 wires them up):

  • EncDotNet.S100.Interoperability.S98DisplayPlane (enum) — the values from §2.3.
  • EncDotNet.S100.Interoperability.LayerStackEntry (record): 
    public sealed record LayerStackEntry(
    Mapsui.Layers.ILayer Layer,
    S98DisplayPlane Plane,
    int WithinPlanePriority,
    DatasetEntry SourceDataset,
    string? SourceSubLayerName,
    string? SourceFeatureType);    // null for whole-layer entries
  • EncDotNet.S100.Interoperability.IInteroperabilityAuthority (interface): 
    public interface IInteroperabilityAuthority
{
    IReadOnlyList<LayerStackEntry> Apply(IReadOnlyList<LayerStackEntry> raw);
}
    The authority is pure — (in) → (out) — so it can be unit-tested without the viewer. PR-L1 ships the default Level0Authority : IInteroperabilityAuthority that just sorts by (Plane, WithinPlanePriority, DatasetOrder). PR-L2 layers a catalogue-driven implementation on top.

The existing DisplayPlane enum (UnderRadar / OverRadar) in Core.Pipelines.Vector.VectorPipeline.cs:315 does not change — it remains the intra-product portrayal concept. The two enums exist side-by-side. Doc the relationship in the new S98DisplayPlane's XML summary.

4.2 Renderer output and pipeline portrayal sources

MapsuiDatasetResult (in src/EncDotNet.S100.Renderers.Mapsui/MapsuiDatasetResult.cs) grows an optional sibling to the existing LayerNames:

public IReadOnlyList<S98DisplayPlane>? LayerPlanes { get; init; }
public IReadOnlyList<int>? LayerWithinPlanePriorities { get; init; }

Both must be the same length as Layers when supplied. Null defaults to (BaseChartOver, 0) for vector products and (OnDemandSurface, 0) for coverage products — chosen by the factory based on Spec.Name so that every existing processor remains source-compatible until it opts into plane assignment.

Per-processor plane defaults (PR-L1 fills these in):

Processor Layer(s) Plane Within-plane priority
S101DatasetProcessor (today) single vector BaseChartOver inherits from DrawingPriority ordering already inside the layer
S101DatasetProcessor (post-split, see below) areas BaseChartUnder 0
S101DatasetProcessor (post-split) lines + points + text BaseChartOver 0
S102DatasetProcessor coverage Bathymetry 0
S104DatasetProcessor colour band OnDemandSurface 0
S111DatasetProcessor colour band (not emitted — the bundled S-111 Ed 2.0.0 PC defines arrows only; reserved plane if one returns) OnDemandSurface 0
S111DatasetProcessor arrow overlay DynamicArrows 10
S122DatasetProcessor vector OtherChartOverlays 0
S124DatasetProcessor vector CautionsAndWarnings 0
S125DatasetProcessor vector OtherChartOverlays 0
S127DatasetProcessor vector OtherChartOverlays 0
S128DatasetProcessor vector OtherChartOverlays 0
S129DatasetProcessor vector OtherChartOverlays 0 (S-129 is in the IC; PR-L2 may move it)
S131DatasetProcessor vector OtherChartOverlays 0
S201DatasetProcessor vector OtherChartOverlays 0
S411DatasetProcessor vector OtherChartOverlays 0
S421DatasetProcessor vector OtherChartOverlays 0
S57DatasetProcessor areas (s57.areas) BaseChartUnder 0
S57DatasetProcessor lines + points + text (s57.linework) BaseChartOver 0

4.2.1 The S-101 split problem

Today S101DatasetProcessor.Render (src/EncDotNet.S100.Datasets.Pipelines/S101DatasetProcessor.cs:83) produces a single Mapsui vector layer that internally is sorted by (DisplayPlane, DrawingPriority, Type) via VectorPipeline.SortByPriority. To honour R-101-102-A (S-102 between S-101 areas and lines), we must split it.

Two options are open for PR-L1:

  • Option A — split inside the renderer. Have MapsuiDisplayListRenderer.Render return two ILayers (areas, then "lines+points+text"). Cleanest, but the renderer becomes coupled to the S-98 partition.
  • Option B — split in the processor. Run the pipeline twice with a pre-filter that selects only area-typed DrawingInstructions the first time and the rest the second time. Doubles symbol-cache reuse cost but isolates the change.

Recommendation: Option B. It keeps the renderer ignorant of S-98 and is fully reverted by toggling the split off. PR-L1 implements it. Cost is one extra style sort + render pass per S-101 dataset; benchmarks expected to be < 5% per Mapsui's measured inner-loop costs.

The split applies only to S-101 (and S-401 and S-57). Coverage products and GML overlays do not have a fill-vs-linework split.

S-57 follow-up. S57DatasetProcessor originally emitted the translated cell as a single s57.main sub-layer on BaseChartOver. That put the cell's DepthArea fills above Bathymetry and OnDemandSurface, so an S-104 water-level band, or an S-102 surface, was painted over completely wherever an S-57 ENC covered it. Because the cell is translated to S-101 / S-401 and portrayed with that catalogue, it now splits along the same AreaInstruction boundary as native S-101: s57.areas (BaseChartUnder, SourceFeatureType = "area") and s57.linework (BaseChartOver). Pinned by tests/EncDotNet.S100.Pipelines.Tests/S98OnDemandSurfaceOrderingTests.cs, which builds the stack from the real S-57, S-101, S-104 and S-111 processors in every load order. S-57 is still excluded from the S-101-keyed suppression rules (R-101-102-B, R-101-104-B land mask), per Annex A §4.1.1.

4.3 src/EncDotNet.S100.Viewer/

4.3.1 Where cross-dataset assembly lives today

DatasetLoaderService.FlattenLayerOrder (src/EncDotNet.S100.Viewer/Services/DatasetLoaderService.cs:473) is the only place layers from different datasets are combined. It walks _entryOrder (user-controlled dataset stacking from the Datasets panel) bottom-up, concatenates each entry's _entryLayers[entry] in dataset-iteration order, and hands the result to IMapLayerCollection.ReplaceDatasetLayers.

That means today every layer of dataset N sits below every layer of dataset N+1, regardless of plane. This is Level 0 in S-98 terms but with a user-defined stacking criterion — closer to "naive overlay" than the §9.2.1 priority-layer list.

4.3.2 Where it moves to

PR-L1 inserts a LayerStackBuilder between the per-entry render result and the map host:

            ┌───────────────────────────────────┐
            │ DatasetLoaderService              │
            │   _entryLayers : entry → ILayer[] │
            │   _entryPlanes : entry → plane[]  │  (NEW)
            └────────────┬──────────────────────┘
                         │
                         ▼
            ┌───────────────────────────────────┐
            │ LayerStackBuilder                 │
            │   raw : LayerStackEntry[]         │
            │   authority.Apply(raw) → ordered  │
            └────────────┬──────────────────────┘
                         │
                         ▼
            ┌───────────────────────────────────┐
            │ IMapLayerCollection.ReplaceDatasetLayers │
            └───────────────────────────────────┘

The builder is owned by DatasetLoaderService (no new singleton). IInteroperabilityAuthority is injected; the default Level-0 implementation is registered in Program.cs / DI bootstrapping. PR-L2 swaps the implementation without touching the builder.

The user's Datasets-panel ordering remains the tie-breaker inside a plane — implemented by passing entryOrderIndex as the last sort key inside the authority. No regression to the existing "drag-to-reorder" UX. This must be tested with a fixture in tests/ that loads two S-101 datasets and confirms drag-reordering still works.

4.3.3 PickService consumption

PickService.ResolveHits (src/EncDotNet.S100.Viewer/Services/PickService.cs:135) walks mapInfo.MapInfoRecords in Mapsui's hit-test order. PR-L1 keeps that contract but adds a stack-order sort key:

hits.OrderByDescending(h => layerStackOrder[h.Layer]).ThenBy(h => …)

…where layerStackOrder is the dictionary produced by the builder. That delivers §6 ("pick should be stack-ordered, topmost first") in PR-L1 without waiting on PR-L2. Mapsui still does the geometric hit test; we only reorder the dedup-survivors.

4.3.4 Other viewer files touched in PR-L1

  • ViewerDatasetCatalog.cs — none expected.
  • MainViewModel.cs — none expected (it never assembles layers).
  • MapsuiMapHost.cs (i.e. IMapLayerCollection.ReplaceDatasetLayers) — none; it already takes a flat ordered list.
  • DatasetsViewModel.cs — no changes; user-perceived ordering still flows through this VM.

4.4 src/EncDotNet.S100.Specifications/content/S98/

S-98 distributes the Interoperability Catalogue as an XML document conforming to the S-100 Part 16 schema (Annex A §4.2.1). When IHO publishes a normative IC we follow the bundled-spec convention already in use for S-127 / S-131 / S-411:

src/EncDotNet.S100.Specifications/
  content/
    S98/
      catalogue/
        S98-Interoperability-Catalogue-1.0.0.xml      (eventual normative)
        S98-Interoperability-Catalogue-1.0.0.xsd
      Adapter/
        defaults.xml                                  (our PR-L1 default
                                                       plane assignments
                                                       in IC-shaped XML
                                                       so the same loader
                                                       can read both)
      README.md

defaults.xml is our content (not byte-identical to upstream) — it encodes the default plane table from §2.3 as a Level-1 IC so the PR-L2 loader works against a single shape. When the IHO publishes the normative IC, catalogue/ is dropped in byte-for-byte and Adapter/defaults.xml becomes the fallback when no normative IC covers a particular product pair (e.g. for S-122 / S-124 / non-Annex-A products).

EncDotNet.S100.Specifications.Specification gains:

public static IInteroperabilityCatalogueSource CreateInteroperabilityCatalogueSource();

PR-L1 publishes the type; PR-L2 wires it into the authority.

4.5 src/EncDotNet.S100.Renderers.Mapsui/

No PR-L1 changes if we adopt Option B (§4.2.1). If we ever adopt Option A, MapsuiDisplayListRenderer.Render returns IReadOnlyList<ILayer> and the area/line+text split moves here.

4.6 Tests

New project (no existing one matches):

tests/EncDotNet.S100.Interoperability.Tests/
  LayerStackBuilderTests.cs
  Level0AuthorityTests.cs
  CatalogueLoaderTests.cs              (PR-L2)
  PredefinedCombinationTests.cs        (PR-L2)

Existing pipeline tests under tests/EncDotNet.S100.Pipelines.Tests/ need new cases:

  • S101DatasetProcessor emits two layers with correct planes when Option B split is on.
  • S102DatasetProcessor declares its coverage layer's plane as Bathymetry.
  • S111DatasetProcessor declares both sub-layer planes correctly.

5. The rule-table shape

5.1 Wire format — XML, IC-shaped, mirroring Part 16

The catalogue must be XML conforming to the S-100 Part 16 IC schema (Annex A §4.2.1, §A-3.2.1.1, §B-3.2.1). TBD-1: we have not yet validated against the real Part 16 XSD; PR-L1 should drop the IHO XSD into content/S98/catalogue/ and validate during build. The element names below are taken from Annex A — they should match the XSD's element names but the exact case / qualification is unconfirmed.

Example Level-1 IC fragment (our Adapter/defaults.xml):

<S100_IC_InteroperabilityCatalogue
    xmlns="http://www.iho.int/s100ic/1.0"
    interoperabilityLevel="1">

  <productCovered>S-101</productCovered>
  <productCovered>S-102</productCovered>
  <productCovered>S-104</productCovered>
  <productCovered>S-111</productCovered>
  <productCovered>S-129</productCovered>

  <S100_IC_DisplayPlane id="BaseChartUnder" order="0"
                        interoperabilityLevel="1">
    <S100_IC_Feature product="S-101" featureCode="DepthArea"/>
    <S100_IC_Feature product="S-101" featureCode="LandArea"/>
    <S100_IC_Feature product="S-101" featureCode="SeaArea"/>
    <!-- … -->
  </S100_IC_DisplayPlane>

  <S100_IC_DisplayPlane id="Bathymetry" order="10"
                        interoperabilityLevel="1">
    <S100_IC_Feature product="S-102" featureCode="BathymetryCoverage"/>
  </S100_IC_DisplayPlane>

  <S100_IC_DisplayPlane id="BaseChartOver" order="30"
                        interoperabilityLevel="1">
    <S100_IC_Feature product="S-101" featureCode="*"/>
    <!-- everything not bound elsewhere -->
  </S100_IC_DisplayPlane>

  <!-- … -->
</S100_IC_InteroperabilityCatalogue>

Level-2 additions (e.g. for R-101-102-B) introduce S100_IC_PredefinedCombination + S100_IC_SuppressedFeatureLayer (Annex A §B-3.1.2, §B-3.1.4):

<S100_IC_PredefinedCombination
    id="S101+S102-bathymetry"
    interoperabilityLevel="2">
  <productSet>S-101,S-102</productSet>
  <S100_IC_SuppressedFeatureLayer product="S-101" featureCode="DepthArea"/>
  <S100_IC_SuppressedFeatureLayer product="S-101" featureCode="DepthContour"/>
</S100_IC_PredefinedCombination>

5.2 In-memory model

public sealed record S98Catalogue(
    int InteroperabilityLevel,
    IReadOnlyList<string> ProductsCovered,
    IReadOnlyList<S98DisplayPlaneEntry> DisplayPlanes,
    IReadOnlyList<S98PredefinedCombination> PredefinedCombinations);

public sealed record S98DisplayPlaneEntry(
    string Id,
    int Order,
    int InteroperabilityLevel,
    IReadOnlyList<S98FeatureBinding> Features);

public sealed record S98FeatureBinding(
    string Product,
    string FeatureCode,           // "*" or a Feature Catalogue code
    string? Filter,               // S-98 §4.3 filter expression, or null
    int? OverrideDrawingPriority);

public sealed record S98PredefinedCombination(
    string Id,
    int InteroperabilityLevel,
    IReadOnlyList<string> ProductSet,
    IReadOnlyList<S98SuppressedFeatureLayer> Suppressed);

public sealed record S98SuppressedFeatureLayer(
    string Product,
    string FeatureCode,
    string? Filter);

The XML loader (S98CatalogueLoader.LoadAsync) returns S98Catalogue. The authority consumes it.

5.3 Evaluation contract

Pure function, as called out in §4.1:

IReadOnlyList<LayerStackEntry> Apply(
    IReadOnlyList<LayerStackEntry> raw);

The default CatalogueDrivenAuthority algorithm:

  1. For each entry in raw, find the first S98DisplayPlaneEntry whose Features matches the entry's (SourceProcessor.Spec.Name, SourceFeatureType). The match semantics:
    • exact product + code, then product + *, then no match (fall through to the entry's Plane default from the processor).
    • filters are not evaluated in v1 because we operate at the layer granularity, not the per-feature level (Annex A §4.3 filters need Level 3+ to act per-instance).
  2. Override entry.Plane with the matched plane's Order-derived S98DisplayPlane. (Order → S98DisplayPlane is a fixed map for the canonical planes; non-canonical IC-declared planes get slotted into the numeric gap closest to their order.)
  3. Apply any per-S100_IC_DrawingInstruction.drawingPriority override (none in v1).
  4. For each S98PredefinedCombination whose ProductSet is a subset of the currently-loaded products, drop every entry whose (Product, FeatureCode) matches a Suppressed row.
  5. Sort by (Plane.Order, WithinPlanePriority, DatasetOrderIndex).

This algorithm is pure and trivially unit-testable.

5.4 Where the catalogue lives

  • Bundled in EncDotNet.S100.Specifications's embedded resources (same pattern as S127/pc/ etc.).
  • Loaded once at startup by Specification.CreateInteroperabilityCatalogueSource().
  • Selected by interop-level: the user picks Level 0 / 1 / 2 in the layer-controls UI (PR-L3). PR-L1 hard-wires Level 1.

6. Pick semantics

6.1 Today

PickService.ResolveHits enumerates mapInfo.MapInfoRecords in Mapsui's order (which is roughly topmost-first within a single hit, but unspecified across layers), dedupes by (processor, featureRef), and presents the survivors to the panel. Status text follows hits[0], with a "+N more" suffix.

6.2 Proposal

S-98 Annex A §A-6.12 (Part A) and §10.12 (Main) note "The Pick Report functionality specification in S-98 is still under development." So pick stack ordering is viewer-side derivation, not a normative cite.

We adopt the natural rule: pick is ordered by the same stack the renderer uses, top-of-stack first.

Concretely in PickService.ResolveHits:

  1. Compute stackOrder : ILayer → int once per IDatasetLoaderService.EntryLayers change (cached in LayerStackBuilder).
  2. Sort MapInfoRecords by descending stackOrder[record.Layer] before the existing dedup walk.
  3. Existing dedup-by-(processor, featureRef) then keeps the topmost hit per logical feature (since the topmost record wins by being first into seen.Add).

Status-text logic is unchanged — hits[0] is now guaranteed to be the topmost hit, which matches user expectation ("the pick panel describes what I clicked on, not what was hidden underneath").

Coverage pick (TryCoveragePick) is unaffected — it triggers only when no vector hit existed at all.

6.3 Future: Pick filtered by IC

Annex A §A-6.12 hints at future "pick report" features tied to the IC. Out of scope for v1 — once the spec lands, we can add a SourceFeatureType-aware filter in the same place.

7. Visual regression and rollout plan

7.1 Snapshot impact

Re-ordering layers will change most rendered snapshots. Specifically:

  • Any test that loads an S-101 + S-102 pair (RenderS102 outputs) will change as S-102 moves between S-101 area fills and S-101 line work.
  • S-104 / S-111 + S-101 snapshots will shift if any S-101 line work was previously below an on-demand surface.
  • Single-product snapshots should not change, because the intra-product sort (VectorPipeline.SortByPriority) is unchanged.

7.2 Rebaseline approach

  1. PR-L1 lands the plumbing but defaults the authority to Level0Authority (no plane reordering) so behaviour is identical to today. Snapshots do not change.
  2. PR-L1.5 (or PR-L2 opener) flips the default to CatalogueDrivenAuthority(Level=1) and explicitly rebaselines the affected snapshots. Each rebaselined image gets a short commit-message note ("S-98 R-101-102-A: S-102 moves between S-101 areas and S-101 line work").
  3. The PR includes a before/after collage image bundle so reviewers can diff visually.
  4. Anything that's not one of the five v1 rules should produce identical output. If a snapshot changes that we did not expect, the change needs a clause cite or a TBD entry — no silent visual shifts.

7.3 PR sequence

PR Scope Files-changed budget
PR-L0 (this) Design note only. docs/design/s98-interoperability.md + docs/toc.yml
PR-L1 plumbing S98DisplayPlane enum, LayerStackEntry, IInteroperabilityAuthority, Level0Authority, processor plane defaults, LayerStackBuilder, S-101 area/line split, pick stack-ordering, tests. No snapshot rebaseline (authority is L0). ~25 files; new project EncDotNet.S100.Interoperability likely warranted
PR-L1.5 Flip default authority to L1 catalogue-driven; load Adapter/defaults.xml; rebaseline snapshots. ~5 src files + ~20 snapshot files
PR-L2 rules S98CatalogueLoader, CatalogueDrivenAuthority, R-101-102-B suppression rule, predefined-combination plumbing, tests. Still no UI. ~15 files
PR-L3 UI Layer-controls panel: interop-level selector, predefined-combination picker, per-plane visibility toggles. viewer-only
PR-L4 S-98 IC Drop normative S-100 Part 16 schema + IHO-published IC into content/S98/catalogue/; deprecate Adapter/defaults.xml. content-only

Each PR is independently mergeable. PR-L1's plumbing is dead code (authority is L0, behaves like today) but it gives us the API surface to land tests against ahead of L2.

8. Open questions / TBDs

The reviewer should resolve these before PR-L1 is opened. Each item is actionable as a focused follow-up session.

  • TBD-1. Validate the proposed XML element names (S100_IC_InteroperabilityCatalogue, S100_IC_DisplayPlane, S100_IC_Feature, S100_IC_DrawingInstruction, S100_IC_PredefinedCombination, S100_IC_SuppressedFeatureLayer) against the actual S-100 Part 16 XSD. Drop the XSD into content/S98/catalogue/ and validate Adapter/defaults.xml during build. (Annex A §4.2.1.)

  • TBD-2 — resolved 2026-10-03: keep the colour band above ENC fills and below ENC line work. Checked against the S-98 Ed 2.0.0 PDFs. S-98 has no clause that places an S-104 or S-111 surface directly; the Interoperability Catalogue is meant to decide (Annex A §8.6, §10.11). The text does constrain the answer, though:

    • Part A §A-6.9.1 / Part B §B-6.9.1. Gridded data goes over the ENC. At Level 0 it obscures all ENC features; at Level 1 and above it obscures only specific ones, by interleaving display planes. The worked Level 1 example (S-102) overwrites depth areas and contours while soundings, aids to navigation and obstructions stay above the grid. Our plane split is that same structure.
    • Main §9.2.1 (informative). Of the priority categories, layer 1 is the top. Layer 6, "official on demand data" (water levels, surface currents), must not obscure the information in layer 2 (official points, curves and surfaces) or layer 5 (official colour-fill areas). So ENC line work, points, symbols and text must stay above the surface. Main §10.8 also says text is drawn last.
    • Annex A §4.4.1–4.4.2. Level 0 overlays may use transparency so lower layers stay visible. Level 1 interleaves feature layers with the ENC "to prevent ENC data from being obscured".

    Conclusion: ENC line work above the surface is supported by every relevant clause, so it stays. Putting the band over the ENC fills follows the Part A/B model. Read literally, Main §9.2.1 would forbid fully obscuring the fills. Annex A §4.4.1 makes transparency the tool for this, so the S-104 band is partly transparent by default: GridCoverageSubLayer.Opacity is S104DatasetProcessor.ColorBandOpacity (0.8). The viewer's map session multiplies that portrayal opacity by the mariner's dataset and sub-layer opacity rather than replacing it, including when R-101-104-B rebuilds the band's layer with a land mask. The headless compositor composites the band at the same opacity. S-102 stays opaque (1.0) because Part A §A-6.9.1 has it replace the ENC depth areas. Two related points: S-98 Main §6.1.1 requires S-104 only for water-level adjustment, so the band is an optional graphical display (gridded S-104 loads hidden). S-98 never calls for an S-111 colour band; the bundled S-111 portrayal catalogue defines arrows only, and they sit on DynamicArrows above the ENC.

  • TBD-3. Decide between S-101 split Option A (renderer) vs Option B (processor double-pass) — §4.2.1. Recommendation is Option B but a microbenchmark on a large ENC cell should confirm the < 5% expectation.

  • TBD-4. Where does S-129 UKC sit by default? S-129 is in Annex A Table 1-1 but Annex A doesn't pin a default plane for it — Section 9.2.1 layer 6 covers it implicitly under "on-demand data". Our table puts S-129 on OtherChartOverlays; arguably it belongs on OnDemandSurface instead. Verify in the eventual IHO-published IC.

  • TBD-5. Filter-expression evaluation (§4.3 of Annex A — strings like "depth>10"). We've punted at the layer granularity for v1. When the layer-controls panel needs to honour S100_IC_PredefinedCombination filters that do discriminate by attribute, we'll need to either re-emit per-feature plane metadata into LayerStackEntry or push filtering into the per-product pipelines. Pick the design and write it up before PR-L2.

  • TBD-6. Skin-of-the-earth handling. R-101-102-B suppresses DepthArea / DepthContour when S-102 is loaded. S-98 Annex A §8.4.1 + §A-6.9.1 NOTE caution that the safety contour is an IMO requirement (MSC.232(82) §5.8) that must continue to be drawn even when S-102 replaces depth shading. Decide whether we preserve the safety contour as an exception to the suppression rule, and where the exception is encoded (in our default IC, or by a "safety-contour synthesiser" later).

  • TBD-7. Pre-defined combination UX: how does the user pick one? S-98 Annex A §15.4 says the user "must be able to select" a PdC. PR-L3 has to design this picker. For now, PR-L2 hard-codes "use the IC's first PdC whose product set is a subset of the loaded products" and ships without UX.

  • TBD-8. Out-of-S-98 products. We use MSC.530(106)/Rev.1 §App.2 as informative default-plane derivation for S-122 / S-124 / S-125 / S-127 / S-128 / S-131 / S-201 / S-411 / S-421. That is not a normative S-98 reading. Decide whether the project wants to (a) keep these on default planes derived from MSC.530, (b) commit to specific planes per product spec, (c) wait for S-98 v3.0.0 to enumerate them.

  • TBD-9. Should S98DisplayPlane be an enum or an open string-id? An enum bakes in our nine canonical values; the IC schema (Annex A §A-3.2.1.1) lets a catalogue declare any identifier. Recommendation: enum for the canonical planes, with an escape hatch string ExtensionId carried on LayerStackEntry for IC-declared custom planes. Decide before PR-L1.

  • TBD-10. Pick across processors. Today PickService.ResolveHits dedupes by (processor, featureRef). Under S-98 Level 3 hybridization, a single visible feature may span two processors (S-101 depth area + S-102 hi-def patch). We don't need to solve this for v1, but the dedup key should be reviewed before PR-L3 lands hybrid-feature pick.

  • TBD-11. Should the design note also commit our reading of Annex A §15.5 "Priority overrides for user-specified settings"? We currently let the user drag-reorder datasets in the panel, which is broadly equivalent to manual plane override. PR-L3 must decide whether that UX survives, lives alongside the plane controls, or is replaced by a single ordered list with grouping.

  • TBD-12. S-98 Annex A §13 metadata: the IC carries S100_ExchangeCatalogue + S100_CataloguePointofContact + signature blocks. Our bundled Adapter/defaults.xml is not a signed IHO publication; clarify in the README that this is a viewer-internal default, not a forged IHO catalogue.

Appendix A — Code-citation index

For convenience to PR-L1's implementer, the locations referenced above:

  • src/EncDotNet.S100.Core/Pipelines/Vector/VectorPipeline.cs:315 — enum DisplayPlane { UnderRadar, OverRadar }.
  • src/EncDotNet.S100.Core/Pipelines/Vector/VectorPipeline.cs:287 — SortByPriority, the intra-product sort.
  • src/EncDotNet.S100.Core/Pipelines/Vector/DrawingInstruction.cs:34 — DrawingPriority per instruction.
  • src/EncDotNet.S100.Core/Pipelines/Vector/Part9DisplayListReader.cs:62-355 — Part 9 drawing-instruction parser (already extracts plane and priority for every GML/XSLT product).
  • src/EncDotNet.S100.Renderers.Mapsui/MapsuiDisplayListRenderer.cs:156-167 — area / pattern fill ordering by DrawingPriority.
  • src/EncDotNet.S100.Datasets.Pipelines/IDatasetProcessor.cs — IDatasetProcessor contract.
  • src/EncDotNet.S100.Renderers.Mapsui/MapsuiDatasetResult.cs — MapsuiDatasetResult (extension point for LayerPlanes / LayerWithinPlanePriorities).
  • src/EncDotNet.S100.Datasets.Pipelines/S101DatasetProcessor.cs:83 — S101DatasetProcessor.Render; the split point for §4.2.1.
  • src/EncDotNet.S100.Viewer/Services/DatasetLoaderService.cs:473 — FlattenLayerOrder; the only place cross-dataset stacking exists today.
  • src/EncDotNet.S100.Viewer/Services/DatasetLoaderService.cs:117-118 — EntryLayers (read-only view of entry → ILayer[]); the builder needs to read this.
  • src/EncDotNet.S100.Viewer/Services/PickService.cs:135-166 — ResolveHits; where stack-order sorting is injected.
  • src/EncDotNet.S100.Specifications/content/S111/pc/portrayal_catalogue.xml:181-196 — example <displayPlanes> block already in a bundled PC.
  • src/EncDotNet.S100.Specifications/content/S111/pc/Rules/SurfaceCurrent.xsl:11-12 — example of a portrayal rule emitting <displayPlane>UnderRadar</displayPlane> and <drawingPriority>10</drawingPriority>.
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