Boundary and Seam Regression Test¶

Regression audit — instantiates Neighborhood-Preserving Substrate Mapping

Re-verifies, after every map change, that continuity still holds across the substrate's edges and seams — the wrap-arounds, tile joins, and chart borders where neighborhood preservation is most fragile and regressions hide.

Boundary and Seam Regression Test is a regression suite scoped deliberately to the map's seams — the places where the substrate wraps around on itself, where two tiles or charts abut, or where the representable domain simply ends. Its defining move is to ignore the interior (well-covered by other checks) and concentrate its assertions on edges and seams, re-running them after every remesh, retrain, or tiling change. Seams are exactly where a small code or data change can silently reintroduce a discontinuity, and exactly where interior metrics never look; this test encodes the boundary-and-seam policy as executable assertions and fails loudly the moment a change tears a seam the policy says must stay continuous.

Example¶

A mapping service renders the world as square tiles in a Web Mercator pyramid. Everything works until an engineer optimizes tile loading and quietly breaks the antimeridian — the ±180° longitude seam — so a shipping route across the Pacific now draws a line all the way back across the whole map. A boundary-and-seam regression test catches this before deploy. It asserts that two points one kilometer apart across the date line still render one kilometer apart, that a polygon spanning the seam stays a single polygon rather than splitting, and that tile edges align to the pixel at every zoom level. Held as fixtures and run in CI on every rendering change, it converts "someone will eventually notice the map looks wrong over the Pacific" into a red build the same afternoon the seam broke.

How it works¶

Enumerate the substrate's seams explicitly — wrap edges, tile joins, chart boundaries, projection singularities such as the poles. For each, write executable assertions on cross-seam continuity, adjacency, and alignment, plus checks that any edge-effect compensation (padding, wrap-around context) is still present. Bind representative cross-seam cases as regression fixtures. Run the whole suite on every map-affecting change and fail with the specific seam named. What distinguishes it from a general preservation test is that it is interior-blind, regression-oriented, and organized by an inventory of seams.

Tuning parameters¶

Seam inventory — which edges are declared seams and checked; any seam left off the list is an invisible blind spot.
Continuity tolerance — how much mismatch across a seam is allowed before the test fails.
Trigger scope — run on every commit versus only at release; more frequent catches regressions sooner at more CI cost.
Edge-compensation assertions — whether the suite also verifies that edge fixes exist, not merely that the seam currently lines up.
Fixture coverage — the specific cross-seam artifacts (a route, a polygon, a cluster) pinned as regression cases.

When it helps, and when it misleads¶

Its strength is that seams are where preservation quietly breaks and where interior fidelity scores are looking the other way; a scoped regression suite makes those failures cheap to catch and hard to reintroduce. Its honest limits: it only tests the seams it knows about — an unenumerated seam is invisible to it — and it verifies that continuity holds, not whether the seam policy is itself the right one. The classic misuse is treating a green seam suite as proof the whole map is sound, when it has said nothing about the interior. The discipline that guards against it is to keep the seam inventory current as the substrate's topology changes and to pair it with interior preservation checks rather than leaning on it alone.^[1]

How it implements the components¶

This mechanism fills the boundary-governing and edge-repair components; it produces no map and no interior metric:

boundary_and_seam_policy — it encodes the policy as executable assertions and is the guardian that the policy is not silently violated by a later change.
edge_effect_compensation — it checks that edge-effect compensations remain in place, regressing loudly the moment one is dropped.

It does not detect interior distortions, folds, or collisions — that is the Map Fold and Collision Scan — and it does not test overall neighbor preservation, which is the Adjacency-Matrix Preservation Test.

Instantiates: Neighborhood-Preserving Substrate Mapping — it protects the archetype's promise exactly where the map is weakest, at its edges and joins.
Sibling mechanisms: Adjacency-Matrix Preservation Test · Map Fold and Collision Scan · Adaptive Remeshing and Reallocation · Calibration Anchor Stimuli · Canary Region Probe · Coverage-Hole Heatmap · Elastic-Net Embedding · Lateral-Interaction Smoothing · Local Ablation or Lesion Probe · Magnification Function · Map Registration and Alignment · Multidimensional Scaling Layout · Neighborhood Trustworthiness and Continuity Metric · Perturbation-Response Map · Receptive-Field Tiling Grid · Self-Organizing Map Training · Topographic Error Measure · Versioned Coordinate Atlas

References¶

[1] The antimeridian (180^th-meridian) problem is a well-documented hazard in geographic software, where geometry crossing ±180° longitude is mishandled. Used here as the real named exemplar of a fragile seam. ↩