diff --git a/decisions/architecture.md b/decisions/architecture.md
index d74707fdf..20d24b0b8 100644
--- a/decisions/architecture.md
+++ b/decisions/architecture.md
@@ -710,7 +710,9 @@ Technical foundation decisions that constrain implementation: engine, client-ser
   - Maps to D-181 signal visibility ladder
 
   **8. Settlement Data Model**
-  - markers.json schema per body: `cities` (name, lat/lon, population_tier, primary_function, gate_terminal, continent_id), `roads` (path polylines, connects), `railroads` (path polylines, connects), `pois` (name, kind, position), plus existing rivers/oceans/mountains with names filled
+  - markers.json is stored in heightmap pixel space — every marker file declares a `grid: { w, h }` header (the heightmap dimensions, typically `1024 × 512`) and every position is `{ x, y }` integer pixels into that grid. This is the canonical storage format used by both the generator and the hand-authored template files (Edict, Vuurkloof, Røros, Cairnside, Estrade). Pixel space is what the heightmap analysis (flood-fill, A* cost grid, city placement) natively operates on; it is deterministic and avoids double-conversion through a projection.
+  - markers.json schema per body: `grid: {w, h}`, `cities` (name, pos `{x,y}`, population_tier, primary_function, gate_terminal, continent_id), `roads` (path polylines as `[{x,y}, ...]`, connects), `railroads` (path polylines, connects), `pois` (name, kind, pos), plus existing rivers/oceans/mountains with names filled
+  - Lat/lon strings are a **display-time derivation**, not a storage format. The atlas UI converts `{x, y}` + `grid: {w, h}` into an equirectangular `lat°N/S, lon°E/W` string for the city data panel and hover tooltips, using the body's radius for any great-circle distances it needs. This keeps the immersive surface without paying conversion cost in the generator, the DB, or the diff churn on hand-authored files.
   - Population tier → city count: `floor(log10(pop/1M))`, modified by `settlement_pattern`
   - Gate terminal POI: at largest population center, sometimes scattered to a smaller one
   - Moons: same depth as planets, scale with population
@@ -737,4 +739,4 @@ Technical foundation decisions that constrain implementation: engine, client-ser
 
 ---
 
-*53 decisions. Last updated: 2026-04-10 (D-191 Atlas of the Reach — Phase 3 scope and pipeline)*
+*53 decisions. Last updated: 2026-04-14 (D-191 §8 — markers.json canonical format is pixel space `{x, y}` with a `grid: {w, h}` header; lat/lon is a display-time derivation)*