# Local origin and projected CRS

Defined 2026-11-06. This is the single place where geographic coordinates become local metres, and where local metres become Unreal units. Every asset, blockout, and manifest refers to it; nothing else may define its own origin.

## Projected CRS

**EPSG:26911 — NAD83 / UTM zone 11N**, metres, grid north.

NAD83 rather than WGS84 is deliberate: it is the CRS the US national elevation and parcel data are published in, and the difference against WGS84 at this latitude (roughly 1 m, varying) is far below what a walkable demo can show. The choice is recorded here so nobody has to re-derive it, and so an elevation source can be brought in without a second reprojection.

## Origin

| Field | Value |
| --- | --- |
| Definition | Hollywood Blvd × North Highland Avenue intersection node, on the boulevard centreline |
| WGS84 | lat 34.101545, lon −118.338718 |
| OSM node | the single shared node between the `Hollywood Boulevard` and `North Highland Avenue` ways in `Geo/raw/osm-api-map-2026-11-06.osm` |
| EPSG:26911 easting / northing | **to be computed** — see below |
| Rationale | a real, data-defined, reproducible point at the demo block's east boundary; no surveyed control point exists for this project |

The origin is not "the hero building" on purpose: a building may later move a metre or two as verification corrects it, while an intersection node is fixed by the source data.

### Pending value

The projected pair is not filled in yet because neither `pyproj` nor the PROJ command line tools are installed on this workstation (`Docs/environment.md`), and inventing a number that later disagrees with QGIS would be worse than leaving it blank. Fill it in the QGIS step:

```bash
python3 -c "from pyproj import Transformer; \
  t=Transformer.from_crs('EPSG:4326','EPSG:26911',always_xy=True); \
  print(t.transform(-118.338718, 34.101545))"
```

Record the printed easting/northing back into this table and note the PROJ version used.

## Axis convention

| Axis | Meaning |
| --- | --- |
| Local X | easting − origin easting, metres, east positive |
| Local Y | northing − origin northing, metres, north positive |
| Local Z | up, metres, elevation relative to the origin's ground height |

The block then spans roughly **X −279 m to 0 m**, Y a few tens of metres either side, which keeps float precision comfortable in Unreal.

## Conversion to Unreal

1 metre → **100 Unreal units**. The ×100 happens exactly once, in the export/import path, and is asserted by the scale test required in `ROADMAP.md`. Blender files are authored in metres; Unreal receives centimetres.

Unreal's own axes differ (X forward, Y right, Z up). If a level rotates the imported block so that "north" is not +X, that rotation is recorded once, in the level's documentation, and never baked into the export.

## Obligations

- Any OSM-derived output keeps "© OpenStreetMap contributors" and the licence link. See `THIRD_PARTY_NOTICES.md`.
- Elevation data, when chosen, must be reprojected into EPSG:26911 with the origin above, and its vertical datum recorded (NAVD88 is the expected one for US sources; ellipsoidal heights are not interchangeable).