# GIS Standards

## CRS standards

- API GeoJSON: EPSG:4326.
- Internal Belgian metric calculations: EPSG:31370.
- Raster native operations preserve source CRS unless reproject requested.
- Store both source CRS and normalized CRS.

## Units

- Area: square meters internally, hectares/km² as display derivatives.
- Length: meters internally, kilometers as display derivative.
- Resolution: meters/pixel where projected CRS allows it; otherwise state units clearly.

## Geometry validity

All vector geometries must pass validation before becoming `READY`.

Allowed repair strategies:

1. `make_valid` where available.
2. zero-width buffer only if documented.
3. reject and require user correction.

Repairs must be recorded in metadata.

## Geometry type normalization

- Polygon inputs may become MultiPolygon.
- LineString inputs may become MultiLineString.
- GeometryCollections require explicit extraction or rejection.

## Spatial operations

All spatial operations must define:

- input CRS;
- output CRS;
- unit assumptions;
- tolerance;
- geometry repair behavior;
- empty-result behavior.

## Precision

Store full precision in database. Simplify only derived visualization layers.

## Boundary handling

For clipping/intersections, include features that intersect area boundary. Record whether metrics use full feature geometry or clipped geometry.

## Source priority for V1 reference workflows

1. GRB-like reference fixtures / GRB when implemented.
2. Official Flemish datasets where available.
3. OSM fallback.
4. User-uploaded reference layer.