Dutch Portolan Demo

# Kadaster — Netherlands Cadastre, Land Registry & Mapping Agency

## What This Is

Three collections from Kadaster, the Dutch government agency responsible for land registry,
cadastral mapping, and geographic data infrastructure. Kadaster maintains several national
key registrations (basisregistraties) and publishes them via PDOK.

## Collections

### bag_light/ — BAG Buildings (Light Extract)
11.4 million buildings (panden) from the BAG (Basisregistratie Adressen en Gebouwen),
the authoritative Dutch building and address registry. Current records only, with
construction year, usage function, floor area, and dwelling unit count. This is the
recommended starting point for building analysis.

- File: `bag-light.parquet` (933 MB)
- CRS: EPSG:28992 (RD New)
- Key fields: `identificatie` (BAG ID), `bouwjaar` (construction year), `gebruiksdoel` (usage), `status`

### bestuurlijke_gebieden/ — Administrative Boundaries
342 municipalities (gemeenten), 12 provinces (provincies), and 1 national territory
(landgebied) — the 2026 edition. Three separate GeoParquet files. The municipality codes
(`identificatie`) are CBS codes used as join keys across all Dutch government data.

- Files: `gemeentegebied.parquet` (8 MB), `provinciegebied.parquet` (1.6 MB), `landgebied.parquet` (0.4 MB)
- CRS: EPSG:28992 (RD New)
- Key fields: `identificatie` (CBS code), `naam` (name), `code` (prefixed code like GM0363)

### inspire_buildings/ — INSPIRE Harmonized Buildings
24.2 million building footprints harmonized to the EU INSPIRE Buildings schema. Includes
version history (current + superseded records). Filter `endLifespanVersion IS NULL` for
current buildings only (~12.6M).

- File: `buildings.parquet` (2.1 GB)
- CRS: EPSG:4258 (ETRS89, near-identical to WGS84)
- Key fields: `localId` (BAG ID), `anyPoint` (construction date as ISO string), `endLifespanVersion` (null = current)

## BAG Light vs INSPIRE Buildings

Both datasets come from the same BAG source, but serve different purposes:

| | BAG Light | INSPIRE Buildings |
|---|-----------|-------------------|
| Rows | 11.4M (current only) | 24.2M (current + history) |
| Attributes | Rich: usage, floor area, dwelling count | Minimal: date + ID only |
| Field names | Dutch native (bouwjaar, status) | INSPIRE names (anyPoint) |
| CRS | EPSG:28992 (metres) | EPSG:4258 (degrees) |
| Construction year | Integer (1975) | ISO string (1975-01-01T00:00:00) |
| File size | 933 MB | 2.1 GB |

**Use BAG Light** for most analysis — it has more attributes, cleaner data, and is smaller.
**Use INSPIRE Buildings** only when you need version history or EU-standard coordinates.

## CBS Municipality Codes

The `identificatie` field in bestuurlijke_gebieden contains CBS municipality codes (e.g.,
`0363` for Amsterdam). These 4-digit codes are the universal key across all Dutch government
data. They appear in:
- BAG building IDs (first 4 digits of `identificatie`)
- CBS statistics (Wijken en Buurten, StatLine)
- BRP (population registry), WOZ (property valuations), and more

Use bestuurlijke_gebieden as the reference geometry for any municipality-level analysis.

## Example Queries

### Count buildings per municipality (joining BAG Light with admin boundaries)

```sql
INSTALL spatial; LOAD spatial;

SELECT g.naam AS gemeente, COUNT(*) AS buildings
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/kadaster/bag_light/bag-light.parquet') b
JOIN read_parquet('https://data.source.coop/cholmes/portolan-nl/kadaster/bestuurlijke_gebieden/gemeentegebied.parquet') g
  ON LEFT(b.identificatie, 4) = g.identificatie
WHERE b.status = 'Pand in gebruik'
GROUP BY g.naam
ORDER BY buildings DESC
LIMIT 10
```

### Compare building counts between BAG Light and INSPIRE

```sql
SELECT 'BAG Light' AS source, COUNT(*) AS buildings
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/kadaster/bag_light/bag-light.parquet')
UNION ALL
SELECT 'INSPIRE (current)', COUNT(*)
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/kadaster/inspire_buildings/buildings.parquet')
WHERE endLifespanVersion IS NULL
```

### List all provinces with municipality count

```sql
SELECT ligt_in_provincie_naam AS province, COUNT(*) AS municipalities
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/kadaster/bestuurlijke_gebieden/gemeentegebied.parquet')
GROUP BY 1
ORDER BY municipalities DESC
```

## See Also

Each collection has its own `llms.txt` with detailed field descriptions, value distributions,
and query patterns:
- `bag_light/llms.txt`
- `bestuurlijke_gebieden/llms.txt`
- `inspire_buildings/llms.txt`