Dutch Portolan Demo

# Water Depth Charts — Rhine Branches — Rijkswaterstaat / Netherlands

## What This Dataset Is

A spatial reference grid of **3,112 hectometer sections** (hectometervakken) along the fairway
(vaargeul) of the Rhine branches in the Netherlands. Each polygon represents a section of the
navigable channel, identified by a section ID.

This is primarily a **spatial indexing grid** — it provides the spatial framework for referencing
locations along the Rhine waterways. The attribute data is minimal; the value lies in the
spatial geometry itself.

**Provider:** Rijkswaterstaat (Directorate-General for Public Works and Water Management)
**License:** Public domain (Dutch government open data)

## How to Access

The data is a GeoParquet file in **EPSG:28992** (Amersfoort / RD New). Use DuckDB with the
spatial extension.

```python
import duckdb

con = duckdb.connect()
con.execute("INSTALL spatial; LOAD spatial;")

URL = 'https://data.source.coop/cholmes/portolan-nl/rijkswaterstaat/hectometervakken_vaargeul/hectometervakken_vaargeul.parquet'

df = con.execute(f"""
    SELECT * FROM read_parquet('{URL}')
    LIMIT 5
""").df()
```

## Schema — Field Meanings

| Field | Type | Meaning |
|-------|------|---------|
| `geometry` | WKB Polygon | Fairway section boundary in **EPSG:28992**. |
| `vak_ID` | string | **Section identifier** — prefix indicates the waterway branch (e.g., `"ij_8784"`, `"wa_9509"`). |
| `Shape__Area` | float64 | Section area in m². |
| `Shape__Length` | float64 | Section perimeter in metres. |

## Useful Query Patterns

### List sections by waterway branch

The `vak_ID` prefix identifies the waterway. Extract it to group sections.

```sql
SELECT SPLIT_PART(vak_ID, '_', 1) AS waterway_prefix,
       COUNT(*) AS sections,
       SUM(Shape__Area) / 1e6 AS total_area_km2
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/rijkswaterstaat/hectometervakken_vaargeul/hectometervakken_vaargeul.parquet')
GROUP BY waterway_prefix
ORDER BY sections DESC
```

### Find a specific section

```sql
SELECT vak_ID, Shape__Area, Shape__Length
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/rijkswaterstaat/hectometervakken_vaargeul/hectometervakken_vaargeul.parquet')
WHERE vak_ID = 'wa_9509'
```

### Summary statistics

```sql
SELECT COUNT(*) AS total_sections,
       SUM(Shape__Area) / 1e6 AS total_area_km2,
       AVG(Shape__Area) AS avg_section_area_m2
FROM read_parquet('https://data.source.coop/cholmes/portolan-nl/rijkswaterstaat/hectometervakken_vaargeul/hectometervakken_vaargeul.parquet')
```

## Geometry Notes

- CRS is **EPSG:28992** (Amersfoort / RD New). Coordinates are in metres.
- All geometries are Polygon.
- Reproject to EPSG:4326 for web maps.


## Visualization Styles

One Mapbox GL v8 style is available for interactive map visualization via the PMTiles file.

Style files are Mapbox GL v8 JSON with relative PMTiles source paths. They can be
used with MapLibre GL JS, OpenLayers (via ol-mapbox-style), or any Mapbox GL v8-compatible renderer.

- **`styles/default.json`** — Dark navy blue fill with darker outlines. Shows fairway channel sections along major Dutch waterways. Navigational-chart appearance.

Style files are at: `https://data.source.coop/cholmes/portolan-nl/rijkswaterstaat/hectometervakken_vaargeul/styles/`
## Caveats

- **Minimal attribute data** — this is primarily a spatial reference grid. The main value
  is the geometry itself and the `vak_ID` for cross-referencing with other Rijkswaterstaat
  datasets (e.g., depth measurements, water quality).
- The `vak_ID` prefix encodes the waterway branch but the exact mapping (e.g., `ij` = IJssel,
  `wa` = Waal) is not included in the dataset.
- `Shape__Area` uses double underscores (ArcGIS convention).