Add database schema docs

docs/database-schema.md

@@ -0,0 +1,74 @@
+---
+layout: page
+title: Database Schema and Layers
+published: true
+---
+
+# Database Schema
+
+The layer schema tries to stay compliant to Mapbox Streets v6 and Mapbox Streets v5.
+The detailed documentation on which database tables are mapped to which feature classes, can be found in the following sections.
+
+## Aeroways, Barriers and Landusages
+
+For some layers linestring and polygon data needs to be mapped into
+tables. The different geometries are then both rendered as vector
+linestrings. The landing strips of airports for example might be a
+linestring or polygon.
+
+![Layers for aeroways, barriers and landusages](/media/aero_barrier_landusage_layer.png)
+
+## Administrative Borders
+
+The administrative area on lower zoom levels is entirely from Natural
+Earth data. Only at higher zoom levels where details are more important
+the OSM borders are rendered.
+
+Natural Earth provides data in several generalization levels. The table
+with highest generalization is used on the lowest zoom levels and on
+higher zoom levels the less generalized tables.
+
+![Layers for administrative areas](/media/admin_layer.png)
+
+## Roads, Bridges and Tunnels
+
+Roads are split up into normal roads, tunnels and bridges after a
+certain zoom level. `z_order` and `layer` attributes are used to order
+the geometries on the right z axis. Road labels however will always
+contain data for tunnels, bridges and normal roads therefore one table
+that is filtered into different views at higher zoom levels is the best
+approach.
+
+![Layers for roads, tunnels and bridges](/media/road_layer.png)
+
+## Points of Interest
+
+Most POIs are in fact points, but buildings tagged with POI attributes
+are often polygons, which is why tables for both points and polygons are
+created.
+
+The `localrank` and `scalerank` of the `#poi_label` layer are calculated
+from the `type` and `area` attributes. The `address` field is pulled
+together from the various address attributes on the tables (`street`,
+`housenumber`, `place`, `city`, `postcode` and `country`).
+  
+![Point of interest label layer](/media/poi_layer.png)
+
+Water
+-----
+
+Water bodies for lower zoom levels are taken from Natural Earth data
+while lakes and rivers are from OpenStreetMap. Big rivers often consist out of
+a water polygons while smaller rivers are only water ways.
+
+![Water bodies and river layers](/media/water_layer.png)
+
+Places
+------
+
+Places are names of cities and villages. To calculate the importance of
+a city the scalerank of the most important cities from Natural Earth
+data is merged into the OpenStreetMap data set. For places that do not have a
+scalerank value a dynamic value is calculated based on the population.
+
+![Place label layer](/media/place_layer.png)

docs/index.md

@@ -32,4 +32,5 @@ Targeted at people who want to reuse our process or want to contribute.
   - [Data Sources of OSM Vector Tiles](/docs/data-sources)
 - Import and Export
   - [Import and Export Process](/docs/import-export-process)
+  - [Database Schema](/docs/database-schema)
   - [Imposm Mapping Schema](/docs/imposm-schema)