Generalization as a function of zoom level - polygons

[ymoisan]

Hi. I downloaded building footprint data (polygons) from 'https://ftp.maps.canada.ca/pub/nrcan_rncan/extraction/auto_building/shp/, combined all shapefiles there into one big geodataframe and saved the result as geoparquet with geometry_encoding='geoarrow'. This gives a table of 11M lines.

I tried opening the parquet file with geoarrow.rust.core read_parquet and use it in a PolygonLayer :

'geoarrow.rust.core._rust.GeoTable' object [table_parquet_geoarrow_rust] has no attribute 'select'

So I ended up opening the parquet file with pyarrow.parquet read_table and now I can get a PolygonLayer. I have a GPU-less computer and got to see parts of the map but display is awfully slow. I would think 11M polygons is just too much to draw.

I'm trying to mimick this map, which shows rough perimeter of areas with data and more and more detail as one zooms in, only with the data in lonboard (and eventually Polars/DuckDB for analysis). Have there been discussions on ways to adapt the number of vertices in polygons as a function of zoom level e.g. take a bbox or some rough perimeter of individual polygons ? Or get some rough perimeter of groups of polygons, a kind of "geometry aggregation", as a function of zoom level ?

[kylebarron]

'geoarrow.rust.core._rust.GeoTable' object [table_parquet_geoarrow_rust] has no attribute 'select'

Regardless, you should ensure you're using the latest version of geoarrow-rust.

At this point, geometry simplification is not in scope of Lonboard, and you should do any geometry simplification you need before passing the data to Lonboard

[ymoisan]

Regardless, you should ensure you're using the latest version of geoarrow-rust

Sorry for that. geoarrow.rust.io 0.3.0 read_parquet works fine.

[kylebarron]

geoarrow-rust-compute has some linestring simplification algorithms you can try: https://geoarrow.org/geoarrow-rs/python/latest/api/compute/functions/#geoarrow.rust.compute.simplify. These won't maintain a polygonal coverage, if you need that