Altair not plotting geojson correctly

[joepropellernet]

I'm trying to plot a geojson in a notebook and while the geopandas plot looks like this;

the Altair output looks like this;

As the data is pretty big, I'll not copy it out in full here! The full notebook and data are here: https://github.com/joepropellernet/Plotting_Police_areas and should be able to replicate the issue.

One thought I had is that the geojson alternates between polygons and multipolygons but even filtering out to either one or the other doesn't solve the issue.

[jakevdp]

Altair (via Vega-Lite) supports topojson for map shapes. I'm not very familiar with the various geographic data formats, so I don't know what to recommend when it comes to shape files. You might ask on the Vega-Lite issue tracker whether there is a way to visualize shapefiles in Vega-Lite; that answer will apply to Altair as well.

[mattijn]

Altair plots by default geographic coordinates in WGS 84 (also known as epsg:4326). Since your coordinates are not coordinates in degrees but in meters and since your data represents England I imagined it is projected in the British National Grid (OSGB 1936, epsg 27700).

Setting the projection to epsg:27700 and reprojecting it to the default epsg:4326 in geopandas before plotting by Altair worked for me:

import altair as alt
import geopandas as gpd

# load raw url in geodataframe
url = 'https://raw.githubusercontent.com/joepropellernet/Plotting_Police_areas/master/Police_areas.geojson'
gdf = gpd.read_file(url)

# set projection and reproject
gdf.crs = {'init' :'epsg:27700'}
gdf = gdf.to_crs({'init': 'epsg:4326'})

# define inline geojson data object
data_geojson = alt.InlineData(values=gdf.to_json(), format=alt.DataFormat(property='features',type='json')) 

# chart object
alt.Chart(data_geojson).mark_geoshape(
).encode(
    color=alt.Color("properties.pfa16nm:N", legend=alt.Legend(columns=2))
)

[mattijn]

Fixed the typos!

[joepropellernet]

That's amazing Matt and Jake! Thanks so much, I had no idea of the different types of geojson and the limitations of Vega-lite. Would it be possible to put the specific projection that Altair prefers in the documentation?

[mattijn]

This issue can be closed since the projection part is documented here: https://altair-viz.github.io/user_guide/data.html#projections

[jakevdp]

Thanks!