add documentation how to load a local topojson file

is this pseudo code supported

import pandas as pd
import altair as alt

counties = alt.topo_feature('https://raw.githubusercontent.com/djouallah/R_leaflet_MAP/master/foundation.json', 'id')

map =alt.Chart(counties).mark_geoshape().encode(
    color='id:Q'
                                                 )
map

all the documentation is using “data.world_110m.url”, how about local topojson or geojson files ?

[Deepakpratap]

@djouallah your problem literally solved my problem.I wanted to plot for the country India…It is sorted now

` import altair as alt

url = “https://raw.githubusercontent.com/deldersveld/topojson/master/countries/india/india-states.json”

source = alt.topo_feature(url, “IND_adm1”)

alt.Chart(source).mark_geoshape().encode( tooltip=‘properties.NAME_1:N’ ) `

[mattijn]

I know some of TopoJSON, as 'm working on a Python package regarding this extension of GeoJSON ( https://github.com/mattijn/topojson)*. Its work in progress. but potentially to be used as an interchange format between geographical packages in the Python ecosystem (eg. geopandas and altair).

Without explaining the format, you are dealing with nested objects. So in the alt.topo_feature(url, feature) you have to define the feature or layer that is present within the objects in your TopoJSON file.

This layer or feature (in your case named foundation) contains often multiple geometries, where each geometry refers to certain top-level stored geometric segments or so-called arcs. Next to the geometry, you have to store some attributes of that geometry, which is stored in another nested object properties.

So in order to access these attribute information you have to add properties to enter this nested object.

{
  "type": "Topology",
  "arcs": [[[641, 1092], [0, 116], [89, 0], [0, -116], [-89, 0]]],
  "objects": {
    "foundation": {
      "type": "GeometryCollection",
      "geometries": [
        {
          "arcs": [[0]],
          "type": "Polygon",
          "properties": {
            "id": 22,
            "width": 0.000558,
            "height": 0.000719,
            "area": 0,
            "perimeter": 0.002555,
            "type": "foundation"
          }
        }
      ]
    }
  }
}

Your issue have been discussed before on the Vega-repo: https://github.com/vega/vega/issues/1319 And more work has also been done in: https://iliatimofeev.github.io/gpdvega/

• @jakevdp In https://github.com/mattijn/topojson/blob/master/topojson/core/extract.py I make use of the singledispatcher method as was discussed in https://github.com/altair-viz/altair/issues/843, which works very well.