Incubator-superset: Please provide documentation for deckgl charts

[songololo]

It is difficult to find any meaningful information about the deckgl charts.

Would it please be possible to add at least rudimentary documentation providing a brief overview of the accepted geospatial formats (e.g. GeoJSON) and any requirements such as coordinate reference system (e.g. epsg 4326?), geometry types (e.g. do Multipolygons work or only Polygons?) etc.

Thanks.

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[junlincc]

@srinify

[mistercrunch]

There's a bit of info here https://docs.preset.io/docs/scatterplot

[songololo]

Thank you. Can anyone give some clues as to what the polygon chart expects in terms of geometry? (e.g. whether geojson polygon in epsg 4326 coordinate reference system works?)

[songololo]

@srinify @mistercrunch @junlincc

I'm able to get it working for point data (e.g. GeoJSON in EPSG 4326 CRS) but not for Polygon data:

Here are some examples that should be fairly easy to reproduce. These make use of the SQL editor and I'm using the amancevice docker container (v0.37.2).

Running a query to create a rectangular polygon (using PostGIS):
SELECT 1 as mock_data, ST_MakeEnvelope(-0.127677, 51.507515, -0.122161, 51.510021, 4326) as geom;

Gives a jagged polygon (instead of rectangular):

Similarly, when casting the geometry to Well Known Text format:
SELECT 1 as mock_data, ST_AsText(ST_MakeEnvelope(-0.127677, 51.507515, -0.122161, 51.510021, 4326)) as geom;

And with GeoJSON it displays nothing:
SELECT 1 as mock_data, ST_AsGeoJSON(ST_MakeEnvelope(-0.127677, 51.507515, -0.122161, 51.510021, 4326)) as geom;

Any information or advice on how to display a polygon would be much appreciated.

Thanks.

[songololo]

I managed to make some headway, posting here for others:

The JSON strings returned from postGIS using the ST_AsGeoJSON function contain only the geometry component of the geoJSON spec. This means that it is necessary to wrap these geometries in a full-fledged geoJSON object before deck.gl can recognise it.

For example:

SELECT *, json_build_object(
  'type',       'Polygon',
  'geometry',   ST_AsGeoJSON(ST_Transform(geom, 4326))::json)::text as geoJSON
FROM <my-schema>.<my-table>;

If you are working with MultiPolygons then you need to extract a Polygon first:

SELECT *, json_build_object(
  'type',       'Polygon',
  'geometry',   ST_AsGeoJSON(ST_Transform((ST_DUMP(geom)).geom::geometry(Polygon, 27700), 4326))::json)::text as geoJSON
FROM <my-schema>.<my-table>;

Note that in both cases, the geoJSON result has to be cast to a text type otherwise superset will have issues when it runs internal queries, which are otherwise unable to sort the results (duplicate checking?) if using JSON.

[mistercrunch]

Yes, I can confirm that we're expecting a geo json object as text. We could do more magic to understand the Postgres/postGIS geoJSON type and convert it accordingly, but it gets tricky for us to support special types that are database specific.