[gdal-dev] Vector Tiles in OGR

[Even Rouault]

On lundi 29 janvier 2018 10:21:04 CET Stefan Keller wrote:
> Hi Even
> 
> Great news about this MVT reader!
> 
> The docs [1] says:
> > The MVT driver only uses the "json" key to retrieve the layer names, their
> > fields and the geometry type, and the "bounds" key for the layer extent.
> I just wanted to point you to the spec. discussion of MBTiles and
> TileJSON metadata [2] and I hope the SW keeps aligned with it.

Yeah, I didn't really found a specification for those. Just my experiments with various MBTiles 
/ directory of tiles found in the wild, and those I produced with tippecanoe.

> 
> > If this file cannot be found, the layer schema is established by scanning
> > the features of the tile(s).
> Q1: So the reader scans all tiles on the one given zoom level and
> generates an internal metadata structure for that zoom level?

Yes. Potentially with a limitation on the number of tiles to explore with the 
TILE_COUNT_TO_ESTABLISH_FEATURE_DEFN open option

> Q2: Option JSON_FIELD=YES says "put all attributes as a serialized
> JSon dictionary"; Can a metadata.json file also be written to disk in
> case it was generated?

Not sure to understand your question. If you use JSON_FIELD=YES, it means that the driver 
will not try to establish a full schema that fit for all features, but just put the key/value pair it 
finds for each feature in a JSon dictionnary. So there's no metadata.json to write.

> Q3: Is this metadata.json accessible from an GDAL/OGR Python binding?

The open option you mean ? Yes:
https://github.com/OSGeo/gdal/blob/trunk/autotest/ogr/ogr_mvt.py#L69

> Q4: Ultimately I'd like to get a metadata.json (TileJSON compatible)
> from the above mentioned MVT input for all zoom levels (at least for
> the json key): How could I achieve this?

Manually. By iterating on the OGR layer feature definition. Are there use cases where 
metadata.json is missing ?


Note: I'm going to work on the write side of the drivers in the coming weeks.


Even

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