[gdal-dev] Vector Tiles in OGR

[Even Rouault]

On mardi 23 janvier 2018 13:10:30 CET Jaak Laineste wrote:
> Hey,
> 
> I read the drv doc, it was not completely clear how OGR handles exactly
> multi-resolution specifics; as OGR data model does not really “support”
> multi-resolution vector in data model level AFAIK.

Indeed, there's no particular handling of multi-resolution in the MVT driver. Meaning it is the 
responsibility of the user to open the zoom level it is interested in.

( For the MBTiles driver, I've added a ZOOM_LEVEL_AUTO=YES open option:
"""Whether to auto-select the zoom level for vector layers according to the spatial filter 
extent. Only for display purpose. Defaults to NO""")

> 
> I guess it works in this way:
> a) if you set Z= option, e.g. Z=10, then it takes data for zoom 10.

No, X, Y, Z open options are only useful if you point to a single .pbf / .mvt file whose path + 
name doesn't contain any hint on the tile coordinate.

To point to a tileset at a particular zoom level, you need to open "directory/{zlevel}"

> b) in other cases it extracts data for all zooms, which means with typical
> multi-zoom/resolution tileset you end up with up to N (=number of zoom
> levels) geometries per object. If geometries would have generated field for
> tile address (xyz) then consumer can decide which copy to be used,
> otherwise it may become quite unusable mess.

No, only one zoom level is opened. See above

> 
> p.s. there is no geometry merging across tiles, right?

Yep, that's what i meant with
"""Note: When opening a zoom level of a tileset, the driver will make no effort of stiching 
together geometries for features that overlap several tiles."""

> I imagine this to be
> useful either based on coordinate connections, and/or unique object id
> field.

Geometry merging could potentially be done, but it was left aside for now, because it can 
involve huge processing time. Imagine in the worst case, you have a single polygon covering 
the whole world at zoom level 15: to read it, you will need to merge the 2^30 tiles ...
So geometry merging should probably be an option not enabled by default, and used 
together with reasonable spatial filter.

Even


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