CODEWITHSUNDEEP
geojsonmapbox-gl-jsvector-tiles
Malcolm
Malcolm

Reputation: 473

Why do points from vector tiles at lower zoom levels not match source data?

I am querying raw vector tiles from zoom level 8 from tiles created to zoom level 11. I then convert the point features in these tiles to geojson. The converted feature geometries do not match the tile geometry at zoom level 11, or tiles from zoom level 11 converted to geojson. I have created the tiles with two different programs (tegola and geojson-vt). I am converting the tiles with vt2geojson. I am trying to determine at what stage in the conversion process the geometry is being manipulated - tile creation, tile conversion back to geojson, or in mapbox gl js. As far as I can tell the coordinates are not being trimmed in the creation or conversion process, but am not 100% positive on that. I get the reason for the simplification of lines and polygons at lower zoom levels, but I do not see any reason to manipulate point geometry.

enter image description here.

As can be seen in the image, the points start to drift apart after the max zoom level of the original tiles. One workaround is to simply filter the vector tiles to only show the features in the resulting geojson features as the properties are still intact or store the coordinates in the properties, but this is not ideal.

Bottom line - if I want to view points as close to the original data as possible, what max tile zoom level should I use (i.e., 11, 12, 13, 14), and at what stage does the geometry get manipulated?

Upvotes: 1

Views: 1175

Answers (1)

Steve Bennett
Steve Bennett

Reputation: 126235

It's a bit hard to tell what your exact question is, but if I'm understanding correctly it is essentially: "Why do I lose spatial accuracy when I overzoom my vector tiles" and the answer is "because you're overzooming them". It's inherent in the way they work.

Your original data probably had 10 significant figures of precision. Each offset for a point in a vector tile is usually encoded as an integer between 1 and 4096.

Your options are basically:

  • increase the spatial accuracy of the generated tiles (eg, tippecanoe's -d flag)
  • generate tiles to a higher zoom level
  • don't overzoom them so much

Upvotes: 1

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