Mapbox-gl-js: Zoom levels 1 less in GL than raster?

[ashleyclough]

It appears that the zoom levels reported in GL are 1 less than the same data source rendered as raster tiles. See the attached images of the same area as GL and in the Mapbox project view:

[incanus]

Yes, tiles are drawn in 512px of screen space on GL, so the zoom level retrieved is off by one.

[jfirebaugh]

We had some extensive discussions about tile size and zoom level which we need to capture in documentation. If I recall correctly, the upshot is that this is the expected behavior: because GL renders 512px tiles, one tile at a given zoom level renders 4x the area as raster tiles, which have historically been 256px.

[ashleyclough]

OK, makes sense, although I've always assumed the zoom value is just an arbitrary value. It does need documenting as it makes translating Mapbox Studio styles a bit more awkward - particularly as the data is exported using the 256px zoom references. How does it work with raster data?

[rouen-sk]

Just found about this issue and I am really surprised. This is not a "minor documentation issue" - you practically decided that Mapbox GL is not slippy map, and it is not compatible with other standardized slippy map things - for example any server side code using standard zoom-to-scale-denominator (13 = 1:100000). You cant use one common datasource-level filtering based on zoom for both raster and vector tiles, you need different styling rules, etc. What a huge standard-breaker... This should be like first thing mentioned in the documentation - in big bold font.

Is there perhaps any way to setup Mapbox GL JS to work in slippy maps mode? 256px tiles, standard zoom levels?

[timiyay]

Hmmmm, is there any chance this explains why I get incorrect results from mapbox/geo-viewport? https://github.com/mapbox/geo-viewport/issues/3

I use Mapbox's Static Image API, and I need to compute the bounding box of those images. Mapbox Support advised me that geo-viewport was written for this very purpose.

If I request a static image at MapboxGL zoom 17, and then ask geo-viewport to compute this bounding box, it gives the bbox for zoom 16 instead. I have to +1 to get the right answer.

I'm trying to learn enough about the underlying mapbox/node-spherical-mercator library to make a fix, but it sounds like it's mostly a misunderstanding on my part. I had absolutely no idea that GL changed the game in terms of zoom levels.

I'm honestly not fussed about the zoom matching with raster-based zooms. I just need a way to compute the bbox of a Mapbox Static Map API image.

[lucaswoj]

I am closing this issue because the GL team feels 👍 about this decision and does not plan to revisit.

[timiyay]

For posterity, the answer to my comment was:

Yes, the 512x512px tile size was the cause of my issue in https://github.com/mapbox/geo-viewport/issues/3

[nrenner]

Documenting this in Mapbox GL JS fundamentals would be helpful.