Supporting multiple offline pmtile maps

[davecraig]

Hi,

We've been working on an Android app that helps people with visual impairment navigate and we're using mapping data in two ways:

1. To drive an audio description of the surroundings via our own code
2. To show a graphical map on the phone using maplibre native

Our initial beta release uses a protomaps server in the cloud to serve up individual tiles and that's used by both our audio code and maplibre. However, one of the big features requested by users is offline maps. The most efficient way of providing these will be to allow downloads of static map extracts covering countries or states which we can generate when building our map. For reasons of cost we don't want to support extracts of user set regions, just the regions that we specify. Obviously that won't work for everyone and if you live on a region border then you might need to download 2 or even 3 extracts - people of Basel I'm looking at you!

We could add code to the app to merge the extracts together when they are downloaded, that's probably doable though it means that double the amount of storage is required and it will need to happen every time the extracts are updated.

What I'd like to explore instead is listing the series of extracts in the style.json and have maplibre simply try to get a tile from each one in turn. This could even have a fallback of using our cloud protomaps server if the tile isn't in any of the extracts. As far as I can tell this use case isn't covered in the maplibre style spec as it can only have a single url entry per source. I suspect that this has been discussed before, so apologies as I haven't found it. Are there arguments against this?

Thanks

Dave Craig.

[louwers]

Additions to the style spec are discussed in the style spec repo. However, I think your suggestion has a pretty low chance of being accepted since I don't think we want very specific tile loading logic to live in a style JSON.

We have had some recent discussions to allow developing plugins for tile loading.

In the meantime, you could perhaps use a server that runs on the device that acts as a proxy between local PMTiles files the hosted ones?

For reasons of cost we don't want to support extracts of user set regions

I think an approach where you populate the offline database with PMTiles extracts is the most sensible approach here. There is an issue that suggests adding functionality for this. The offline functionality we have right now is quite limited. A lot of users have build their own custom solutions.

[davecraig]

That's very helpful, thank you for the detailed response.

The populating of the offline database is possibly a good approach here, so long as it doesn't double the amount of storage required - we still need to use the pmtiles directly for our audio generation. Otherwise, the proxy approach is doable too. Just wanted to make sure I wasn't missing anything,

Thanks again,

Dave.

[louwers]

Something like queryRenderedFeatures may be useful to retrieve data from tiles for audio generation.