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TileMill doesn't seem particularly well suited for that either. I did a rough calculation and it looks like getting a usable map of Washington D.C. would run several hundred gigabytes. I'm wondering if the only way to do something like this realistically on a mobile device is to move the rendering to the client.

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Maybe your definition of usable is a bit different than mine, but we've done two maps of DC:

http://tiles.mapbox.com/mapbox#!/map/dc-bright http://tiles.mapbox.com/mapbox#!/map/dc-nightvision

The first is 17MB, the second is 57MB. One could probably get to sidewalk-level without cracking 1GB here. Where did hundreds of gigabytes come from?

That said - we'd like to start doing rendering on-client, and it would be great - the question is how to slice and dice data so that the data itself isn't gigantic and rendering it isn't slow.

I probably made a mistake when estimating this. I exported 21 tiles to MBTiles at 429KB, and extrapolated based on the 5.7 billion tiles it would take to do D.C. at 13-16.

I'm still waiting for a larger job to complete, so I don't have a sense of the file size there.

Edit: Got it. The default export is the entire world. Whoops.

Though perhaps not optimized for an urban solution like DC, we have done work around tile de-duping and tile data compression to combat things like this.

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