[alprc]

I don't have the exact size comparison between this approach vs PNG compression, but i guess the difference would be similar to the difference between DEFLATE vs brotli.

I'm curious about what you just asked of me though, i will make the actual measurements, and will update this page with the results when i got time.

[pheelicks]

Great thanks - perhaps obvious, but worth running the PNGs through something like pngcrush.

I'm thinking about it the other way also, that is could your approach be used to reduce the size of DEMs encoded as PNGs. While I can see brotli being more efficient, by not using a image compression algorithm you perhaps lose out on exploiting the 2D nature of the data, as if I understood correctly when you're compressing the data you treat the tile as a 1D blob of binary data.