Please please please include a description in your title. Just a couple of words will do.
It's obviously something about paper size A5.
On reading the comments it's about the healing powers of dodecahedrons.
Show HN: A5
21–30 of 31 posts
Re: Show HN: A5
#22Re: Show HN: A5
#23Is it primarily useful just for data visualization? Would there be an potential performance benefits for something like searching a database for nearby data?
Re: Show HN: A5
#24What's the advantage of this over HEALPix projection? https://en.wikipedia.org/wiki/HEALPix
The base platonic solid that Healpix is based on is the octahedron ( https://en.wikipedia.org/wiki/Octahedron ), which A5 uses the dodecahedron( https://en.wikipedia.org/wiki/Regular_dodecahedron ). The octahedron has a much higher angular defect ( https://en.wikipedia.org/wiki/Angular_defect ) than the dodecahedron, and thus when it is projected onto the sphere the cells are warped a lot. So while their areas may be…
Not sure I understand—healpix starts from the rhombic dodecahedron and then bisects the generalizations of the 12 squares each time. Where do octahedra come into play?
Re: Show HN: A5
#25Is it primarily useful just for data visualization? Would there be an potential performance benefits for something like searching a database for nearby data?
Yes, such indices (S2 & H3) are widely used for providing a index in databases, so geospatial features that are close by in the world and stored in nearby databases rows. https://cloud.google.com/bigquery/docs/grid-systems-spatial-...
Re: Show HN: A5
#26Is it primarily useful just for data visualization? Would there be an potential performance benefits for something like searching a database for nearby data?
Yes, such indices (S2 & H3) are widely used for providing a index in databases, so geospatial features that are close by in the world and stored in nearby databases rows. https://cloud.google.com/bigquery/docs/grid-systems-spatial-...
Thanks. I always enjoy when geospatial topics show up on here. My background it geo, but unfortunately I have slowly drifted away. Geohash is about where I left off in the same general realm of concepts, so S2 / H3 are essentially new to me as well.
Re: Show HN: A5
#27Can you give some examples of when this might be better to use than H3? The ones that seem obvious: - You need very high resolution. H3 is also 64 bit I think, but it seems like A5 highest resolution is about 4 orders of magnitude higher. - Equal cell size: are the cells exactly equal in size (in m2)? H3 they vary by up to ~2x. What are the downsides? The shapes are irregular, distances between centroids are not unif…
Yes, those are the obvious ones. This example: https://a5geo.org/examples/airbnb shows why the equal area is valuable in practice, while https://a5geo.org/examples/area shows the area variation vs h3. The downsides are the characteristics that make h3 or s2 useful. For h3, the single neighbor type means it is well suited to flow analysis and S2 having exact cell subdivision means it is great for simplifying geometry.…
Haha that colour scale on the area variance page makes it a bit hard to see whether nearby H3 hexagons are very different in size...? I've never really investigated, but my baseless assumption was that nearby hexagons (at a high zoom level) would be pretty similar size? But maybe that's completely wrong.
But yeah, will definitely reach for A5 at some point just for the aesthetics!
My favourite DGGS (this is a new term to me) is water basins as created by HydroSheds [1]. Different area, unpredictable shape, basically no usefull properties but they conform to topography! Can get a feel for them with this little thing I made several years ago [2] (your Cells example reminded me of this).
Re: Show HN: A5
#28[dead]
Re: Show HN: A5
#29Earlier quoted context omitted.
The base platonic solid that Healpix is based on is the octahedron ( https://en.wikipedia.org/wiki/Octahedron ), which A5 uses the dodecahedron( https://en.wikipedia.org/wiki/Regular_dodecahedron ). The octahedron has a much higher angular defect ( https://en.wikipedia.org/wiki/Angular_defect ) than the dodecahedron, and thus when it is projected onto the sphere the cells are warped a lot. So while their areas may be…
Not sure I understand—healpix starts from the rhombic dodecahedron and then bisects the generalizations of the 12 squares each time. Where do octahedra come into play?
My mistake, you are correct. The base solid is indeed the rhombic dodecahedron. I believe the point about the angular defect is still valid though.
Re: Show HN: A5
#30The description of the algorithm notes that each irregular pentagon is divided into four sub-pentagons. Eyeballing the maps, I don't see any group of 4 pentagons forming a similar larger pentagon.
I noticed that you had an analog to the H3 landing page on your landing page, allowing zooming in. If you could also steal the next-higher / next-smaller overlay like they did on the H3 landing page, it would make it clearer the relationship between the larger and smaller pentagons.
I've used H3 extensively, and one of the things that always bugged me about it was that each large hexagon was _mostly_ covered by a group of the next smaller ones, but because geometry, the edges have some overlap with the neighbor large hexagons. So I can't just truncate an integer mapping, for example, to get the ID of the next-largest.