Show HN: A5
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Show HN: A5
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Re: Show HN: A5
#2Can 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 uniform...
Re: Show HN: A5
#3Can 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.
However, there a number of use cases where choosing a spatial index is a more stylistic choice, like for visualization.
The aim of A5 is not to replace S2/H3 but rather to offer an alternative that has different strengths and weaknesses compared to existing solutions
Re: Show HN: A5
#4A5 uses pentagons, Uber's H3 uses hexagons:
H3: Uber’s Hexagonal Hierarchical Spatial Index https://www.uber.com/en-DE/blog/h3/
Re: Show HN: A5
#5A5 uses pentagons, Uber's H3 uses hexagons: H3: Uber’s Hexagonal Hierarchical Spatial Index https://www.uber.com/en-DE/blog/h3/
Also check S2: http://s2geometry.io/, created at Google before H3, which uses squares and underpins the fast indexing in BigQuery amongst many other things
Re: Show HN: A5
#6I once made a DGG without knowing that it was called a DGG so I could look up how to actually do it in the literature.
I ended up making it an icosahedron and recursively subdividing each face into four new ones by inscribing a new triangle. The project went nowhere for different reasons, so I never figured out if it would have worked, and given this isn’t one of the examples I suspect it wouldn’t have.
Re: Show HN: A5
#7So if I've understood correctly:
Google's S2 is all about performance and prefix-matching.
Uber's H3 makes the math a bit more complex to prioritize less variation in centerpoint-to-centerpoint distances (because they care most about driving times).
This makes the math even more complex to prioritize less variation in area covered by far away tiles (most applicable to e.g. analyzing density of something).
Re: Show HN: A5
#8So if I've understood correctly: Google's S2 is all about performance and prefix-matching. Uber's H3 makes the math a bit more complex to prioritize less variation in centerpoint-to-centerpoint distances (because they care most about driving times). This makes the math even more complex to prioritize less variation in area covered by far away tiles (most applicable to e.g. analyzing density of something).
As a user, you generally don’t care about the math (and S2 is hardly simple either, as it warps the squares prior to projection). You just call the API and use the indices for spatial joins or computations.
The primary benefit is indeed the ability to treat cells as if they are equal areas. This is something people do currently with H3, but it introduces a bias. Contrary to popular belief, this is not only an issue near the poles or in the ocean.
The other difference is aesthetics, people generally find H3 more pleasing to look at than S2, which is why it gets used in visualization more. You can make the same argument for A5, although of course it is a matter of taste!
Finally, you are correct that H3 was originally developed at Uber for their specific use case, however it has since been used in many other contexts and I think it doesn’t hurt to have some alternatives as conceptually S2/H3/A5 are similar
Re: Show HN: A5
#9For a visual explanation of how the system works, as well as interactive examples, check out the project website at https://a5geo.org/examples/
Re: Show HN: A5
#10H3 and S2 are supported out of the box in ClickHouse and have reference libraries in C and C++. But it looks like A5 only has a reference implementation in TypeScript. Porting would not be a problem, though...