thc — The H3 Compressor

This library allows to compress an H3 cell set into a compacted space-efficient representation.

This is especially useful for on-disk storage or on-wire transmission.

Compression results

Dense set

Test data: the 54,812 H3 cells at resolution 11 that covers Paris (contiguous shape, no holes).

Dense set composed of contiguous indexes are the easier to compress. We can see that the compact routine from H3 is really efficient and performs as well as THC by encoding an index on ~2 bits.

Now, the interesting part is that by first compacting the indexes set and then applying THC we can reduce the size even more (the resulting payload is ~470x smaller than the original one)!

This is due to the fact that H3 compaction, unlike THC, doesn't really compress the data (an index is always encoded on 64-bit) but reduces the number of indexes by replace groups of children by their ancestors.

The denser the set is, the higher is the compression: mainland France at resolution 11 (267,532,208 indexes, 1.99 Gio) is compacted and compressed down to 100.93Kio!

Sparse set

Test data: the 690,451 H3 cells at resolution 11 that covers cycle lanes in mainland France.

Sparse sets like this one are harder to compress because there is less redundancy to exploit.

H3 compaction completely falls apart here (only shaving 480 indexes from the 600k), since it cannot find enough complete groups of children to replace them by their ancestors.

THC, on the other hand, still produces remarkable results (though not as good as the one on the dense sets), producing a payload 10x smaller than the original one and using ~6.25 bits/index.

Installation

Cargo

• Install the rust toolchain in order to have cargo installed by following this guide.
• run cargo install thc

Usage

Load the shape of a city, compute the H3 coverage at resolution 10 and save the compressed result on disk:

use geojson::GeoJson;
use h3o::{
    geom::{Geometry, ToCells},
    CellIndex, Resolution,
};
use std::{fs::File, io::BufReader};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let file = BufReader::new(File::open("city.geojson")?);
    let geojson = GeoJson::from_reader(file)?;
    let geometry = Geometry::try_from(&geojson)?;

    let mut file = File::create("coverage.thc")?;
    thc::compress(
        &mut file,
        CellIndex::compact(geometry.to_cells(Resolution::Ten))?,
    )?;

    Ok(())
}

License

BSD 3-Clause