H3_postgis

Your Nile database arrives with h3_postgis extension and its dependencies h3 and postgis already installed and enabled.

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H3_postgis Functions and Usage

The h3_postgis extension provides several functions that enable interoperability between H3 and PostGIS.

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Converting a Geometry to an H3 Index

To convert a PostGIS geometry (Point) into an H3 index at a given resolution:

SELECT h3_lat_lng_to_cell(ST_SetSRID(ST_MakePoint(-122.4194, 37.7749), 4326), 9);

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Converting an H3 Index to a Geometry

To convert an H3 index into a PostGIS polygon geometry:

SELECT h3_cell_to_geometry('8928308280fffff');

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Finding Neighboring H3 Cells as Geometries

To get the neighboring H3 cells as PostGIS geometries:

SELECT h3_cell_to_geometry(neighbor) 
FROM h3_grid_disk('8928308280fffff', 1) AS neighbor;

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Checking if a Point is Within an H3 Cell

To check if a given point lies within an H3 indexed cell:

SELECT ST_Contains(h3_cell_to_geometry('8928308280fffff'), 
                   ST_SetSRID(ST_MakePoint(-122.4194, 37.7749), 4326));

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Example: Storing H3 Indices and Geometries in a Table

A common use case is storing H3 indices alongside geospatial data:

CREATE TABLE locations (
    id SERIAL PRIMARY KEY,
    name TEXT,
    geom GEOMETRY(Point, 4326),
    h3_index H3INDEX
);

INSERT INTO locations (name, geom, h3_index)
VALUES ('San Francisco', ST_SetSRID(ST_MakePoint(-122.4194, 37.7749), 4326), 
        h3_lat_lng_to_cell(ST_SetSRID(ST_MakePoint(-122.4194, 37.7749), 4326), 9));

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Querying Locations by Proximity

To find all locations within a given H3 ring distance:

SELECT DISTINCT l.name 
FROM locations l
CROSS JOIN LATERAL (
    SELECT h3_grid_disk(h3_lat_lng_to_cell(l.geom, 9), 1) AS nearby_cell
) disk
WHERE l.h3_index = disk.nearby_cell;

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Use Cases

• Efficient geospatial indexing using hexagonal grids.
• Spatial analysis integrating H3 with PostGIS functions.
• Fast proximity searches based on geospatial cells rather than bounding boxes.

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Limitations

• H3 cells approximate real-world geography and do not align with political or administrative boundaries.
• Higher-resolution H3 cells increase data granularity but also computational costs.

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Conclusion

The h3_postgis extension in PostgreSQL enables seamless integration between H3 spatial indexing and PostGIS geometry operations, making it an excellent choice for advanced geospatial analysis and location-based applications.

For more details, refer to the h3-pg GitHub repository.