ENH: Implement a "finer polyfill"

[DahnJ]

A common use-case is to generate all hexagons that intersect with a given polygon. That's not straightforward to do using H3.

One approximate solution is to first generate a finer resolution grid using polyfill and then return to the desired resolution using h3_to_parent.

The H3-Pandas API could thus contain a convenience function that performs both of these operations in a single pass.

A quick demonstration of the idea:

import geopandas as gpd
import h3pandas

gdf = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))

# Resample to H3 cells
gdf = gdf.h3.polyfill_resample(4)
gdf = gdf.h3.h3_to_parent_aggregate(2)

Sidenote

The above code yields a "strange" result:

This is because a number of the hexagons cross the anti-meridian, and get rendered wrongly. Since this case might be reasonably common, H3-Pandas could perhaps provide a fix, something like

import numpy as np
from shapely.geometry import Polygon

def fix(cell):
    cell_coords = np.array(cell.boundary.coords)
    lngs = cell_coords[:, 0]
    if not (np.any(lngs < 0) and np.any(lngs > 0)) or not (np.any(np.abs(lngs) > 170)):
        return cell
    
    negative = lngs < 0
    lngs[negative] = lngs[negative] + 360
    return Polygon(cell_coords)

Applying the fix to the dataframe's geometries

gdf['geometry'] = gdf['geometry'].apply(fix)

fixes the visual output

[Rimasko]

Please can you help. For which object to use the fix function?
when i use it for gdf, i get an error
AttributeError: 'GeoSeries' object has no attribute 'coords'

[DahnJ]

Please can you help. For which object to use the fix function? when i use it for gdf, i get an error AttributeError: 'GeoSeries' object has no attribute 'coords'

The fix must be applied to the geometries themselves. I have added the relevant line to my post above.

[andresfchamorro]

Hi @DahnJ

Long time on this issue but..

I've modified polyfill with the idea suggested above (not sure if you were planning on making changes).

This solution below does two things:

1. If a given polygon is too small (returns no h3 cells), check iteratively with higher resolutions and return the parent cells at the res requested.
2. Add option for overfill returning also the outer edge cases using h3.k_ring.

I can submit a PR if you think this could be a useful change!

andresfchamorro@c4a7852