H3 polyfill is skipping the areas near the boundary I am trying to polyfill. (Python)

Is there a way to polyfill edge to edge or as much as possible (Res 8) without increasing the resolution. Black boundary is the polygon boundary I am trying to polyfill completely

Why this happens

This happens because polyfill only returns cells whose centroid falls into the polygon.

What to do instead

In your case, you could use polyfill on a finer resolution and then use h3_to_parent to convert back to your desired resolution.

Keep in mind that this isn't guaranteed to cover the whole geometry. But in practice, using sufficiently fine resolution for polyfill usually does the trick (and the fact that some cells with only a tiny intersection might not generated is often even useful).

Example

Here is an example using Geopandas and H3-Pandas, but of course, the logic works with any implementation of the H3 API.

import geopandas as gpd
import h3pandas

# Choose a random country boundary (Tanzania)
gdf = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres')).iloc[1:2]

Simple polyfill

gdf.h3.polyfill_resample(2)

"Finer" polyfill

gdf.h3.polyfill_resample(4).h3.h3_to_parent_aggregate(2)