Why Does D3.js Display a Black Rectangle When Visualizing GeoJSON Regions with Inconsistent Winding Order?

Understanding D3.js's Quirky GeoJSON Interpretation

Issue:

When attempting to visualize Russian regions using D3.js, users encounter a perplexing issue where the map appears as a single black rectangle instead of the anticipated outline of the regions.

Investigation:

Upon closer examination of the resulting SVG paths, it becomes apparent that the winding order of the coordinates in the GeoJSON data is inconsistent. D3.js, unlike many other tools that treat GeoJSON with Cartesian coordinates, utilizes ellipsoidal coordinates. This introduces sensitivity to winding order, leading to an "inverted polygon" effect where areas not corresponding to the target feature are also covered.

Solution:

To resolve this issue, it is necessary to ensure that all coordinates have the correct winding order. This can be achieved using a library like turf.js, which can rewind the coordinates to conform to D3.js's unconventional winding order.

However, due to this quirk, the winding order of GeoJSON features must be reversed during the rewind process.

Improved Visualization:

By applying the rewind operation and adjusting projection settings, a visually appealing map of Russian regions can be achieved. This demonstrates the intricacies of winding order considerations when working with GeoJSON in D3.js.

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