Fixing D3.js GeoJSON Drawing Issue: How to Correct the Winding Order?

D3.js Incorrectly Drawing GeoJSON: The Winding Order Problem

When attempting to visualize Russian regions using geoJSON data, programmers have encountered a problem where D3.js draws a single black rectangle instead of the desired map outlines. This discrepancy arises from an issue with the winding order of the coordinates in the geoJSON file.

Understanding Winding Order

GeoJSON coordinates can be arranged in either clockwise or counterclockwise order, representing the inside or outside of a shape. Most tools and validators ignore this order, but D3.js uses ellipsoidal math, which requires correct winding to avoid creating inverted polygons that cover the entire globe.

Identifying the Issue

Upon inspecting the SVG paths, it becomes evident that some paths are accurately drawn while others cover the entire planet except for the intended space. This is because the data contains a combination of both clockwise and counterclockwise windings, leading D3.js to treat the inverted polygon as everything on the planet that is not the target region.

Resolving the Issue

To resolve this problem, the coordinates must be re-ordered. A convenient solution is to utilize the turf.js library:

<code class="js">var fixed = features.map(function(feature) {
    return turf.rewind(feature,{reverse:true});
});</code>

This rewinds each polygon to follow the correct winding order, as specified by the geoJSON spec. However, D3.js uses the opposite winding order, so the reverse parameter is set to true.

Adjusting the Map View

After fixing the winding order, further adjustments can be made to enhance the map's appearance. By adding a fitSize method to the projection, the map can be scaled and translated to achieve a more suitable view.

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