How to Fix Incorrect GeoJSON Rendering in D3.js

When attempting to visualize geoJSON data in D3.js, it's not uncommon to encounter discrepancies between the expected and actual rendering. One particular issue arises when the output displays a large black rectangle instead of the intended geographical features.

Issue Analysis

Upon inspecting the code provided in the question, it becomes apparent that the error lies in the winding order of the coordinates. D3.js utilizes ellipsoidal math, which requires a specific winding order for coordinates. Unfortunately, the geoJSON file contains a mixture of both "clockwise" and "counter-clockwise" winding orders, leading to the incorrect rendering.

Solution

To rectify this issue, it's necessary to ensure that all of the coordinates in the geoJSON file have the correct winding order. This can be achieved manually by reordering the coordinates or by using a library like turf.js, which provides a convenient method for rewinding features.

Code Example

Utilizing the turf.rewind() method to correct the winding order:

var fixed = features.map(function(feature) {
  return turf.rewind(feature,{reverse:true});
});

Additional Considerations

While correcting the winding order resolves the black rectangle issue, it's worth noting another quirk in D3.js. Unlike the geoJSON specification, D3.js uses the opposite winding order (counter-clockwise for clockwise and vice versa). As a result, it might be necessary to reverse the winding order before rewinding to ensure proper rendering.

Improved Visualization

In addition to fixing the winding order, adjusting the projection settings can enhance the visualization by fitting the features within the designated space. This can be achieved using the fitSize method to scale and translate the features:

// Fit the features to the screen
var projection = d3.geoMercator().fitSize([width, height], features);

By following these steps, you can correct the incorrect rendering of geoJSON data in D3.js and achieve an accurate visualization of the desired geographical features.