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Winding order

In almost all cases, you won't need to worry about winding order, but if you're trying to use GeoJSON that you export from Placemark with d3-geo, this is the page for you.

Tl;dr

Winding order option screenshot

If you're trying to use GeoJSON that you export from Placemark in d3-geo and encountering issues, set the winding order to "D3" when you export.

What is winding order?

Winding order is the order of the coordinates along the ring of a polygon geometry. Some specifications require these coordinates to be ordered clockwise, others counter-clockwise. Winding order can be useful in some narrow circumstances to determine what's the "outside" of a Polygon (or MultiPolygon) and what is the inside.

The Winding Order Option

Placemark gives you the choice of winding order for one format: GeoJSON. Other formats are either unopinionated about winding order or have a well-established standard for which order in which to wind coordinates.

GeoJSON is unique, in that it has three ideas:

  • In the original GeoJSON specification, there was no stipulation of winding order. You could wind rings however you'd like.
  • In the fully-standardized revised GeoJSON¬†specification (aka RFC 7946), polygons should follow the right-hand rule - exterior rings are counterclockwise, interior clockwise. However, for backward-compatibility, this is not required. Polygons with differently-wound coordinates should still work fine.
  • The d3-geo tools require the opposite of the specification - exterior rings should be clockwise wound. And if you have them incorrectly wound, then many functions in d3-geo will not function properly.

Spherical polygons

The reason why d3-geo is so particular about winding order is that it supports spherical polygons - polygons that might cover the whole earth’s surface except for one small area. Imagine a polygon of all the earth except for a city outline: in this case the outer ring of that polygon has the same coordinates as a polygon covering just the city, but the polygon should be interpreted as everything except that area. Using winding order you can represent this. Placemark currently doesn't support this kind of polygon - you can represent this instead by creating a polygon with an outer ring of the full world extent and an inner ring of the city's shape.

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