Wrong (sampled) projected border?

[ondras]

Hi,

I am using d3's geoRobinson to render country polygons. The USA-Canada border looks wrong:

(the line shall be mostly straight)

I am not sure how/why this happens; it is related to the projection's precision (setting a high value makes the line straight), but I have no idea why (the projection is supposed to make lines of latitude parallel anyway). The screenshot above was created using precision(.1).

[ondras]

Update: my bad, of course. That is the line along the great circle, obviously, which is correct. Sorry.

[Fil]

Now you've invaded Canada :) The border between the two countries truly is on the 49th parallel north (small circle).

The interaction of D3's great circle drawing with your GeoJSON created this diplomatic incident.

See d3/d3-geo-projection#75

Other refs:

• https://beta.observablehq.com/@fil/wgs84-resampling
• Support adaptive resampling for any transform #134 (comment)

[ondras]

Now you've invaded Canada :) The border between the two countries truly is on the 49th parallel north (small circle).

Apparently getting from the eastern part of the US to the western part is faster if you take the route through Canada ;)

Thanks for the clarification and relevant resources!