Polygon intersection with line fails for geography

[radu123]

The polygon and line should intersect, but ST_Intersects fails:

select ST_intersects(ST_GeogFromText('POLYGON((-107.61362 40.589806,-106.89783 40.278473,-106.37725 39.967155,-106.31217 39.240715,-106.76768 37.528435,-107.35333 36.12747,-108.329414 35.764263,-109.04517 36.075596,-109.24042 36.750122,-109.24042 37.89165,-108.78492 40.122803,-107.93898 40.434128,-107.61362 40.589806))'), ST_GeogFromText('LINESTRING(-120 40, -96.6 40)'));

st_intersects

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f

I attach PNG which shows the shapes.

[radu123]

Line and Polygon should intersect

[radu123]

We noticed that it works if we change the line a bit:

select ST_intersects(ST_GeogFromText('POLYGON((-107.61362 40.589806,-106.89783 40.278473,-106.37725 39.967155,-106.31217 39.240715,-106.76768 37.528435,-107.35333 36.12747,-108.329414 35.764263,-109.04517 36.075596,-109.24042 36.750122,-109.24042 37.89165,-108.78492 40.122803,-107.93898 40.434128,-107.61362 40.589806))'), ST_GeogFromText('LINESTRING(-120 40, -96.7 40)'));

st_intersects

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t

[radu123]

It also works if we cast the objects to geometry, so it seems to be something related to geography.

select ST_intersects(ST_GeogFromText('POLYGON((-107.61362 40.589806,-106.89783 40.278473,-106.37725 39.967155,-106.31217 39.240715,-106.76768 37.528435,-107.35333 36.12747,-108.329414 35.764263,-109.04517 36.075596,-109.24042 36.750122,-109.24042 37.89165,-108.78492 40.122803,-107.93898 40.434128,-107.61362 40.589806))')::geometry, ST_GeogFromText('LINESTRING(-120 40, -96.6 40)')::geometry);

st_intersects

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t

(1 row)

[pramsey]

Your line isn't straight like in your picture, it's actually straight in a different way, as a great circle arc (which, in your picture would be a northerly curve). Think about how an airplane would get between the ends of your line. Reasoning about geography on flat maps leads to mistakes like this.