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On 6/19/2021 2:52 PM, Andrew Bell wrote:
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<div dir="auto">The X and Y dimensions are assumed to lie on a
plane. All intersection points are also assumed to lie on the
same plane as the polygon. Z values are assigned after the fact.</div>
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<div dir="ltr" class="gmail_attr">On Sat, Jun 19, 2021, 4:40 PM
David Strip <<a href="mailto:qgis-user@stripfamily.net">qgis-user@stripfamily.net</a>>
wrote:<br>
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<div>On 6/19/2021 1:34 PM, Andrew Bell wrote:<br>
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<blockquote type="cite">These are done in 2D, without regard
to the spatial reference.</blockquote>
This still doesn't answer the question about great circles.
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After some head-scratching and playing in QGIS, I realized that what
Andrew is saying is that vertices are treated as Cartesian
coordinates with lon/lat values. QGIS appears to always draw a
straight line between any two vertices regardless of the active
projection. This leads to some un-intuitive outcomes. Consider the
map below in an Albers projection. The intersection of the two green
lines is computed as the pink point. In EPSG:4326, the northern
border of the US and the green line are coincident, and the
intersection point lies on the visual intersection of the two lines.
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Densification of the line solves the problem, since each vertex is
projected, creating the appearance of a curved line. <br>
And for a different use case, there is a Geodesic Densification
plug-in to create great circle lines between vertices.<br>
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<img src="cid:part2.2E5137DC.DAA0EEEA@stripfamily.net" alt="">
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