Subject: [Geojson] Implications of Cartesian constraint for lines cross the antimeridian

From: Sean Gillies <[email-masked]>
Date: Tue, 1 Mar 2016 16:01:16 -0700

It's been pointed out to me that in section 3.1.1 (
https://circle-artifacts.com/gh/geojson/draft-geojson/56/artifacts/0/home/ubuntu/draft-geojson/draft.html#rfc.section.3.1.1)
the following does not hold for a line segment that crosses the
antimeridian.

> In other words, every point on a line between a point (lon0, lat0) and
(lon1, lat1) can be calculated as
>
> F(lon, lat) = (lon0 + (lon1 - lon0) * t, lat0 + (lat1 - lat0) * t)
>
> with t a real number greater or equal to 0 and smaller or equal to 1.

One existing practice (you can see it in the USGS's Landsat path/row
shapefiles at https://landsat.usgs.gov/tools_wrs-2_shapefile.php) is to cut
geometries at the antimeridian, e.g., turning polygons into multipolygon
pairs. The feature isn't cut in two, but its spatial extent is.

We could recommend that practice for GeoJSON geometries or we could write a
special case of the above in section 3.1.1 for single part geometries that
cross the antimeridian.

I prefer the former. It's an existing practice and doesn't require any
coordinate system transformations or gymnastics.

-- 
Sean Gillies

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From: Martin Thomson <[email-masked]>
Date: Wed, 2 Mar 2016 12:42:11 +1100

> coordinate system transformations or gymnastics.

I think that asking users of GeoJSON to change their practices isn't
likely to work.  I would rather that you recommend the split, but note
that the formula, such as it is, does not work for antimeridian
crossings.  You don't have to specify a formula for the aberrant case,
I think, if you recommend against creating documents that rely on it.

---

From: Sean Gillies <[email-masked]>
Date: Wed, 2 Mar 2016 00:30:11 -0700

> I think, if you recommend against creating documents that rely on it.

https://github.com/geojson/draft-geojson/pull/162 has a recommendation for
geometry cutting at the antimeridian with two examples. See

https://circle-artifacts.com/gh/geojson/draft-geojson/59/artifacts/0/home/ubuntu/draft-geojson/draft.html#rfc.section.10.4PR

-- 
Sean Gillies

---

From: Martin Thomson <[email-masked]>
Date: Wed, 2 Mar 2016 21:51:43 +1100

> geometry cutting at the antimeridian with two examples. See

I think that it would pay to add text to the section on line
interpolation highlighting that the formula does not work for lines
that cross the antimeridian.