Clarify that current CrossTrackDistance uses spherical (Haversine) earth model, add Ellipsoidal (Vincenty) version

[urschrei]

https://docs.rs/geo/latest/geo/algorithm/cross_track_distance/trait.CrossTrackDistance.html

One approach would be:

1. Deprecate cross_track_distance in favour of cross_track_distance_haversine
2. Add cross_track_distance_geodesic (I think we have all the necessary pieces between geo and geographiclib_rs)
3. Note the advantages / caveats related to WGS-84 (specifically: most WGS-84 coordinates won't be accurate to less than 1 m anyway)

[frewsxcv]

A PR was opened for this here: #1140

[frewsxcv]

(Didn't mean to close)

[michaelkirk]

This would be a good candidate to revisit in light of the line measure work I've been doing.

I'd guess the API would look something like:

line.cross_track_distance::<Geodesic>(point)
line.cross_track_distance::<Haversine>(point)

Also, just to make sure I'm understanding correctly, isn't cross_track_distance exactly the same as Point x Line distance in a particular metric space?

e.g. we already have implementations:

impl Distance<Point, Point> for Haversine { ... }
impl Distance<Point, Point> for Geodesic { ... }

If we also had these implementations:

impl Distance<Line, Point> for Haversine { ... }
impl Distance<Line, Point> for Geodesic { ... }

Than these two are precisely the same thing:

line.distance::<Haversine>(point);
line.cross_track_distance::<Haversine>(point)

Is that right? Please correct me if I'm wrong, I've not worked with "cross track distance" as a concept by name before.

Regardless, even if they are equivalent, it might be worth having it filed under both names since "cross_track_distance" is apparently a term that people know and will try to find.

[michaelkirk]

Than these two are precisely the same thing:

line.distance::<Haversine>(point);
line.cross_track_distance::<Haversine>(point)

Is that right? Please correct me if I'm wrong, I've not worked with "cross track distance" as a concept by name before.

That is wrong! CrossTrackDistance is measuring the distance to an infinite line passing through the segment. I mistakenly thought it was measuring the distance to the given line segment.