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From "Martin Desruisseaux (JIRA)" <>
Subject [jira] [Resolved] (SIS-155) Area calculation on ellipsoid
Date Thu, 21 Jul 2016 21:58:20 GMT


Martin Desruisseaux resolved SIS-155.
    Resolution: Fixed

The proposed approach is to project the geometry to the Lambert Cylindrical Equal Area projection,
then compute the area in Cartesian coordinates.

> Area calculation on ellipsoid
> -----------------------------
>                 Key: SIS-155
>                 URL:
>             Project: Spatial Information Systems
>          Issue Type: New Feature
>          Components: Referencing
>            Reporter: Martin Desruisseaux
>            Assignee: Martin Desruisseaux
>             Fix For: 0.8
> We need a method for calculating the area inside a polygon on the ellipsoid. Some useful
> * [Algorithm to find the area of a polygon|]
in Cartesian coordinate system.
> * [Ellipsoidal area computations of large terrestrial objects|]
> * [Some algorithms for polygons on a sphere|]
> * [Addenda for C. F. F. Karney, Algorithms for Geodesics|]
> This algorithm for Cartesian coordinate system can be adapted to spherical coordinate
systems by replacing the area sum by (note that this replacement uses vertical strips instead
than horizontal ones):
> {code:java}
> s += (λ2 - λ1) * (2 + sin(φ1) + sin(φ2));
> {code}
> and the final answer by:
> {code:java}
> area = abs(s * r² / 2);
> {code}
> The _r_ value could be approximated to the authalic radius (the radius of a hypothetical
sphere having the same surface than the ellipsoid). However the _Ellipsoidal Area Computations
of Large Terrestrial Objects_ article seems to use a more local approximation, where _a_ and
_b_ are semi-major and semi-minor axis lengths:
> {code:java}
> s = sin(φ)
> c = cos(φ)
> r = (a²b) / (a²c² + b²s²)
> {code}
> This task is for writing down some ideas. We probably need to read the above-cited article
and other internet resources more carefully. In particular we need some more analytical analysis
for determining how [rhumb lines|] are handled in the
above-cited resources. This would affect polygon segments of more than 100 km.

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