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Departure of longitudes at specified latitudes

`dist = departure(long1,long2,lat)dist = departure(long1,long2,lat,ellipsoid)dist = departure(long1,long2,lat,`

`dist = departure(long1,long2,lat)` computes
the departure distance from `long1` to `long2` at
the input latitude `lat`. Departure is the distance
along a specific parallel between two meridians. The output `dist` is
returned in degrees of arc length on a sphere.

`dist = departure(long1,long2,lat,ellipsoid)` computes
the departure assuming that the input points lie on the ellipsoid
defined by the input `ellipsoid`. `ellipsoid` is
a `referenceSphere`, `referenceEllipsoid`, or `oblateSpheroid` object, or a vector
of the form `[semimajor_axis eccentricity]`.

`dist = departure(long1,long2,lat,units)` uses
the input string

`dist = departure(long1,long2,lat,geoid,units)` is
a valid calling form. In this case, the departure is computed in the
same units as the semimajor axes of the ellipsoid.

*Departure* is the distance along a parallel
between two points. Whereas a degree of latitude is always the same
distance, a degree of longitude is different in length at different
latitudes. In practice, this distance is usually given in nautical
miles.

On a spherical Earth, the departure is proportional to the cosine of the latitude:

distance = departure(0, 10, 0) distance = 10 distance = departure(0, 10, 60) distance = 5

When an ellipsoid is used, the result is more complicated. The distance at 60º is not exactly twice the 0º value:

distance = departure(0, 10, 0, referenceEllipsoid('earth', 'nm')) distance = 601.0772 distance = departure(0, 10, 60, referenceEllipsoid('earth', 'nm')) distance = 299.7819

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