The shortest distance between two points on an ellipsoid (the 'geodetic'), according to the 'Meeus' method. distGeo
should be more accurate.
distMeeus(p1, p2, a=6378137, f=1/298.257223563)
longitude/latitude of point(s), in degrees 1; can be a vector of two numbers, a matrix of 2 columns (first one is longitude, second is latitude) or a SpatialPoints* object
as above; or missing, in which case the sequential distance between the points in p1 is computed
numeric. Major (equatorial) radius of the ellipsoid. The default value is for WGS84
numeric. Ellipsoid flattening. The default value is for WGS84
Distance value in the same units as parameter a
of the ellipsoid (default is meters)
Parameters from the WGS84 ellipsoid are used by default. It is the best available global ellipsoid, but for some areas other ellipsoids could be preferable, or even necessary if you work with a printed map that refers to that ellipsoid. Here are parameters for some commonly used ellipsoids:
ellipsoid 
a 
f 

WGS84 
6378137 
1/298.257223563 

GRS80 
6378137 
1/298.257222101 

GRS67 
6378160 
1/298.25 

Airy 1830 
6377563.396 
1/299.3249646 

Bessel 1841 
6377397.155 
1/299.1528434 

Clarke 1880 
6378249.145 
1/293.465 

Clarke 1866 
6378206.4 
1/294.9786982 

International 1924 
6378388 
1/297 

Krasovsky 1940 
6378245 
1/298.2997381 
Meeus, J., 1999 (2nd edition). Astronomical algoritms. WillmanBell, 477p.
distGeo, distVincentyEllipsoid, distVincentySphere, distHaversine, distCosine
# NOT RUN {
distMeeus(c(0,0),c(90,90))
# on a 'Clarke 1880' ellipsoid
distMeeus(c(0,0),c(90,90), a=6378249.145, f=1/293.465)
# }
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