geodDist {oce}

Compute Geodesic Distance on Surface of Earth

Description

This calculates geodesic distance, in km, between points on the earth, i.e. distance measured along the (presumed ellipsoidal) surface. The method involves the solution of the geodetic inverse problem, using Vincenty's (1975) modification of Rainsford's method with Helmert's elliptical terms.

Usage

geodDist(
  longitude1,
  latitude1 = NULL,
  longitude2 = NULL,
  latitude2 = NULL,
  alongPath = FALSE
)

Arguments

longitude1

longitude or a vector of longitudes, or a section object, from which longitude and latitude are extracted and used instead of the next three arguments

latitude1

latitude or vector of latitudes (ignored if longitude1 is a section object)

longitude2

optional longitude or vector of longitudes (ignored if alongPath=TRUE)

latitude2

optional latitude or vector of latitudes (ignored if alongPath=TRUE)

alongPath

boolean indicating whether to compute distance along the path, as opposed to distance from the reference point. If alongPath=TRUE, any values provided for latitude2 and longitude2 will be ignored.

Details

The function may be used in several different ways.

Case 1: longitude1 is a section object. The values of latitude1, longitude2, and latitude2 arguments are ignored, and the behaviour depends on the value of the alongPath argument. If alongPath=FALSE, the return value contains the geodetic distances of each station from the first one. If alongPath=TRUE, the return value is the geodetic distance along the path connecting the stations, in the order in which they are stored in the section.

Case 2: longitude1 is a vector. If longitude2 and latitude2 are not given, then the return value is a vector containing the distances of each point from the first one, or the distance along the path connecting the points, according to the value of alongPath. On the other hand, if both longitude2 and latitude2 are specified, then the return result depends on the length of these arguments. If they are each of length 1, then they are taken as a reference point, from which the distances to longitude1 and latitude1 are calculated (ignoring the value of alongPath). However, if they are of the same length as longitude1 and latitude1, then the return value is the distance between corresponding (longitude1,latitude1) and (longitude2,latitude2) values.

Value

Vector of distances in kilometres.

Author(s)

Dan Kelley based this on R code sent to him by Darren Gillis, who in 2003 had modified Fortran code that, according to comments in the source, had been written in 1974 by L. Pfeifer and J. G. Gergen.

References

Vincenty, T. "Direct and Inverse Solutions of Geodesics on the Ellipsoid with Application of Nested Equations." Survey Review 23, no. 176 (April 1, 1975): 88-93. https://doi.org/10.1179/sre.1975.23.176.88.

See Also

geodXy()

Other functions relating to geodesy: geodGc(), geodXyInverse(), geodXy()

Examples

library(oce)
km <- geodDist(100, 45, 100, 46)
data(section)
geodDist(section)
geodDist(section, alongPath = TRUE)