geodDist {oce} | R Documentation

## Compute Geodesic Distance on Surface of Earth

### Description

This calculates geodesic distance 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 T. Vincenty's 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 distancce 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

T. Vincenty, "Direct and Inverse Solutions of Ellipsoid on the Ellipsoid with Application of Nested Equations", Survey Review, April 1975. (As of early 2009, this document is available at http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf.)

### 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)

[Package oce version 0.9-22 Index]