geodXy {oce} | R Documentation

## Convert From Geographical to Geodesic Coordinates

### Description

The method, which may be useful in determining coordinate systems for a mooring array or a ship transects, calculates (x,y) from distance calculations along geodesic curves. See “Caution”.

### Usage

    geodXy(longitude, latitude, longitudeRef, latitudeRef,
      debug = getOption("oceDebug"))

### Arguments

`longitude, latitude` | vector of longitude and latitude
`longitudeRef, latitudeRef` | numeric reference location. Poor results will be returned if these values are not close to the locations described by `longitude` and `latitude`. A sensible approach might be to set `longitudeRef` to `longitude[1]`, etc.

`debug`

an integer specifying whether debugging information is to be printed during the processing. This is a general parameter that is used by many `oce` functions. Generally, setting `debug=0` turns off the printing, while higher values suggest that more information be printed. If one function calls another, it usually reduces the value of `debug` first, so that a user can often obtain deeper debugging by specifying higher `debug` values.

### Details

The calculation is as follows. Consider the `i`-th point in the `longitude` and `latitude` vectors. To calculate `x[i]`, `geodDist` is used is to find the distance along a geodesic curve connecting (`longitude[i]`, `latitude[i]`) with (`longitudeRef`, `latitude[i]`). The resultant distance is multiplied by -1 if `longitude[i]-longitudeRef` is negative, and the result is assigned to `x[i]`. A similar procedure is used for `y[i]`.

### Value

`geodXy` returns a data frame of `x` and `y`, geodesic distance components, measured in metres.

### Caution

This scheme is without known precedent in the literature, and users should read the documentation carefully before deciding to use it.

### Change history

On 2015-11-02, the names of the arguments were changed from `lon`, etc., to `longitude`, etc., to be in keeping with other oce functions.

On 2017-04-05, four changes were made. 1. Default values of `longitudeRef` and `latitudeRef` were removed, since the old defaults were inappropriate to most work. 2. The argument called `rotate` was eliminated, because it only made sense if the mean resultant x and y were zero. 3. The example was made more useful. 4. Pointers were made to `lonlat2utm`, which may be more useful.

### Author(s)

Dan Kelley

### See Also

`geodDist`

Other functions relating to geodesy: `geodDist`, `geodGc`, `geodXyInverse`

### Examples

    ## Not run:
    # Develop a transect-based axis system for final data(section) stations
    library(oce)
    data(section)
    lon <- tail(section[["longitude", "byStation"]], 26)
    lat <- tail(section[["latitude", "byStation"]], 26)
    lonR <- tail(lon, 1)
    latR <- tail(lat, 1)
    data(coastlineWorld)
    mapPlot(coastlineWorld, proj="+proj=merc",
            longitudelim=c(-75,-65), latitudelim=c(35,43), col="gray")
    mapPoints(lon, lat)
    XY <- geodXy(lon,lat,mean(lon), mean(lat))
    angle <- 180/pi*atan(coef(lm(y~x, data=XY))[2])
    mapCoordinateSystem(lonR, latR, 500, angle, col=2)
    # Compare UTM calculation
    UTM <- lonlat2utm(lon, lat, zone=18) # we need to set the zone for this task!
    angleUTM <- 180/pi*atan(coef(lm(northing~easting, data=UTM))[2])
    mapCoordinateSystem(lonR, latR, 500, angleUTM, col=3)
    legend("topright", lwd=1, col=2:3, bg="white", title="Axis Rotation Angle",
           legend=c(sprintf("geod: %.1f deg", angle), sprintf("utm: %.1f deg",angleUTM)))

    ## End(Not run)

[Package oce version 0.9-22 Index]