oce (version 0.9-17)

map2lonlat: Convert from x-y to longitude and latitude

Description

Convert from x-y coordinates to longitude and latitude

Usage

map2lonlat(x, y, init=c(0,0))

Arguments

x

a vector containing the x component of points in the projected space, or a list containing items named x and y, in which case the next argument is ignored.

y

a vector containing the y coordinate of points in the projected space (ignored if x is a list, as described above).

init

a vector containing the initial guesses for longitude and latitude, used only for mapproj-style projections (see mapPlot. If the default produces inaccurate results, try sett

Value

• A list containing longitude and latitude, with NA values indicating points that are off the globe as displayed.

Bugs

The mapproj-style case can yield NA for longitude and latitude, even for what would seem to be perfectly benign conditions. Since map2lonlat is called by many mapping functions, this can yield errors of various types, e.g. unaesthetic axis labelling. In many cases those errors can be circumvented by switching to the equivalent proj4-style projection.

Details

A projection must already have been set up, by a call to mapPlot or lonlat2map. The values of x and y have a clear meaning for proj4-style projections. This is not true for mapproj-style projections, since mapproject in the mapproj package shifts the x component of its return value to have mean value equal to 0, up to a calculation error.

The accuracy of map2lonlat tends to be higher for proj4-style projections than for mapproj-style projections. This is because the proj4 package provides inverse functions, while mapproj does not, so that in the latter case the longitude and latitude are determined by a minimization procedure using optim. See Bugs.

References

http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system, downloaded May 31, 2014.

See Also

lonlat2map does the inverse operation.

Examples

library(oce)
## Cape Split, in the Minas Basin of the Bay of Fundy
cs <- list(longitude=-64.49657,latitude=45.33462)
xy <- lonlat2map(cs, projection="+proj=merc")
map2lonlat(xy)