Locate points on an existing map.
This uses map2lonlat() to infer the location in
geographical space, so it suffers the same
limitations as that function.
mapLocator(n = 512, type = "n", ...)number of points to locate; see locator().
type of connector for the points; see locator().
extra arguments passed to locator() (and either
mapPoints() or mapLines(), if appropriate) if
type is not 'n'.
A map must first have been created with mapPlot().
Other functions related to maps: 
formatPosition(),
lonlat2map(),
lonlat2utm(),
map2lonlat(),
mapArrows(),
mapAxis(),
mapContour(),
mapCoordinateSystem(),
mapDirectionField(),
mapGrid(),
mapImage(),
mapLines(),
mapLongitudeLatitudeXY(),
mapPlot(),
mapPoints(),
mapPolygon(),
mapScalebar(),
mapText(),
mapTissot(),
oceCRS(),
shiftLongitude(),
usrLonLat(),
utm2lonlat()