Trace along the plot box, converting from xy coordinates to lonlat coordinates. The results are used by mapGrid() and mapAxis() to ignore out-of-frame grid lines and axis labels.

usrLonLat(n = 25, debug = getOption("oceDebug"))

## Arguments

n number of points to check along each side of the plot box 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.

## Value

a list containing lonmin, lonmax, latmin, latmax, and ok; the last of which indicates whether at least one point on the plot box is invertible. Note that longitude are expressed in the range from -180 to 180 degrees.

## Details

Note: this procedure does not work for projections that have trouble inverting points that are "off the globe". For this reason, this function examines .Projection()\$projection and if it contains the string "wintri", then the above-stated procedure is skipped, and the return value has each of the numerical quantities set to NA, and ok set to FALSE.

Other functions related to maps: formatPosition(), lonlat2map(), lonlat2utm(), map2lonlat(), mapArrows(), mapAxis(), mapContour(), mapCoordinateSystem(), mapDirectionField(), mapGrid(), mapImage(), mapLines(), mapLocator(), mapLongitudeLatitudeXY(), mapPlot(), mapPoints(), mapPolygon(), mapScalebar(), mapText(), mapTissot(), oceCRS(), shiftLongitude(), utm2lonlat()