Plot coordinates as a map, using one of the subset of projections provided by the rgdal package. The projection information specified with the mapPlot call is stored so that can be retrieved by related functions, making it easy to add points, lines, text, images or contours to an existing map.

mapPlot(
longitude,
latitude,
longitudelim,
latitudelim,
grid = TRUE,
bg,
fill,
border = NULL,
col = NULL,
clip = TRUE,
type = "polygon",
axes = TRUE,
cex,
cex.axis = 1,
mgp = c(0, 0.5, 0),
drawBox = TRUE,
showHemi = TRUE,
polarCircle = 0,
lonlabels = TRUE,
latlabels = TRUE,
projection = "+proj=moll",
tissot = FALSE,
trim = TRUE,
debug = getOption("oceDebug"),
...
)

## Arguments

longitude either a numeric vector of longitudes of points to be plotted, or something (an oce object, a list, or a data frame) from which both longitude and latitude may be inferred (in which case the latitude argument is ignored). If longitude is missing, both it and latitude are taken from coastlineWorld(). numeric vector of latitudes of points to be plotted (ignored if the first argument contains both latitude and longitude). optional numeric vector of length two, indicating the longitude limits of the plot. This value is used in the selection of longitude lines that are shown (and possibly labelled on the axes). In some cases, e.g. for polar views, this can lead to odd results, with some expected longitude lines being left out of the plot. Altering longitudelim can often help in such cases, e.g. longitudelim=c(-180, 180) will force the drawing of lines all around the globe. optional vector of length two, indicating the latitude limits of the plot. This, together with longitudelim (and, importantly, the geometry of the plot device) is used in the selection of map scale. either a number (or pair of numbers) indicating the spacing of longitude and latitude lines, in degrees, or a logical value (or pair of values) indicating whether to draw an auto-scaled grid, or whether to skip the grid drawing. In the case of numerical values, NA can be used to turn off the grid in longitude or latitude. Grids are set up based on examination of the scale used in middle 10 percent of the plot area, and for most projections this works quite well. If not, one may set grid=FALSE and add a grid later with mapGrid(). color of the background (ignored). is a deprecated argument; see oce-deprecated. color of coastlines and international borders (ignored unless type="polygon". either the color for filling polygons (if type="polygon") or the color of the points and line segments (if type="p", type="l", or type="o"). If col=NULL then a default will be set: no coastline filling for the type="polygon" case, or black coastlines, for type="p", type="l", or type="o". logical value indicating whether to trim any coastline elements that lie wholly outside the plot region. This can prevent e.g. a problem of filling the whole plot area of an Arctic stereopolar view, because the projected trace for Antarctica lies outside all other regions so the whole of the world ends up being "land". Setting clip=FALSE disables this action, which may be of benefit in rare instances in the line connecting two points on a coastline may cross the plot domain, even if those points are outside that domain. indication of type; may be "polygon", for a filled polygon, "p" for points, "l" for line segments, or "o" for points overlain with line segments. logical value indicating whether to draw longitude and latitude values in the lower and left margin, respectively. This may not work well for some projections or scales. See also lonlabels and latlabels, which offer more granular control of labelling. character expansion factor for plot symbols, used if type='p' or any other value that yields symbols. axis-label expansion factor (see par()). three-element numerical vector describing axis-label placement, passed to mapAxis(). logical value indicating whether to draw a box around the plot. This is helpful for many projections at sub-global scale. logical value indicating whether to show the hemisphere in axis tick labels. a number indicating the number of degrees of latitude extending from the poles, within which zones are not drawn. An optional logical value or numeric vector that controls the labelling of longitude values using mapAxis(). There are four possibilities for the value of lonlabels: (1) If lonlabels is TRUE (the default), then reasonable values are inferred and axes are drawn accordingly with both ticks and longitudes alongside those ticks; (2) if lonlabels is FALSE, then ticks are drawn by but not numbers; (3) if lonlabels is NULL, then no axis ticks or numbers are are drawn; and (4) if lonlabels is a vector of finite numerical values, then tick marks are placed at those longitudes, and labels are put alongside them. In cases 1 and 4, overdrawing of numbers is avoided, so some ticks may not have numbers alongside them. See also latlabels, and note that setting axes=FALSE ensures that no longitude or latitude axes will be drawn regardless of the values of lonlabels and latlabels. As lonlabels, but for latitude, on the left plot axis. optional indication of projection, in one of two forms. First, it may be a character string in the "CRS" format that is used by the rgdal package (and in much of modern computer-based cartography). For example, projection="+proj=merc" specifies a Mercator projection. The second format is the output from sp::CRS() in the sp package, which is an object with a slot named projarg that gets used as a projection string. See “Details”. logical value indicating whether to use mapTissot() to plot Tissot indicatrices, i.e. ellipses at grid intersection points, which indicate map distortion. logical value indicating whether to trim islands or lakes containing only points that are off-scale of the current plot box. This solves the problem of Antarctica overfilling the entire domain, for an Arctic-centred stereographic projection. It is not a perfect solution, though, because the line segment joining two off-scale points might intersect the plotting box. a flag that turns on debugging. Set to 1 to get a moderate amount of debugging information, or to 2 to get more. optional arguments passed to some plotting functions. This can be useful in many ways, e.g. Example 5 shows how to use xlim etc to reproduce a scale exactly between two plots.

## Details

Creates a map using the indicated projection. As noted in the information on the projection argument, projections are specified in the notation used by project() in the rgdal package; see “Available Projections” for a list of possibilities.

Further details on map projections are provided by references 1 and 11, an exhaustive treatment that includes many illustrations, an overview of the history of the topic, and some notes on the strengths and weaknesses of the various formulations. See especially pages 2 through 7, which define terms and provide recommendations. Reference 2 is also useful, especially regarding datum shifts; references 3 and 4 are less detailed and perhaps better for novices. See reference 8 for a gallery of projections.

## Available Projections

Map projections are provided by the rgdal package, but not all projections in that package are available. The available list is given in the table below. The cartographic community has set up a naming scheme in a coded scheme, e.g. projection="+proj=aea" selects the Albers equal area projection.

The allowed projections include those PROJ.4 projections provided by rgdal that have inverses, minus a few that cause problems: alsk overdraws coastlineWorld, and is a niche projection for Alaska; calcofi is not a real projection, but rather a coordinate system; gs48 overdraws coastlineWorld, and is a niche projection for the USA; gs50 overdraws coastlineWorld, and is a niche projection for the USA; gstmerc overdraws coastlineWorld; isea causes segmentation faults on OSX systems; krovak overdraws coastlineWorld, and is a niche projection for the Czech Republic; labrd returns NaN for most of the world, and is a niche projection for Madagascar; lee_os overdraws coastlineWorld; and nzmg overdraws coastlineWorld.

The information in the table is reformatted from the output of the unix command proj -lP, where proj is provided by version 4.9.0 of the PROJ.4 system. Most of the arguments listed have default values. In addition, most projections can handle arguments lon_0 and lat_0, for shifting the reference point, although in some cases shifting the longitude can yield poor filling of coastlines.

Further details of the projections and the controlling arguments are provided at several websites, because PROJ.4 has been incorporated into rgdal and other R packages, plus many other software systems; a good starting point for learning is reference 6.

See “Examples” for suggested projections for some common applications, and reference 8 for a gallery indicating how to use every projection.

 Projection Code Arguments Albers equal area aea lat_1, lat_2 Azimuthal equidistant aeqd lat_0, guam Aitoff aitoff - Mod. stererographics of Alaska alsk - Bipolar conic of western hemisphere bipc - Bonne Werner bonne lat_1 Cassini cass - Central cylindrical cc - Equal area cylindrical cea lat_ts Collignon collg - Craster parabolic Putnins P4 crast - Eckert I eck1 - Eckert II eck2 - Eckert III eck3 - Eckert IV eck4 - Eckert V eck5 - Eckert VI eck6 - Equidistant cylindrical plate (Caree) eqc lat_ts, lat_0 Equidistant conic eqdc lat_1, lat_2 Euler euler lat_1, lat_2 Extended transverse Mercator etmerc lat_ts, lat_0 Fahey fahey - Foucault fouc - Foucault sinusoidal fouc_s - Gall stereographic gall - Geostationary satellite view geos h General sinusoidal series gn_sinu m, n Gnomonic gnom - Goode homolosine goode - Hatano asymmetrical equal area hatano - HEALPix healpix - rHEALPix rhealpix north_square, south_square Interrupted Goode homolosine igh - Kavraisky V kav5 - Kavraisky VII kav7 - Lambert azimuthal equal area laea - Longitude and latitude lonlat - Longitude and latitude longlat - Longitude and latitude latlon - Lambert conformal conic lcc lat_1, lat_2, lat_0 Lambert equal area conic leac lat_1, south Loximuthal loxim Space oblique for Landsat lsat lsat, path McBryde-Thomas flat-polar sine, no. 1 mbt_s McBryde-Thomas flat-polar sine, no. 2 mbt_fps McBryde-Thomas flat-polar parabolic mbtfpp McBryde-Thomas flat-polar quartic mbtfpq McBryde-Thomas flat-polar sinusoidal mbtfps Mercator merc lat_ts Miller oblated stereographic mil_os Miller cylindrical mill Mollweide moll Murdoch I murd1 lat_1, lat_2 Murdoch II murd2 lat_1, lat_2 murdoch III murd3 lat_1, lat_2 Natural earth natearth Nell nell Nell-Hammer nell_h Near-sided perspective nsper h New Zealand map grid nzmg General oblique transformation ob_tran o_proj, o_lat_p, o_lon_p, o_alpha, o_lon_c o_lat_c, o_lon_1, o_lat_1, o_lon_2, o_lat_2 Oblique cylindrical equal area ocea lat_1, lat_2, lon_1, lon_2 Oblated equal area oea n, m, theta Oblique Mercator omerc alpha, gamma, no_off, lonc, lon_1, lat_1, lon_2, lat_2 Orthographic ortho - Perspective conic pconic lat_1, lat_2 Polyconic American poly - Putnins P1 putp1 - Putnins P2 putp2 - Putnins P3 putp3 - Putnins P3' putp3p - Putnins P4' putp4p - Putnins P5 putp5 - Putnins P5' putp5p - Putnins P6 putp6 - Putnins P6' putp6p - Quartic authalic qua_aut - Quadrilateralized spherical cube qsc - Robinson robin - Roussilhe stereographic rouss - Sinusoidal aka Sanson-Flamsteed sinu - Swiss. oblique Mercator somerc - Stereographic stere lat_ts Oblique stereographic alternative sterea - Transverse cylindrical equal area tcea - Tissot tissot lat_1, lat_2 Transverse Mercator tmerc - Two point equidistant tpeqd lat_1, lon_1, lat_2, lon_2 Tilted perspective tpers tilt, azi, h Universal polar stereographic ups south Urmaev flat-polar sinusoidal urmfps n Universal transverse Mercator utm zone, south van der Grinten I vandg - Vitkovsky I vitk1 lat_1, lat_2 Wagner I Kavraisky VI wag1 - Wagner II wag2 - Wagner III wag3 lat_ts Wagner IV wag4 - Wagner V wag5 - Wagner VI wag6 - Werenskiold I weren - Winkel I wink1 lat_ts Winkel Tripel wintri lat_ts

## Available ellipse formulations

In the PROJ.4 system of specifying projections, the following ellipse models are available: MERIT, SGS85, GRS80, IAU76, airy, APL4.9, NWL9D, mod_airy, andrae, aust_SA, GRS67, bessel, bess_nam, clrk66, clrk80, clrk80ign, CPM, delmbr, engelis, evrst30, evrst48, evrst56, evrst69, evrstSS, fschr60, fschr60m, fschr68, helmert, hough, intl, krass, kaula, lerch, mprts, new_intl, plessis, SEasia, walbeck, WGS60, WGS66, WGS72, WGS84, and sphere (the default). For example, use projection="+proj=aea +ellps=WGS84" for an Albers Equal Area projection using the most recent of the World Geodetic System model. It is unlikely that changing the ellipse will have a visible effect on plotted material at the plot scale appropriate to most oceanographic applications.

## Available datum formulations

In the PROJ.4 system of specifying projections, the following datum formulations are available: WGS84, GGRS87, Greek_Geodetic_Reference_System_1987, NAD83, North_American_Datum_1983, NAD27, North_American_Datum_1927, potsdam, Potsdam, carthage, Carthage, hermannskogel, Hermannskogel, ire65, Ireland, nzgd49, New, OSGB36, and Airy. It is unlikely that changing the datum will have a visible effect on plotted material at the plot scale appropriate to most oceanographic applications.

## Choosing a projection

The best choice of projection depends on the application. Readers may find projection="+proj=moll" useful for world-wide plots, ortho for hemispheres viewed from the equator, stere for polar views, lcc for wide meridional ranges in mid latitudes, and merc in limited-area cases where angle preservation is important.

## Problems

Map projection is a complicated matter that is addressed here in a limited and pragmatic way. For example, mapPlot tries to draw axes along a box containing the map, instead of trying to find spots along the edge'' of the map at which to put longitude and latitude labels. This design choice greatly simplifies the coding effort, freeing up time to work on issues regarded as more pressing. Chief among those issues are (a) the occurrence of horizontal lines in maps that have prime meridians (b) inaccurate filling of land regions that (again) occur with shifted meridians and (c) inaccurate filling of Antarctica in some projections. Generally, issues are tackled first for commonly used projections, such as those used in the examples.

There are also systematic problems on i386/windows machines, owing to problems with rgdal on such systems. This explains why example("mapPlot") does not try to create maps on such machines. However, rgdal is in continue development, so it is reasonable to hope that oce map projections may start working at some time. As of rgdal version 1.4-3 (in March 2019), however, mapPlot does not work on i386/windows machines.

## Changes

• 2019-03-20: the test code provided the “Examples” section is disabled on i386/windows machines, on which the requisite rgdal package continues to fail on common projections.

• 2017-11-19: imw_p removed, because it has problems doing inverse calculations. This is a also problem in the standalone PROJ.4 application version 4.9.3, downloaded and built on OSX. See https://github.com/dankelley/oce/issues/1319 for details.

• 2017-11-17: lsat removed, because it does not work in rgdal or in the latest standalone PROJ.4 application. This is a also problem in the standalone PROJ.4 application version 4.9.3, downloaded and built on OSX. See https://github.com/dankelley/oce/issues/1337 for details.

• 2017-09-30: lcca removed, because its inverse was wildly inaccurate in a Pacific Antarctic-Alaska application (see https://github.com/dankelley/oce/issues/1303).

## References

1. Snyder, John P., 1987. Map Projections: A Working Manual. USGS Professional Paper: 1395 (available at https://pubs.er.usgs.gov/publication/pp1395).

4. Radical Cartography website http://www.radicalcartography.net/?projectionref (This URL worked prior to Nov 16, 2016, but was found to fail on that date.)

5. The PROJ.4 website is http://trac.osgeo.org/proj, and it is the place to start to learn about the code.

6. PROJ.4 projection details were once at http://www.remotesensing.org/geotiff/proj_list/ but it was discovered on Dec 18, 2016, that this link no longer exists. Indeed, there seems to have been significant reorganization of websites related to this. The base website seems to be https://trac.osgeo.org/geotiff/ and that lists only what is called an unofficial listing, on the wayback web-archiver server http://web.archive.org/web/20160802172057/http://www.remotesensing.org/geotiff/proj_list/

7. A gallery of map plots is provided at http://dankelley.github.io/r/2015/04/03/oce-proj.html.

8. A fascinating historical perspective is provided by Snyder, J. P. (1993). Two thousand years of map projections. University of Chicago Press.

Points may be added to a map with mapPoints(), lines with mapLines(), text with mapText(), polygons with mapPolygon(), images with mapImage(), and scale bars with mapScalebar(). Points on a map may be determined with mouse clicks using mapLocator(). Great circle paths can be calculated with geodGc(). See reference 8 for a demonstration of the available map projections (with graphs).

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

## Examples

canProject <- .Platform\$OS.type!="windows"&&requireNamespace("rgdal")
if (canProject) {
library(oce)
data(coastlineWorld)

# Example 1.
# Mollweide (referenc 1 page 54) is an equal-area projection that works well
# for whole-globe views.
mapPlot(coastlineWorld, projection="+proj=moll", col='gray')

# Example 2.
# Note that filling is not employed (col is not
# given) when the prime meridian is shifted, because
# this causes a problem with Antarctica
cl180 <- coastlineCut(coastlineWorld, lon_0=-180)
mapPlot(cl180, projection="+proj=moll +lon_0=-180")

# Example 3.
# Orthographic projections resemble a globe, making them attractive for
# non-technical use, but they are neither conformal nor equal-area, so they
# are somewhat limited for serious use on large scales.  See Section 20 of
# reference 1. Note that filling is not employed because it causes a problem with
# Antarctica.
par(mar=c(3, 3, 1, 1))
mapPlot(coastlineWorld, projection="+proj=ortho +lon_0=-180")

# Example 4.
# The Lambert conformal conic projection is an equal-area projection
# recommended by reference 1, page 95, for regions of large east-west extent
# away from the equator, here illustrated for the USA and Canada.
par(mar=c(3, 3, 1, 1))
mapPlot(coastlineCut(coastlineWorld, -100),
longitudelim=c(-130,-55), latitudelim=c(35, 60),
projection="+proj=lcc +lat_0=30 +lat_1=60 +lon_0=-100", col='gray')

# Example 5.
# The stereographic projection (reference 1, page 120) is conformal, used
# below for an Arctic view with a Canadian focus.  Note the trick of going
# past the pole: the second latitudelim value is 180 minus the first, and the
# second longitudelim is 180 plus the first; this uses image points "over"
# the pole.
par(mar=c(3, 3, 1, 1))
mapPlot(coastlineCut(coastlineWorld, -135),
longitudelim=c(-130, 50), latitudelim=c(70, 110),
proj="+proj=stere +lat_0=90 +lon_0=-135", col='gray')

# Example 6.
# Spinning globe: create PNG files that can be assembled into a movie
if (FALSE) {
png("globe-%03d.png")
lons <- seq(360, 0, -15)
par(mar=rep(0, 4))
for (i in seq_along(lons)) {
p <- paste("+proj=ortho +lat_0=30 +lon_0=", lons[i], sep="")
if (i == 1) {
mapPlot(coastlineCut(coastlineWorld, lons[i]),
projection=p, col="lightgray")
xlim <- par("usr")[1:2]
ylim <- par("usr")[3:4]
} else {
mapPlot(coastlineCut(coastlineWorld, lons[i]),
projection=p, col="lightgray",
xlim=xlim, ylim=ylim, xaxs="i", yaxs="i")
}
}
}
}