mapPlot: Draw a Map

Description

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.

Usage

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,
  lonlabel = NULL, latlabel = NULL, sides = NULL,
  projection = "+proj=moll", tissot = FALSE, trim = TRUE,
  debug = getOption("oceDebug"), ...)

Arguments

longitude

either a 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.

latitude

vector of latitudes of points to be plotted (ignored if the first argument contains both latitude and longitude).

longitudelim

optional 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.

latitudelim

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.

grid

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).

fill

(deprecated) is a deprecated argument; see oce-deprecated.

border

color of coastlines and international borders (ignored unless type="polygon".

col

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".

clip

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.

type

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.

axes

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.

cex

character expansion factor for plot symbols, used if type='p' or any other value that yields symbols.

cex.axis

axis-label expansion factor (see par).

mgp

three-element numerical vector describing axis-label placement, passed to mapAxis.

drawBox

logical value indicating whether to draw a box around the plot. This is helpful for many projections at sub-global scale.

showHemi

logical value indicating whether to show the hemisphere in axis tick labels.

polarCircle

a number indicating the number of degrees of latitude extending from the poles, within which zones are not drawn.

lonlabel, latlabel, sides

Optional vectors of longitude and latitude to label on the indicated sides of plot, passed to plot,coastline-method. Using these arguments permits reasonably simple customization. If they are are not provided, reasonable defaults will be used.

projection

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 CRS in the sp package, which is an object with a slot named projarg that gets used as a projection string. See “Details”.

tissot

logical value indicating whether to use mapTissot to plot Tissot indicatrices, i.e. ellipses at grid intersection points, which indicate map distortion.

trim

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.

debug

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.

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 [6].

See “Examples” for suggested projections for some common applications, and [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`	-
Foucaut	`fouc`	-
Foucaut 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).

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 [1,11], 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; [3] and [4] are less detailed and perhaps better for novices. See [8] for a gallery of projections.

References

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

2. Natural Resources Canada https://www.nrcan.gc.ca/earth-sciences/geography/topographic-information/maps/9805

3. Wikipedia page https://en.wikipedia.org/wiki/List_of_map_projections

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.

Examples

Run this code

# NOT RUN {
canProject <- .Platform$OS.type!="windows"&&requireNamespace("rgdal")
if (canProject) {
    library(oce)
    data(coastlineWorld)

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

    # Example 2.
    # Note that filling is not employed (\code{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")
    mtext("Mollweide with coastlineCut", adj=1)

    # 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
    # [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")
    mtext("Orthographic", adj=1)

    # Example 4.
    # The Lambert conformal conic projection is an equal-area projection
    # recommended by [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')
    mtext("Lambert conformal", adj=1)

    # Example 5.
    # The stereographic projection [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')
    mtext("Stereographic", adj=1)

    # Example 6.
    # Spinning globe: create PNG files that can be assembled into a movie
# }
# NOT RUN {
    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")
        }
    }
# }
# NOT RUN {
}
# }

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