mapPlot: Draw a Map

Description Usage Arguments Details Available Projections Available ellipse formulations Available datum formulations Choosing a projection Issues Author(s) References See Also Examples

View source: R/map.R

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 more items so the map, including points, lines, text, images and contours.

Usage

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mapPlot(longitude, latitude, longitudelim, latitudelim, grid = TRUE, bg, fill,
  border = NULL, col = NULL, 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.

bg

colour of the background (ignored).

fill

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

border

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

col

either the colour for filling polygons (if type="polygon") or the colour 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".

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.

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.

Once a projection is set, other map* functions may be used to add to the map.

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.

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 -
Int'l map of the world polyconic imw_p lat_1, lat_2, lon_1
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 conformal conic alternative lcca 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.

Issues

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.

Author(s)

Dan Kelley and Clark Richards

References

1. Snyder, John P., 1987. Map Projections: A Working Manual. USGS Professional Paper: 1395 (available at pubs.usgs.gov/pp/1395/report.pdf).

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

3. Wikipedia page http://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.

See Also

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 [8] for a demonstration of the available map projections (with graphs).

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

Examples

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## Not run: 
library(oce)
data(coastlineWorld)

# Example 1.
# Mollweide ([1] page 54) is an equal-area projection that works well
# for whole-globe views, below shown in a Pacific-focus view.
# Note that filling is not employed when the prime meridian
# is shifted, because this causes a problem with Antarctica
par(mfrow=c(2, 1), mar=c(3, 3, 1, 1))
mapPlot(coastlineWorld, projection="+proj=moll", col='gray')
mtext("Mollweide", adj=1)
cl180 <- coastlineCut(coastlineWorld, lon_0=-180)
mapPlot(cl180, projection="+proj=moll +lon_0=-180")
mtext("Mollweide", adj=1)
par(mfrow=c(1, 1))

# Example 2.
# 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 3.
# 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(coastlineWorld, 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 4.
# 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(coastlineWorld, longitudelim=c(-130, 50), latitudelim=c(70, 110),
        proj="+proj=stere +lat_0=90 +lon_0=-135", col='gray')
mtext("Stereographic", adj=1)

# Example 5.
# Spinning globe: create PNG files that can be assembled into a movie
png("539B-\%03d.png")
lons <- seq(360, 0, -15)
ilon <- seq_along(lons)
par(mar=rep(0, 4))
for (i in ilon) {
    p <- paste("+proj=ortho +lat_0=30 +lon_0=", lons[i], sep="")
    if (i == 1) {
        mapPlot(coastlineWorld, projection=p, col="blue", lwd=1.4)
        xlim <- par("usr")[1:2]
        ylim <- par("usr")[3:4]
    } else {
        mapPlot(coastlineWorld, projection=p, col="blue", lwd=1.4,
                xlim=xlim, ylim=ylim, xaxs="i", yaxs="i")
    }
}
dev.off()

## End(Not run)

oce documentation built on Aug. 29, 2017, 1:05 a.m.