linesGC: Add Great Circle lines on a map

Description

linesGC draws Great Circle lines between a set of start and end points on an existing map.

Usage

linesGC(start.points, end.points, n = 10, antimeridian = FALSE, ...)

Arguments

start.points: Two-column data.frame or matrix of longitudes and latitudes for start points.
end.points: Two-column data.frame or matrix of longitudes and latitudes for end points. The dimensions of start.points and end.points must be compatible (i.e. they must have the same number of rows).
n: Numeric. The number of intermediate points to add along the great circle line between the start end end points.
antimeridian: Logical indicating if the map on which the great circle lines will be plotted covers the antimeridian region. The antimeridian (or antemeridian) is the 180th meridian and is located in the middle of the Pacific Ocean, east of New Zealand and Fidji, west of Hawaii and Tonga.
...: Further arguments to be passed to lines to control the aspect of the lines to draw.

Author

Benoit Simon-Bouhet

Details

linesGCD takes advantage of the gcIntermediate function from package geosphere to plot lines following a great circle. When working with marmap maps encompassing the antimeridian, longitudes are numbered from 0 to 360 (as opposed to the classical numbering from -180 to +180). It is thus critical to set antimeridian=TRUE to avoid plotting incoherent great circle lines.

Examples

Run this code

# Load NW Atlantic data and convert to class bathy
data(nw.atlantic)
atl <- as.bathy(nw.atlantic)

# Create vectors of latitude and longitude
lon <- c(-70, -65, -63, -55, -48)
lat <- c(33, 35, 40, 37, 33)

# Compute distances between each point and the -200m isobath
d <- dist2isobath(atl, lon, lat, isobath = -200)
d

# Create a nice palette of bleus for the bathymetry
blues <- c("lightsteelblue4","lightsteelblue3","lightsteelblue2","lightsteelblue1")

# Visualize the great circle distances
plot(atl, image=TRUE, lwd=0.1, land=TRUE, 
	 bpal = list(c(0,max(atl),"grey"), c(min(atl),0,blues)))
points(lon,lat, pch=21, col="orange4", bg="orange2", cex=.8)
linesGC(d[2:3],d[4:5])

# Load aleutians data and plot the map
data(aleutians)
plot(aleutians, image=TRUE, lwd=0.1, land=TRUE, 
	 bpal = list(c(0,max(aleutians),"grey"), c(min(aleutians),0,blues)))

# define start and end points
start <- matrix(c(170,55, 190, 60), ncol=2, byrow=TRUE, dimnames=list(1:2, c("lon","lat")))
end <- matrix(c(200, 56, 201, 57), ncol=2, byrow=TRUE, dimnames=list(1:2, c("lon","lat")))
start
end

# Add points and great circle distances on the map
points(start, pch=21, col="orange4", bg="orange2", cex=.8)
points(end, pch=21, col="orange4", bg="orange2", cex=.8)
linesGC(start, end, antimeridian=TRUE)