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Generate coordinates of small circles

Source: R/sc.R
sc_shape.Rd

Creating points along small circles

Usage

sc_shape(
  x = "random",
  r = NULL,
  r.ex = NULL,
  r.rad = NULL,
  r.deg = NULL,
  n = NULL,
  breaks = 100,
  radius = 6371.007,
  origin = c(0, 0, 0),
  output = "polar",
  sf.type = "polygon",
  sf.wrap.dateline = TRUE,
  drop = TRUE
)

Arguments

x

Numeric matrix or character string. Coordinates of the centers of the small circles

r

The surface radius of the small circle expressed as great circle distance of the small circle's points from the center of the small circles.

r.ex

The extrinsic radius of the small circles, i.e. the radius of the circle in the plane of the small circle.

r.rad

The angle of the small circle in radians.

r.deg

The angle of the small circle in degrees.

n

Integer or NULL. only used if "x= random"

breaks

Integer. The number of points to create from the small circle.

radius

The radius of the sphere, defaults to the authalic radius of Earth - relevant only for Cartesian output.

origin

The center of the sphere (don't touch this, unless you really think you know what you are doing!).

output

Output structure for the function. The value output="polar" will return the polar (longitude-latitude) coordinates of the small circles in an array. The setting output="cartesian" will return the 3D cartesian coordinates of the small circles in an array. The option output="sf" will return an sfc geometry collection.

sf.type

The type of sf object to be returned.

sf.wrap.dateline

Argument indicating whether returned sf object should be wrapped around the dateline.

drop

If there is a single small circle to be generated, should its array wrapper be dropped?

Value

Either a numeric array or and sfc geometry collection.

Details

The function generates a single reference (standard) small circle around the (0,0) long-lat coordinates using a given radius. This reference circle is then rotated to match a given center. The function is iterated for multiple radiiand centers (separately!).

Examples

set.seed(1)
# generate a random small circle
central <- sc_shape(r=6000, breaks=40)
plot(NULL, xlim=c(-180, 180), ylim=c(-90,90), xlab="longitude", ylab="latitude")
points(central, pch=17, col="gray")
# repeat, same radius multiple centers
smaller <- sc_shape(x=central, r=1000, breaks=100)
for(i in 1:nrow(central)) points(smaller[,,i], col=i, pch=16)


# Random circles with different radii
diff200 <- sc_shape(r=runif(200, 100, 10000))
plot(NULL, xlim=c(-180, 180), ylim=c(-90,90), xlab="", ylab="", axes=FALSE)
for(i in 1:(dim(diff200)[3])){
 points(diff200[,,i], col=i,pch=16)
}


# Same center, different radius
cent <- central[10, ,drop=FALSE]
outward <- sc_shape(cent, r=seq(100, 15000, length.out=20)) 
plot(NULL, xlim=c(-180, 180), ylim=c(-90,90), xlab="", ylab="", axes=FALSE)
for(i in 1:(dim(outward)[3])){
 points(outward[,,i], col=i,pch=16)
}