Calculate ellipse polygon — st

The function calculates ellipse polygons, given centroid locations and sizing on the x and y axes.

st_ellipse(pnt, ex, ey, res = 30)

Arguments

pnt: Object of class sf or sfc (type "POINT") representing centroid locations
ex: Size along x-axis, in CRS units
ey: Size along y-axis, in CRS units
res: Number of points the ellipse polygon consists of (default 30)

Value

Object of class sfc (type "POLYGON") containing ellipse polygons

References

Based on StackOverflow answer by user fdetsch:

https://stackoverflow.com/questions/35841685/add-an-ellipse-on-raster-plot-in-r

Examples

# Sample data
dat = data.frame(
  x = c(1, 1, -1, 3, 3),
  y = c(0, -3, 2, -2, 0),
  ex = c(0.5, 2, 2, 0.3, 0.6),
  ey = c(0.5, 0.2, 1, 1, 0.3),
  stringsAsFactors = FALSE
)
dat = st_as_sf(dat, coords = c("x", "y"))
dat
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: -1 ymin: -3 xmax: 3 ymax: 2
#> CRS:           NA
#>    ex  ey     geometry
#> 1 0.5 0.5  POINT (1 0)
#> 2 2.0 0.2 POINT (1 -3)
#> 3 2.0 1.0 POINT (-1 2)
#> 4 0.3 1.0 POINT (3 -2)
#> 5 0.6 0.3  POINT (3 0)

# Plot 1
plot(st_geometry(dat), graticule = TRUE, axes = TRUE, main = "Input")
text(st_coordinates(dat), as.character(1:nrow(dat)), pos = 2)


# Calculate ellipses
el = st_ellipse(pnt = dat, ex = dat$ex, ey = dat$ey)

# Plot 2
plot(el, graticule = TRUE, axes = TRUE, main = "Output")
plot(st_geometry(dat), pch = 3, add = TRUE)
text(st_coordinates(dat), as.character(1:nrow(dat)), pos = 2)