polygon_area: Polygon Area (using the Shoelace Formula)

In iemisc: Irucka Embry's Miscellaneous Functions

Polygon Area (using the Shoelace Formula)

Description

Calculate the area of a polygon using the shoelace formula.

Usage

polygon_area(x, y, plot = c(0, 1), fill = NULL, color = NULL)

Arguments

x	numeric vector that contains the x coordinates of the vertices. Regardless of rather the user starts in a clockwise or a counter-clockwise direction, the result will be positive.
y	numeric vector that contains the y coordinates of the vertices. Regardless of rather the user starts in a clockwise or a counter-clockwise direction, the result will be positive.
plot	integer vector that contains 0, 1 only. 0 represents do not plot the polygon and 1 is for plotting the polygon.
fill	character vector that contains the inside color of the polygon. The possible colors are those that are accepted by ggplot2. The default fill color is black.
color	character vector that the border color of the polygon. The possible colors are those that are accepted by ggplot2. The default fill color is black.

Value

the area of the polygon as a positive numeric vector.

Author(s)

John D Page for the JavaScript code, Irucka Embry (R code)

References

John D Page, From Math Open Reference: Algorithm to find the area of a polygon. See https://web.archive.org/web/20221006001150/https://www.mathopenref.com/coordpolygonarea2.html. Provided by Internet Archive: Wayback Machine to avoid the connection timeout.

See Also

polyarea and polyarea

Examples

# Example 1 from Source 2

library(iemisc)

x <- c(4,  4,  8,  8, -4, -4)
y <- c(6, -4, -4, -8, -8, 6)

polygon_area(x, y, plot = 1)

# compare with pracma's and geometry's polyarea

pracma::polyarea(x, y)

geometry::polyarea(x, y)





# Example 2

library(iemisc)

type38 <- construction_decimal("46'-10 1/2\"", result = "traditional", output = "vector")

x38 <- c(0, 25, sum(25, type38, 10), sum(25, type38, 10, 25))

y38 <- c(0, rep((3 + 1 / 3), 2), 0)

polygon_area(x38, y38, plot = 1, fill = "darkseagreen3", color = "aquamarine4")

# compare with pracma's and geometry's polyarea

pracma::polyarea(x38, y38)

geometry::polyarea(x38, y38)





# Example 3

install.load::load_package("iemisc", "data.table")

coords <- fread("
X,   Y
0,	0
34,	4
58,	4
84,	6.7
184,	0", header = TRUE)

polygon_area(coords$X, coords$Y, plot = 1, color = "#00abff", fill = NA)
# "Use NA for a completely transparent colour." (from ggplot2 color function)

# compare with pracma's and geometry's polyarea

pracma::polyarea(coords$X, coords$Y)

geometry::polyarea(coords$X, coords$Y)





# Example 4 from pracma

library(iemisc)

Xx <- c(0, 4, 4, 0)

Yy <- c(0, 0, 4, 4)

polygon_area(Xx, Yy, 1, color = "goldenrod1", fill = "#00abff")

# compare with pracma's and geometry's polyarea

pracma::polyarea(Xx, Yy)

geometry::polyarea(Xx, Yy)





# Example 5 from pracma

library(iemisc)

Xx1 <- c(0, 4, 2)

Yy1 <- c(0, 0, 4)

polygon_area(Xx1, Yy1, 1, color = "rosybrown", fill = "papayawhip")

# compare with pracma's and geometry's polyarea

pracma::polyarea(Xx1, Yy1)

geometry::polyarea(Xx1, Yy1)

iemisc documentation built on Sept. 25, 2023, 5:09 p.m.