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Simplify the geometry of a vector

Source: R/simplifyGeom.r
simplifyGeom.Rd

simplifyGeom() reduces the number of vertices used to represent a vector (i.e., to save memory or disk space). There are several methods available.

Usage

# S4 method for class 'GVector'
simplifyGeom(x, tolerance = NULL, method = "VR", prop = 0.5)

Arguments

x

A GVector.

tolerance

Numeric >= 0: Threshold distance in map units (degrees for unprojected, usually meters for projected). If NULL, then 2% of the minimum of the x-, y-, and z-extent will be used.

method

Character: Method used to reduce the number of vertices. Partial matching is used, and case does not matter:

  • "VR": Vertex reduction (default, simplest): If two points p1 and p2 on the same line are closer than the threshold, remove p2. The tolerance argument represents this threshold distance.

  • "DP": Douglas-Peucker (AKA Ramer-Douglas-Peucker) algorithm: Simply stated, for points p1, p2, and p3 on a line, this method constructs a line segment between p1 and p3. If p2 is closer than the threshold to the line segment, it is removed. In this example, the tolerance argument refers to the maximum distance between p2 and the line segment.

  • "DPR": Douglas-Peucker algorithm with reduction: As the Douglas-Pueker method, but each geometry is thinned so that in the end it has only a given proportion of the starting number of points. The prop argument refers to this proportion of remaining points.

  • "RW: Reumann-Witkam algorithm: For points p1, p2, p3, and p4 on a line, constructs two line segments parallel to the line segment defined by p1 and p4. These are placed tolerance distance one either side of the p1-p4 line segment. If the line segment p1-p2 or p3-p4 falls entirely within the bounds of the two outer parallel segments, p2 and p3 are removed, leaving just p1 and p4.

prop

Positive value between 0 and 1: Proportion of points that will be retained for each geometry when the Douglas-Peucker algorithm with reduction is applied (ignored otherwise). Default is 0.5 (retain 50% of vertices).

Value

A GVector.

See also

smoothGeom(), geometry cleaning, terra::simplifyGeom(), GRASS manual page for module v.generalize (see grassHelp("v.generalize"))

Examples

if (grassStarted()) {

# Setup
library(sf)
library(terra)

# Example data
madRivers <- fastData("madRivers")
rivers <- fast(madRivers)
soam <- rivers[rivers$NAM == "SOAMIANINA"] # select one river for illustration

### Simplify geometry (remove nodes)

vr <- simplifyGeom(soam, tolerance = 2000)
dp <- simplifyGeom(soam, tolerance = 2000, method = "dp")
dpr <- simplifyGeom(soam, tolerance = 2000, method = "dpr", prop = 0.5)
rw <- simplifyGeom(soam, tolerance = 2000, method = "rw")

plot(soam, col = "black", lwd = 3)
plot(vr, col = "blue", add = TRUE)
plot(dp, col = "red", add = TRUE)
plot(dpr, col = "chartreuse", add = TRUE)
plot(rw, col = "orange", add = TRUE)

legend("bottom",
   xpd = NA,
   legend = c(
    "Original",
      "Vertex reduction",
      "Douglas-Peucker",
      "Douglas-Peucker reduction",
      "Reumann-Witkam"
  ),
  col = c("black", "blue", "red", "chartreuse", "orange"),
  lwd = c(3, 1, 1, 1, 1)
)

### Smooth geometry

hermite <- smoothGeom(soam, dist = 2000, angle = 3)
chaiken <- smoothGeom(soam, method = "Chaiken", dist = 2000)

plot(soam, col = "black", lwd = 2)
plot(hermite, col = "blue", add = TRUE)
plot(chaiken, col = "red", add = TRUE)

legend("bottom",
   xpd = NA,
   legend = c(
    "Original",
      "Hermite",
      "Chaiken"
  ),
  col = c("black", "blue", "red"),
  lwd = c(2, 1, 1, 1, 1)
)

### Clean geometry

# Has no effect on this vector!
noDangs <- removeDangles(soam, tolerance = 10000)

plot(soam, col = "black", lwd = 2)
plot(noDangs, col = "red", add = TRUE)

legend("bottom",
   xpd = NA,
   legend = c(
    "Original",
      "No dangles"
  ),
  lwd = c(2, 1),
  col = c("black", "red")
)

}