Fix issues with geometries of a vector

These functions are intended to help fix geometric issues with a GVector. Note that the functionality of the snap() and removeAreas() functions can also be implemented when using fast() to create a GVector.

• breakPolys(): Break topologically clean areas. This is similar to fixLines(), except that it does not break loops. Topologically clean vectors may occur if the vector was imported from a format that does not enforce topology, such as a shapefile. Duplicate geometries are automatically removed after breaking.

• fixBridges(): Change "bridges" to "islands" (which are topologically incorrect) within geometries to lines.

• fixDangles(): Change "dangles" hanging off boundaries to lines if shorter than tolerance distance. If tolerance is <0, all dangles will be changed to lines. Units of tolerance are in map units, or in degrees for unprojected CRSs. If tolerance <0, all dangles are removed, and the function will retain only closed loops and lines connecting loops. Dangles will be removed from longest to shortest.

• fixLines(): Break lines at intersections and lines that form closed loops.

• remove0(): Remove all boundaries and lines with a length of 0.

• removeAngles(): Collapse lines that diverge at an angle that is computationally equivalent to 0. This tool often needs to be followed with the break() and removeDups() methods.

• removeBridges(): Remove "bridges" to "islands" (which are topologically incorrect) within geometries.

• removeDangles(): Remove "dangling" lines if shorter than tolerance distance. If tolerance is <0, all dangles will be removed. Units of tolerance are in map units, or in degrees for unprojected CRSs. If tolerance <0, all dangles are removed, and the function will retain only closed loops and lines connecting loops. Dangles will be removed from longest to shortest.

• removeDupCentroids(): Remove duplicated area centroids. In GRASS, closed polygons have their attributes mapped to a (hidden) centroid of the polygon.

• removeDups(): Remove duplicated features and area centroids.

• removeSmallPolys(): Remove polygons smaller than tolerance. Units of tolerance are in square meters (regardless of the CRS).

• snap(): Snap lines/boundaries to each other if they are less than tolerance apart. Subsequent removal of dangles may be needed. Units of tolerance are map units, or degrees for unprojected CRSs.

Usage

# S4 method for class 'GVector'
breakPolys(x)

# S4 method for class 'GVector'
fixBridges(x)

# S4 method for class 'GVector'
fixDangles(x, tolerance = -1)

# S4 method for class 'GVector'
fixLines(x)

# S4 method for class 'GVector'
remove0(x)

# S4 method for class 'GVector'
removeAngles(x)

# S4 method for class 'GVector'
removeBridges(x)

# S4 method for class 'GVector'
removeDangles(x, tolerance = -1)

# S4 method for class 'GVector'
removeDupCentroids(x)

# S4 method for class 'GVector'
removeDups(x)

# S4 method for class 'GVector'
removeSmallPolys(x, tolerance)

# S4 method for class 'GVector'
snap(x, tolerance)

Arguments

x

A GVector.

tolerance

Numeric or NULL (default): Minimum distance in map units (degrees for unprojected, usually meters for projected) or minimum area (in meters-squared, regardless of projection).

Value

A GVector.

See also

terra::topology(), fillHoles(), terra::removeDupNodes(), Details section in fast(), simplifyGeom(), smoothGeom(), GRASS manual page for module v.clean (see grassHelp("v.clean"))

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")
)

}