Grid2Polygons: Convert Spatial Grids to Polygons

Description

This function converts sp spatial objects from class 'SpatialGridDataFrame' to 'SpatialPolygonsDataFrame'. Spatial polygons can then be transformed to a different projection or datum with spTransform in package rgdal. Image files created with spatial polygons are reduced in size and result in a much "cleaner" version of your image.

Usage

Grid2Polygons(grd, zcol = 1, level = FALSE, at, cuts = 20,
  pretty = FALSE, xlim = NULL, ylim = NULL, ply = NULL)

Arguments

grd

'SpatialGridDataFrame'. Spatial grid data frame

zcol

'character' or 'integer'. Attribute name or column number in attribute table.

level

'logical'. If true, a set of levels is used to partition the range of z, its default is false.

'numeric'. A vector giving breakpoints along the range of z.

cuts

'integer'. Number of levels the range of z would be divided into.

pretty

'logical'. Whether to use pretty cut locations.

xlim

'numeric'. Vector of length 2 giving left and right limits of the spatial grid, data outside these limits is excluded.

ylim

'numeric'. Vector of length 2 giving lower and upper limits of the spatial grid, data outside these limits is excluded.

ply

'SpatialPolygons', or 'SpatialGridDataFrame'. Cropping polygon

Value

Returns an object of 'SpatialPolygonsDataFrame'. The objects data slot is a data frame, number of rows equal to the number of Polygons objects and a single column containing values of z. If level is true, z values are set equal to the midpoint between breakpoints. The status of the polygon as a hole or an island is taken from the ring direction, with clockwise meaning island, and counter-clockwise meaning hole.

References

A general explanation of the algorithm provided here; inspiration provided here.

Examples

Run this code

# NOT RUN {
# Example 1

z <- c(1.1,  1.5,  4.2,  4.1,  4.3,  4.7,
       1.2,  1.4,  4.8,  4.8,   NA,  4.1,
       1.7,  4.2,  1.4,  4.8,  4.0,  4.4,
       1.1,  1.3,  1.2,  4.8,  1.6,   NA,
       3.3,  2.9,   NA,  4.1,  1.0,  4.0)
m <- 5
n <- 6
x <- rep(0:n, m + 1)
y <- rep(0:m, each = n + 1)
xc <- c(rep(seq(0.5, n - 0.5, by = 1), m))
yc <- rep(rev(seq(0.5, m - 0.5, by = 1)), each = n)
grd <- data.frame(z = z, xc = xc, yc = yc)
sp::coordinates(grd) <- ~ xc + yc
sp::gridded(grd) <- TRUE
grd <- as(grd, "SpatialGridDataFrame")
image(grd, col = gray.colors(30), axes = TRUE)
grid(col = "black", lty = 1)
points(x = x, y = y, pch = 16)
text(cbind(xc, yc), labels = z)
text(cbind(x = x + 0.1, y = rev(y + 0.1)), labels = 1:((m + 1) * (n + 1)), cex = 0.6)

at <- 1:ceiling(max(z, na.rm = TRUE))
plys <- Grid2Polygons(grd, level = TRUE, at = at)
cols <- rainbow(length(plys), alpha = 0.3)
sp::plot(plys, add = TRUE, col = cols)
zz <- plys[[1]]
legend("top", legend = zz, fill = cols, bty = "n", xpd = TRUE,
       inset = c(0, -0.1), ncol = length(plys))

p1 <- sp::Polygon(rbind(c(1.2, 0.5), c(5.8, 1.7), c(2.5, 5.1), c(1.2, 0.5)),
                  hole = FALSE)
p2 <- sp::Polygon(rbind(c(2.5, 2.5), c(3.4, 1.8), c(3.7, 3.1), c(2.5, 2.5)),
                  hole = TRUE)
p3 <- sp::Polygon(rbind(c(-0.3, 3.3), c(1.7, 5.1), c(-1.0, 7.0), c(-0.3, 3.3)),
                  hole = FALSE)
p <- sp::SpatialPolygons(list(sp::Polygons(list(p1, p2, p3), 1)))
plys <- Grid2Polygons(grd, level = TRUE, at = at, ply = p)
cols <- rainbow(length(zz), alpha = 0.6)[zz %in% plys[[1]]]
sp::plot(plys, col = cols, add = TRUE)

# Example 2

data(meuse.grid, package = "sp")
sp::coordinates(meuse.grid) <- ~ x + y
sp::gridded(meuse.grid) <- TRUE
meuse.grid <- as(meuse.grid, "SpatialGridDataFrame")
meuse.plys <- Grid2Polygons(meuse.grid, "dist", level = FALSE)
op <- par(mfrow = c(1, 2), oma = rep(0, 4), mar = rep(0, 4))
sp::plot(meuse.plys, col = heat.colors(length(meuse.plys)))
title("level = FALSE", line = -7)

meuse.plys.lev <- Grid2Polygons(meuse.grid, "dist", level = TRUE)
sp::plot(meuse.plys.lev, col = heat.colors(length(meuse.plys.lev)))
title("level = TRUE", line = -7)
par(op)

# }