Grid2Polygons: Convert Spatial Grids to Polygons
In inlmisc: Miscellaneous Functions for the USGS INL Project Office

Description Usage Arguments Value Note Author(s) See Also Examples

Convert gridded spatial data to spatial polygons. Image files created with spatial polygons are reduced in size, can easily be transformed from one coordinate reference system to another, and result in much "cleaner" images when plotted.

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

`grd`	'SpatialGridDataFrame', 'SpatialPixelsDataFrame', or 'Raster*'. Spatial grid
`zcol`	'character' string or 'integer' count. Layer to extract from a multi-layer spatial grid.
`level`	'logical' flag. If true, a set of levels is used to partition the range of attribute values, its default is false.
`at`	'numeric' vector. Breakpoints along the range of attribute values.
`cuts`	'integer' count. Number of levels the range of attribute values would be divided into.
`pretty`	'logical' flag. Whether to use pretty cut locations.
`xlim`	'numeric' vector of length 2. Left and right limits of the spatial grid, data outside these limits is excluded.
`ylim`	'numeric' vector of length 2. Lower and upper limits of the spatial grid, data outside these limits is excluded.
`zlim`	'numeric' vector of length 2. Minimum and maximum limits of the attribute variable, data outside these limits is excluded.
`ply`	'SpatialPolygons', or 'SpatialGridDataFrame'. Cropping polygon
`check_validity`	'logical' flag. If true (default), check the validity of polygons. If any of the polygons are invalid, try making them valid by zero-width buffering.

An object of class 'SpatialPolygonsDataFrame'. The objects data slot is a data frame, number of rows equal to the number of Polygons objects and a single column containing attribute values. If level is true, attribute 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.

The traditional R graphics model does not draw polygon holes correctly, holes overpaint their containing 'Polygon' object using a user defined background color (white by default). Polygon holes are now rendered correctly using the plot method for spatial polygons (SpatialPolygons-class), see polypath for more details. The Trellis graphics model appears to rely on the traditional method so use caution when plotting with spplot.

J.C. Fisher, U.S. Geological Survey, Idaho Water Science Center

As an alternative, consider using the rasterToPolygons function in the raster package, setting dissolve = TRUE.

# 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, stringsAsFactors = TRUE)
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 <- GetColors(length(plys), scheme = "bright", 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))

v1 <- rbind(c( 1.2, 0.5), c(5.8, 1.7), c( 2.5, 5.1), c( 1.2, 0.5))
v2 <- rbind(c( 2.5, 2.5), c(3.4, 1.8), c( 3.7, 3.1), c( 2.5, 2.5))
v3 <- rbind(c(-0.3, 3.3), c(1.7, 5.1), c(-1.0, 7.0), c(-0.3, 3.3))
p1 <- sp::Polygon(v1, hole = FALSE)
p2 <- sp::Polygon(v2, hole = TRUE)
p3 <- sp::Polygon(v3, hole = FALSE)
p <- sp::SpatialPolygons(list(sp::Polygons(list(p1, p2, p3), 1)))
plys <- Grid2Polygons(grd, level = TRUE, at = at, ply = p)
cols <- GetColors(length(zz), scheme = "bright", alpha = 0.6)
cols <- cols[zz %in% plys[[1]]]
sp::plot(plys, col = cols, add = TRUE)
text(cbind(xc, yc), labels = z)

# 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)

# Example 3
m <- datasets::volcano
m <- m[nrow(m):1, ncol(m):1]
x <- seq(from = 2667405, length.out = ncol(m), by = 10)
y <- seq(from = 6478705, length.out = nrow(m), by = 10)
r <- raster::raster(m, xmn = min(x), xmx = max(x), ymn = min(y),
                    ymx = max(y), crs = "+init=epsg:27200")
plys <- Grid2Polygons(r, level = TRUE)
cols <- GetColors(length(plys), scheme = "DEM screen")
sp::plot(plys, col = cols, border = "#515151")