Convert isolines or isobands to an sf geometry collection (sfg) object. Further downstream processing needs to happen via the sf package.

## Usage

iso_to_sfg(x)

## Arguments

x

The object to convert.

## Details

The function iso_to_sfg() is a generic that takes an object created by either isolines() or isobands() and turns it into a simple features (sf) geometry collection. Importantly, the isobanding algorithm can produce polygons that do not represent valid simple features. This happens usually when the lower limit of an isoband is exactly equal to some data values (see examples for a demonstration). This can be worked around either by slightly shifting the data or band limits (e.g., round all data values and then shift them by a value smaller than the rounding error) or by fixing the geometries using the function st_make_valid().

## Examples

if (requireNamespace("sf", quietly = TRUE)) {
library(sf)
library(ggplot2)

# Example 1: simple 5x5 matrix
m <- matrix(c(0, 2, 2, 2, 0,
0, 1, 0, 1, 0,
0, 1, 0, 0, 0,
0, 1, 0, 1, 0,
0, 0, 0, 0, 0), 5, 5, byrow = TRUE)

z <- isolines(1:ncol(m), nrow(m):1, m, c(0.5, 1.5))
lines <- iso_to_sfg(z)
x <- st_sf(level = names(lines), geometry = st_sfc(lines))
ggplot(x) + geom_sf(aes(color = level))

# Example 2: volcano dataset
m <- volcano
b <- isobands((1:ncol(m))/(ncol(m)+1), (nrow(m):1)/(nrow(m)+1), m,
10*9:19, 10*10:20)
bands <- iso_to_sfg(b)
x <- st_sf(level = as.numeric(sub(":.*", "", names(bands))), geometry = st_sfc(bands))
ggplot(x) + geom_sf(aes(color = level, fill = level))

# Example 3: invalid simple features
m <- matrix(c(1.5, 1.5, 1.5, 1.5, 0.6,
0.5, 1.5, 1.5,   0,   0,
0,   1,   0,   1,   1,
0,   1,   0, 0.7,   0,
0.9, 1.3, 1.8, 1.4, 0.4), 5, 5, byrow = TRUE)

raw <- isobands(1:5, 5:1, m, levels_low = 0:1, levels_high = 1:2)
bands <- iso_to_sfg(raw)

iso <- st_sf(
id = factor(1:length(bands)),
geometry = st_sfc(bands)
)

# the geometries are not valid
st_is_valid(iso, reason = TRUE)
# this doesn't prevent us from plotting them
ggplot(iso, aes(fill = id)) + geom_sf()

# make all geometries valid, requires GEOS >= 3.8.0
if (sf_extSoftVersion()["GEOS"] >= "3.8.0") {
iso2 <- st_make_valid(iso)
st_is_valid(iso2, reason=TRUE)
# the plot should be unchanged
ggplot(iso2, aes(fill = id)) + geom_sf()
}

# alternatively, if we shift all data values by a tiny
# amount (here, 1e-10) so they don't coincide with the band
# limits, no invalid geometries are generated.
raw <- isobands(1:5, 5:1, m + 1e-10, levels_low = 0:1, levels_high = 1:2)
bands <- iso_to_sfg(raw)
iso <- st_sf(id = factor(1:length(bands)), geometry = st_sfc(bands))
st_is_valid(iso, reason = TRUE)
}
#> Linking to GEOS 3.10.2, GDAL 3.4.1, PROJ 8.2.1; sf_use_s2() is TRUE
#> [1] "Valid Geometry" "Valid Geometry"