iso_to_sfg {isoband} | R Documentation

## Convert isolines or isobands to sfg object

### Description

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 `lwgeom::st_make_valid()`.

### Examples

    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
    iso2 <- lwgeom::st_make_valid(iso)
    st_is_valid(iso2, reason=TRUE)
    # the plot looks unchanged (this is not always the case)
    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)

[Package isoband version 0.2.0 Index]