Create transects across polygon zones

Builds regularly spaced straight transects through polygon features.

Usage

make_transects(
  area,
  spacing,
  angle = NULL,
  bathy = NULL,
  orientation = c("auto", "surface", "bbox", "manual"),
  orientation_weight = c("slope", "none"),
  min_slope = 0,
  length = NULL,
  id_field = NULL,
  as = c("SpatVector", "sf")
)

Arguments

area

A terra::SpatVector, local vector path, or optional sf polygon object.

spacing

Distance between transects in map units.

angle

Optional transect direction in degrees counterclockwise from the projected x axis. When supplied, this manual value overrides orientation estimation.

bathy

Optional bathymetry raster used to estimate a terrain-based transect orientation when angle = NULL.

orientation

Orientation strategy. "auto" uses surface orientation when bathy is supplied and otherwise falls back to the historical horizontal line angle with a warning. "surface" requires bathy, "bbox" uses the polygon bounding-box axis, and "manual" requires angle.

orientation_weight

Weighting for surface orientation. "slope" gives steeper cells more influence; "none" averages aspect components equally.

min_slope

Minimum slope, in degrees, used when orientation_weight = "slope".

length

Optional transect length in map units. If NULL, a length based on the polygon bounding box is used.

id_field

Optional field in area used as the zone identifier.

as

Output type: "SpatVector" or "sf".

Value

A terra::SpatVector by default. If as = "sf", an sf object is returned and the optional sf package must be installed.

Details

Transect spacing and length are interpreted in map units. Use a projected CRS for metric distances. Candidate lines are created through each polygon bounding box and clipped to the polygon with terra::intersect().

With angle = NULL and a supplied bathy raster, the default transect direction is estimated from the mean surface aspect within each polygon. Aspect is converted to northness and eastness, averaged as circular components, and converted to the mathematical line angle used for transect generation. For example, a south-facing mean aspect near 180 degrees yields a transect angle near 90 degrees, producing north-south transects in projected coordinates. The estimated angle, source metadata, and circular resultant length are stored on the output lines. Resultant lengths near zero indicate weakly concentrated aspects and an unstable mean direction.

See also

estimate_surface_orientation(), sample_transects(), extract_cross_sections()

Examples

zones <- terra::vect(blueterra_example("zones"))
bathy <- read_bathy(blueterra_example("bathy"))
transects <- make_transects(zones[1, ], spacing = 50, bathy = bathy)
transects
#> class       : SpatVector
#> geometry    : lines
#> dimensions  : 6, 15  (geometries, attributes)
#> extent      : 137487.8, 137761.1, 205591, 205891  (xmin, xmax, ymin, ymax)
#> coord. ref. : NAD83 / Puerto Rico & Virgin Is. (EPSG:32161)
#> names       : site_id        site_name    feature_type      source_name width_m height_m angle_deg zone_id   offset angle_source   (and 5 more)
#> type        :   <chr>            <chr>           <chr>            <chr>   <num>    <num>     <num>   <chr>    <num>        <chr>
#> values      :    hitw Hole-in-the-Wall sampling_recta~ Hole In the Wall     300      300    94.311       1 -124.264      surface
#>                  hitw Hole-in-the-Wall sampling_recta~ Hole In the Wall     300      300    94.311       1 -74.2641      surface
#>                  hitw Hole-in-the-Wall sampling_recta~ Hole In the Wall     300      300    94.311       1 -24.2641      surface
#>               ...

manual <- make_transects(zones[1, ], spacing = 50, angle = 90)
manual[, c("angle_deg", "angle_source")]
#> class       : SpatVector
#> geometry    : lines
#> dimensions  : 6, 2  (geometries, attributes)
#> extent      : 137499.5, 137749.5, 205591, 205891  (xmin, xmax, ymin, ymax)
#> coord. ref. : NAD83 / Puerto Rico & Virgin Is. (EPSG:32161)
#> names       : angle_deg angle_source
#> type        :     <num>        <chr>
#> values      :        90       manual
#>                      90       manual
#>                      90       manual
#>               ...