Extract contour iso-lines from a streamed raster

Traces contour lines at one or more levels from a .vec raster with marching squares, reading the raster one tile-row strip at a time (each strip expanded by one row so a cell straddling the strip boundary is traced once). Each strip contributes line segments, which are accumulated into a lazy vectra_node carrying a level column and hex-WKB geometry. With merge = TRUE the segments of each level are joined into continuous lines.

Usage

contours(x, levels, band = 1L, merge = TRUE, crs = NA, flush_rows = NULL)

Arguments

x

A vectra_raster (from vec_open_raster()) or a path to a .vec raster.

levels

Numeric vector of contour levels to trace.

band

Band to contour (1-based). Default 1.

merge

If TRUE (default) join each level's segments into continuous lines with sf::st_line_merge(); if FALSE return the raw per-cell segments.

crs

Coordinate reference system recorded on the node. Defaults to the raster's EPSG, else unknown.

flush_rows

Rows buffered before a spill flush. Defaults to getOption("vectra.spatial_flush", 5e5).

Value

A vectra_node with a level column and a hex-WKB geometry column, materialise it with collect_sf().

Details

Extraction is the sort / partition tier of the spatial toolbox: bounded to one haloed strip at a time. The optional final merge collects the segment set, which is small relative to the raster, and joins it per level; this is the small all-to-all step on the output, not on the grid. Geometry assembly and the merge are delegated to sf (an optional dependency).

See also

polygonize() for area features, terrain() for the DEM derivatives contours often accompany, collect_sf() to materialise as sf.

Examples

z <- outer(1:20, 1:20, function(r, c) r + c)
f <- tempfile(fileext = ".vec")
vec_write_raster(z, f, dtype = "f64", extent = c(0, 0, 20, 20))

iso <- contours(f, levels = c(15, 25, 35))
collect_sf(iso)
unlink(f)