Seafloor profile along a transect, sampled at a regular interval

Samples the seafloor between the supplied positions, not only at them.

Usage

cc_transect_bathy(
  lon,
  lat,
  dist_km = NULL,
  interval_m = 500,
  bathy = cc_bathy(),
  land_depth_m = 0
)

Arguments

lon, lat

positions to sample between, in order along the transect.

dist_km

optional ruler value at each position — the section's own x-axis. Interpolated linearly within each leg, so the profile lands on the caller's axis and is anchored exactly at every station. Defaults to cumulative great-circle distance from the first position.

interval_m

spacing along the track, metres (default 500).

bathy

raster from cc_bathy().

land_depth_m

at or below this depth a sample is reported on_land (default 0, the raster's land clamp).

Value

Tibble of dist_km, lon, lat, depth_m, on_land, one row per sample, ordered along the transect. Positions outside the raster carry NA depth; the caller decides whether that is a gap or an error.

Why the interval matters

Sampling at stations alone and joining those soundings with straight lines does not simplify the terrain, it invents different terrain. Line 86.7 is the case: station 50 sits on a Channel Islands bank at 80 m between neighbours at 1,654 m and 1,190 m that are 37 km away on either side, so station-only sampling drew that bank as one triangle 74 km wide rising 1.5 km off the seafloor — at the exact depths where someone is reading the thermocline.

Too coarse an interval fails the same way, more quietly. At 2 km, Fortymile Bank on line 93.3 is four soundings (385, 344, 238, 370 m) and draws as a spike; at 500 m it is what it really is, a ~14 km rise from 652 m to a 178 m crest with flanks on both sides. The default sits just above GEBCO's ~390 m cell: fine enough to keep every cell the track crosses, coarse enough not to imply detail the grid does not have.

Crossing land

A cruise track — as opposed to a CalCOFI line — zigzags, so a leg between two consecutive casts can cross an island. Those samples come back at depth 0 with on_land = TRUE rather than being dropped or smoothed away: the track really did cross land, and a consumer that draws a filled silhouette should break the polygon there instead of drawing a wall to the surface.