Spatial derivatives are only defined on a grid, so the points have to be put back onto one. Gridded ocean products come as a regular lon/lat lattice, and datamatch's access functions flatten that lattice to points without moving them, so the grid can be recovered exactly from the unique coordinates. Copernicus, HYCOM, CCMP and most ERDDAP grids are of that kind.
Value
a terra::SpatRaster with one layer per variable
Details
An unstructured mesh is not. datamatch::accessFVCOM() returns one row per
mesh node, and those nodes are irregularly spaced by design, which is the
point of a mesh: resolution follows the coastline rather than a lattice.
Irregular points are rejected rather than interpolated, whether they come
from a mesh or from scattered observations. Silently gridding them would
produce a gradient field that looks plausible and is mostly interpolation
artifact. The derivations that need no lattice – every temporal one, plus
distance_to_shore() and box_anomaly() – work on a mesh unchanged.