{
  "name": "z_at",
  "type": "function",
  "groups": ["GeometryGroup"],
  "description": "Retrieves a z coordinate of the geometry, or NULL if the geometry has no z value.",
  "arguments": [{
    "arg": "geometry",
    "description": "geometry object"
  }, {
    "arg": "vertex",
    "description": "index of the vertex of the geometry (indices start at 0; negative values apply from the last index, starting at -1)"
  }],
  "examples": [{
    "expression": "z_at(geom_from_wkt('LineStringZ(0 0 0, 10 10 5, 10 10 0)'), 1)",
    "returns": "5"
  }],
  "tags": ["retrieves", "coordinate", "3D"]
}