Automatically detect grid registration from xarray data source

For xarray grids read from disk, there is an 'encoding' dictionary, with a 'source' key that gives the path to the file. Running `grdinfo` on that file can give us the grid registration. This works on the earth_relief grids. For grids that are not read from disk, we still default to assuming gridline registration ("GMT_GRID_NODE_REG").

pygmt/base_plotting.py

@@ -18,6 +18,7 @@
     use_alias,
     kwargs_to_strings,
 )
+from .modules import grdinfo
 
 
 class BasePlotting:
@@ -56,6 +57,22 @@ def _preprocess(self, **kwargs):  # pylint: disable=no-self-use
         """
         return kwargs
 
+    def autodetect_registration(self, grid):
+        """
+        Function to automatically detect whether the NetCDF source of an
+        xarray.DataArray grid uses gridline or pixel registration. Defaults to
+        gridline registration if grdinfo cannot find a source file.
+        """
+        registration = "GMT_GRID_NODE_REG"  # default to gridline registration
+
+        try:
+            if "Pixel node registration used" in grdinfo(grid.encoding["source"]):
+                registration = "GMT_GRID_PIXEL_REG"
+        except KeyError:
+            pass
+
+        return registration
+
     @fmt_docstring
     @use_alias(
         R="region",
@@ -282,7 +299,8 @@ def grdcontour(self, grid, **kwargs):
             if kind == "file":
                 file_context = dummy_context(grid)
             elif kind == "grid":
-                file_context = lib.virtualfile_from_grid(grid)
+                registration = self.autodetect_registration(grid)
+                file_context = lib.virtualfile_from_grid(grid, registration)
             else:
                 raise GMTInvalidInput("Unrecognized data type: {}".format(type(grid)))
             with file_context as fname:
@@ -314,7 +332,8 @@ def grdimage(self, grid, **kwargs):
             if kind == "file":
                 file_context = dummy_context(grid)
             elif kind == "grid":
-                file_context = lib.virtualfile_from_grid(grid)
+                registration = self.autodetect_registration(grid)
+                file_context = lib.virtualfile_from_grid(grid, registration)
             else:
                 raise GMTInvalidInput("Unrecognized data type: {}".format(type(grid)))
             with file_context as fname:
@@ -410,7 +429,8 @@ def grdview(self, grid, **kwargs):
             if kind == "file":
                 file_context = dummy_context(grid)
             elif kind == "grid":
-                file_context = lib.virtualfile_from_grid(grid)
+                registration = self.autodetect_registration(grid)
+                file_context = lib.virtualfile_from_grid(grid, registration)
             else:
                 raise GMTInvalidInput(f"Unrecognized data type for grid: {type(grid)}")
 
@@ -420,7 +440,10 @@ def grdview(self, grid, **kwargs):
                     drapegrid = kwargs["G"]
                     if data_kind(drapegrid) in ("file", "grid"):
                         if data_kind(drapegrid) == "grid":
-                            drape_context = lib.virtualfile_from_grid(drapegrid)
+                            registration = self.autodetect_registration(grid)
+                            drape_context = lib.virtualfile_from_grid(
+                                drapegrid, registration
+                            )
                             drapefile = stack.enter_context(drape_context)
                             kwargs["G"] = drapefile
                     else:

pygmt/clib/conversion.py

@@ -7,7 +7,7 @@
 from ..exceptions import GMTInvalidInput
 
 
-def dataarray_to_matrix(grid):
+def dataarray_to_matrix(grid, registration="GMT_GRID_NODE_REG"):
     """
     Transform an xarray.DataArray into a data 2D array and metadata.
 
@@ -27,6 +27,9 @@ def dataarray_to_matrix(grid):
     grid : xarray.DataArray
         The input grid as a DataArray instance. Information is retrieved from
         the coordinate arrays, not from headers.
+    registration : str
+        Either one of "GMT_GRID_PIXEL_REG" for pixel registration, or
+        "GMT_GRID_NODE_REG" for gridline registration [Default].
 
     Returns
     -------
@@ -102,7 +105,10 @@ def dataarray_to_matrix(grid):
                     dim
                 )
             )
-        region.extend([coord.min(), coord.max()])
+        if registration == "GMT_GRID_PIXEL_REG":
+            region.extend([coord.min() - coord_inc / 2, coord.max() + coord_inc / 2])
+        elif registration == "GMT_GRID_NODE_REG":
+            region.extend([coord.min(), coord.max()])
         inc.append(coord_inc)
 
     if any([i < 0 for i in inc]):  # Sort grid when there are negative increments

pygmt/clib/session.py

@@ -1238,7 +1238,7 @@ def virtualfile_from_grid(self, grid, registration="GMT_GRID_NODE_REG"):
         # collected and the memory freed. Creating it in this context manager
         # guarantees that the copy will be around until the virtual file is
         # closed. The conversion is implicit in dataarray_to_matrix.
-        matrix, region, inc = dataarray_to_matrix(grid)
+        matrix, region, inc = dataarray_to_matrix(grid, registration)
         family = "GMT_IS_GRID|GMT_VIA_MATRIX"
         geometry = "GMT_IS_SURFACE"
         gmt_grid = self.create_data(