pygmt.grdproject: Raise an exception when both 'unit' and 'scaling' are given

pygmt/src/grdproject.py

@@ -120,6 +120,10 @@ def grdproject(  # noqa: PLR0913
         msg = "Parameter 'projection' must be specified."
         raise GMTInvalidInput(msg)
 
+    if kwargs.get("M", unit) is not None and kwargs.get("F", scaling) is not False:
+        msg = "Cannot use both 'unit' and 'scaling'."
+        raise GMTInvalidInput(msg)
+
     aliasdict = AliasSystem(
         C=Alias(center, name="center", sep="/", size=2),
         D=Alias(spacing, name="spacing", sep="/", size=2),

pygmt/tests/test_grdproject.py

@@ -81,6 +81,22 @@ def test_grdproject_no_outgrid(grid, projection, expected_grid):
     xr.testing.assert_allclose(a=result, b=expected_grid)
 
 
+def test_grdproject_unit_scaling(grid):
+    """
+    Test that the input validation to prevent passing both 'unit' and
+    'scaling' is performed.
+    """
+    with pytest.raises(GMTInvalidInput):
+        grdproject(
+            grid=grid,
+            projection="M10c",
+            spacing=3,
+            unit="i",
+            scaling="k",
+            region=[-53, -51, -20, -17],
+        )
+
+
 def test_grdproject_fails(grid):
     """
     Check that grdproject fails correctly.