Improve agreement with GEMPAK

* Adjust to change to radians at the last possible time. At least in the
GFS case, this keeps the data exactly representable in floating point
(e.g. 0.5, 0.25 spacing), which lets the math work better, or at least
provide uniform output spacing given uniform input.

* Working in degrees makes everything consistent with our handling of
forward_az in the aftermath of the function. If radians=True, then
forward_az comes out in radians.

src/metpy/calc/tools.py

@@ -868,21 +868,18 @@ def nominal_lat_lon_grid_deltas(longitude, latitude, geod=None):
     else:
         g = geod
 
+    print(g.a)
     if longitude.ndim != 1 or latitude.ndim != 1:
         raise ValueError(
             'Cannot calculate nominal grid spacing from longitude and latitude arguments '
             'that are not one dimensional.'
         )
 
-    dx = units.Quantity(
-        geod.a * np.diff(longitude.m_as('radian')),
-        'meter'
-    )
-    lat = latitude.m_as('radian')
-    lon_meridian_diff = np.zeros(len(lat) - 1)
-    forward_az, _, dy = geod.inv(
-        lon_meridian_diff, lat[:-1], lon_meridian_diff, lat[1:], radians=True
-    )
+    dx = units.Quantity(g.a * np.diff(longitude).m_as('radian'), 'meter')
+    lat = latitude.m_as('degree')
+    lon_meridian_diff = np.zeros(len(lat) - 1, dtype=lat.dtype)
+    forward_az, _, dy = g.inv(lon_meridian_diff, lat[:-1], lon_meridian_diff, lat[1:],
+                              radians=False)
     dy[(forward_az < -90.) | (forward_az > 90.)] *= -1
     dy = units.Quantity(dy, 'meter')
 
@@ -1028,7 +1025,7 @@ def wrapper(f, **kwargs):
 
 def parse_grid_arguments(func):
     """Parse arguments to functions involving derivatives on a grid.
-    
+
     TODO: use this to completely replace add_grid_arguments_from_xarray
     """
     # u, v, *, dx=None, dy=None, x_dim=-1, y_dim=-2, longitude=None, latitude=None, crs=None,
@@ -1148,7 +1145,7 @@ def wrapper(*args, **kwargs):
                 except:
                     # Fall back to cartesian calculation if CRS is not provided or invalid
                     cartesian = True
-                
+
                 lat = bound_args.arguments['latitude']
                 lon = bound_args.arguments['longitude']
 
@@ -1524,7 +1521,7 @@ def _vector_derivative(
 ):
     # Determine which derivatives to calculate
     derivatives = {
-        component: None 
+        component: None
         for component in ('du/dx', 'du/dy', 'dv/dx', 'dv/dy')
         if (return_only is None or component in return_only)
     }