Stabilize derivatives in fatiando.gravmag

cookbook/gravmag_transform_deriv.py

@@ -33,40 +33,40 @@
 mpl.subplot(2, 3, 1)
 mpl.title("x deriv (contour) + true (color map)")
 mpl.axis('scaled')
-levels = mpl.contourf(xp, yp, gxz_true, shape, 12)
+levels = mpl.contourf(yp, xp, gxz_true, shape, 12)
 mpl.colorbar(shrink=0.7)
-mpl.contour(xp, yp, gxz, shape, 12, color='k')
+mpl.contour(yp, xp, gxz, shape, 12, color='k')
 mpl.m2km()
 mpl.subplot(2, 3, 2)
 mpl.title("y deriv (contour) + true (color map)")
 mpl.axis('scaled')
-levels = mpl.contourf(xp, yp, gyz_true, shape, 12)
+levels = mpl.contourf(yp, xp, gyz_true, shape, 12)
 mpl.colorbar(shrink=0.7)
-mpl.contour(xp, yp, gyz, shape, 12, color='k')
+mpl.contour(yp, xp, gyz, shape, 12, color='k')
 mpl.m2km()
 mpl.subplot(2, 3, 3)
 mpl.title("z deriv (contour) + true (color map)")
 mpl.axis('scaled')
-levels = mpl.contourf(xp, yp, gzz_true, shape, 8)
+levels = mpl.contourf(yp, xp, gzz_true, shape, 8)
 mpl.colorbar(shrink=0.7)
-mpl.contour(xp, yp, gzz, shape, levels, color='k')
+mpl.contour(yp, xp, gzz, shape, levels, color='k')
 mpl.m2km()
 mpl.subplot(2, 3, 4)
 mpl.title("Difference x deriv")
 mpl.axis('scaled')
-mpl.pcolor(xp, yp, (gxz_true - gxz), shape)
+mpl.pcolor(yp, xp, (gxz_true - gxz), shape)
 mpl.colorbar(shrink=0.7)
 mpl.m2km()
 mpl.subplot(2, 3, 5)
 mpl.title("Difference y deriv")
 mpl.axis('scaled')
-mpl.pcolor(xp, yp, (gyz_true - gyz), shape)
+mpl.pcolor(yp, xp, (gyz_true - gyz), shape)
 mpl.colorbar(shrink=0.7)
 mpl.m2km()
 mpl.subplot(2, 3, 6)
 mpl.title("Difference z deriv")
 mpl.axis('scaled')
-mpl.pcolor(xp, yp, (gzz_true - gzz), shape)
+mpl.pcolor(yp, xp, (gzz_true - gzz), shape)
 mpl.colorbar(shrink=0.7)
 mpl.m2km()
 mpl.show()

cookbook/grid_surfer.py

@@ -8,8 +8,8 @@
 # Will download the archive and save it with the default name
 archive = datasets.fetch_bouguer_alps_egm()
 
-# Load the GRD file and convert in three numpy-arrays (y, x, bouguer)
-y, x, bouguer, shape = gridder.load_surfer(archive, fmt='ascii')
+# Load the GRD file and convert in three numpy-arrays (x, y, bouguer)
+x, y, bouguer, shape = gridder.load_surfer(archive, fmt='ascii')
 
 mpl.figure()
 mpl.axis('scaled')

doc/api/gravmag.transform.rst

@@ -1,6 +1,6 @@
 .. _fatiando_gravmag_transform:
 
-Space domain processing (``fatiando.gravmag.transform``)
+Potential field transformations (``fatiando.gravmag.transform``)
 ============================================================================
 
 .. automodule:: fatiando.gravmag.transform

doc/changelog.rst

@@ -10,6 +10,20 @@ Version (development)
 
 **Changes**:
 
+* **IMPORTANT BUG FIX**: ``fatiando,gridder.regular`` and many other places in
+  Fatiando where using the wrong convention for x, y dimensions.
+  x should point North and y East. Thus, a data matrix (regular grid) should
+  have x varying in the lines and y varying in the columns. This is **oposite**
+  what we had. This fix also changes the ``shape`` argument to be ``(nx, ny)``
+  instead of ``(ny, nx)``. **Users should be aware of this and double check
+  their code.**
+  (`PR 196 <https://github.com/fatiando/fatiando/pull/196>`__)
+* More stable derivatives in ``fatiando.gravamag.transform``. The horizontal
+  derivatives default to central finite-differences for greater stability. The
+  FFT based derivatives use a grid padding to avoid edge effects.
+  Thanks to `Matteo Niccoli <https://mycarta.wordpress.com/>`__ for suggesting
+  this fix.
+  (`PR 196 <https://github.com/fatiando/fatiando/pull/196>`__)
 * **Renamed** ``fatiando.gravmag.fourier.ansig`` to
   ``fatiando.gravmag.transform.tga``
   (`PR 186 <https://github.com/fatiando/fatiando/pull/186>`__)

fatiando/gravmag/__init__.py

@@ -43,8 +43,8 @@
 * :mod:`~fatiando.gravmag.normal_gravity`: Compute normal gravity and
   reductions.
 * :mod:`~fatiando.gravmag.eqlayer`: Equivalent layer processing
-* :mod:`~fatiando.gravmag.transform`: Space domain potential field
-  transformations, like upward continuation
+* :mod:`~fatiando.gravmag.transform`: Potential field transformations,
+  like upward continuation, derivatives, etc
 * :mod:`~fatiando.gravmag.imaging`: Imaging methods for potential fields for
   estimating physical property distributions
 * :mod:`~fatiando.gravmag.tensor`: Utilities for operating on the gradient