From 29a59cadb9d1371479477af517f3431784e880f9 Mon Sep 17 00:00:00 2001
From: Kristian Evers <kristianevers@gmail.com>
Date: Thu, 14 Feb 2019 12:00:45 +0100
Subject: Reverse direction of deformation
 operations/transformations/deformation

Changed the direction of dt-calculation to follow the same convention as
helmert. Changed from dt = t_c - t_obs to dt = t_obs - t_c, which
effectively reverses the direction of the operation. Existing projstrings
using deformation can simply reverse the direction of the operation to
achieve the same results as before this commit.
---
 .../operations/transformations/deformation.rst     | 20 ++++++++++----------
 docs/source/usage/differences.rst                  | 22 ++++++++++++++++++++++
 2 files changed, 32 insertions(+), 10 deletions(-)

(limited to 'docs/source')

diff --git a/docs/source/operations/transformations/deformation.rst b/docs/source/operations/transformations/deformation.rst
index 87b08453..0a772547 100644
--- a/docs/source/operations/transformations/deformation.rst
+++ b/docs/source/operations/transformations/deformation.rst
@@ -127,7 +127,7 @@ Mathematically speaking, application of a deformation model is simple. The defor
 represented as a grid of velocities in three dimensions. Coordinate corrections are
 applied in cartesian space. For a given coordinate, :math:`(X, Y, Z)`, velocities
 :math:`(V_X, V_Y, V_Z)` can be interpolated from the gridded model. The time span
-between :math:`t_c` and :math:`t_{obs}` determine the magnitude of the coordinate
+between :math:`t_{obs}` and :math:`t_c` determine the magnitude of the coordinate
 correcton as seen in eq. :eq:`apply_velocity` below.
 
 .. math::
@@ -144,7 +144,7 @@ correcton as seen in eq. :eq:`apply_velocity` below.
             Y \\
             Z \\
         \end{pmatrix}_A +
-        (t_c - t_{obs})
+        (t_{obs} - t_c)
         \begin{pmatrix}
             V_X \\
             V_Y \\
@@ -154,14 +154,14 @@ correcton as seen in eq. :eq:`apply_velocity` below.
 
 Corrections are done in cartesian space.
 
-Coordinates of the gridded model are in ENU (east, north, up) space because it would
-otherwise require an enormous 3 dimensional grid to handle the corrections in cartesian
-space. Keeping the correction in lat/long space reduces the complexity of the grid
-significantly. Consequently though, the input coordinates needs to be converted to
-lat/long space when searching for corrections in the grid. This is done with the *cart*
-operation. The converted grid corrections can then be applied to the input coordinates
-in cartesian space. The conversion from ENU space to cartesian space is done in the
-following way:
+Coordinates of the gridded model are in ENU (east, north, up) space because it
+would otherwise require an enormous 3 dimensional grid to handle the corrections
+in cartesian space. Keeping the correction in lat/long space reduces the
+complexity of the grid significantly. Consequently though, the input coordinates
+needs to be converted to lat/long space when searching for corrections in the
+grid. This is done with the :ref:`cart<cart>` operation. The converted grid
+corrections can then be applied to the input coordinates in cartesian space. The
+conversion from ENU space to cartesian space is done in the following way:
 
 .. math::
     :label: enu2xyz
diff --git a/docs/source/usage/differences.rst b/docs/source/usage/differences.rst
index 5694e4d0..caab865c 100644
--- a/docs/source/usage/differences.rst
+++ b/docs/source/usage/differences.rst
@@ -69,3 +69,25 @@ Before PROJ 6, the proj_def.dat was used to provide general and per-projection
 parameters, when +no_defs was not specified. It has now been removed. In case,
 no ellipsoid or datum specification is provided in the PROJ string, the
 default ellipsoid is GRS80 (was WGS84 in previous PROJ versions).
+
+Changes to :ref:`deformation<deformation>`
+------------------------------------------------------------------
+
+In the initial version of the of :ref:`deformation<deformation>` operation the time span between :math:`t_{obs}` and :math:`t_c` was determine by the expression
+
+.. math::
+
+    dt = t_c - t_{obs}
+
+With version 6.0.0 this has been reversed in order to behave similarly to
+the :ref:`Helmert operation<helmert>`, which determines time differences as
+
+.. math::
+
+    dt = t_{obs} - t_c
+
+Effectively this means that the direction of the operation has been reversed,
+so that what in PROJ 5 was a forward operation is now an inverse operation and
+vice versa.
+
+Pipelines written for PROJ 5 can be migrated to PROJ 6 by adding :option:`+inv` to forward steps involving the deformation operation. Similarly :option:`+inv` should be removed from inverse steps to be compatible with PROJ 6.
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