Merge pull request #1264 from kbevers/remove-t_obs

Clean up time handling in helmert and deformation

3 files changed, 85 insertions, 40 deletions

diff --git a/docs/source/operations/transformations/deformation.rst b/docs/source/operations/transformations/deformation.rst
index 87b08453..3a9d025c 100644
--- a/docs/source/operations/transformations/deformation.rst
+++ b/docs/source/operations/transformations/deformation.rst
@@ -31,9 +31,9 @@ they represent the physical world better. In PROJ this is done with the deformat
 operation.
 
 The horizontal grid is stored in CTable2 format and the vertical grid is stored in the
-GTX format. Both grids are expected to contain grid-values in units of mm/year.
-Details about the formats can be found in the GDAL documentation. GDAL both reads and
-writes both file formats. Using GDAL for construction of new grids is recommended.
+GTX format. Both grids are expected to contain grid-values in units of
+mm/year. GDAL both reads and writes both file formats. Using GDAL for
+construction of new grids is recommended.
 
 Example
 -------------------------------------------------------------------------------
@@ -59,13 +59,14 @@ to the Danish realisation of ETRS89 is in PROJ described as::
     step    proj = deformation t_epoch = 2000.0
             xy_grids = ./nkgrf03vel_realigned_xy.ct2
             z_grids  = ./nkgrf03vel_realigned_z.gtx
+            inv
             # NKG_ETRF@2000.0 -> ETRF92@2000.0
     step    proj=helmert convention=position_vector s = -0.009420e
             x = 0.03863 rx = 0.00617753
             y = 0.147   ry = 5.064e-05
             z = 0.02776 rz = 4.729e-05
             # ETRF92@2000.0 -> ETRF92@1994.704
-    step    proj = deformation t_epoch = 1994.704 t_obs = 2000.0
+    step    proj = deformation dt = -5.296
             xy_grids = ./nkgrf03vel_realigned_xy.ct2
             z_grids  = ./nkgrf03vel_realigned_z.gtx
 
@@ -86,10 +87,6 @@ component.
 Parameters
 -------------------------------------------------------------------------------
 
-Required
-################################################################################
-
-
 .. option:: +xy_grids=<list>
 
     Comma-separated list of grids to load. If a grid is prefixed by an `@` the
@@ -110,15 +107,22 @@ Required
 
 .. option:: +t_epoch=<value>
 
-    Central epoch of transformation given in decimalyears.
+    Central epoch of transformation given in decimalyears. Will be used in
+    conjunction with the observation time from the input coordinate to
+    determine :math:`dt` as used in eq. :eq:`apply_velocity` below.
+
+    .. note:: :option:`+t_epoch` is mutually exclusive with :option:`+dt`
+
+.. option:: +dt=<value>
 
-Optional
-################################################################################
+    .. versionadded:: 6.0.0
 
-.. option:: +t_obs=<value>
+    :math:`dt` as used in eq. :eq:`apply_velocity` below. Is useful when
+    no observation time is available in the input coordinate or when
+    a deformation for a specific timespan needs to be applied in a
+    transformation. :math:`dt` is given in units of decimalyears.
 
-    Observation time of coordinate(s) given in decimalyears. If not specified,
-    the observation time from the temporal component of 4D input points is used.
+    .. note:: :option:`+dt` is mutually exclusive with :option:`+t_epoch`
 
 Mathematical description
 -------------------------------------------------------------------------------
@@ -127,7 +131,7 @@ 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 +148,7 @@ 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 +158,14 @@ 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
@@ -182,3 +186,9 @@ following way:
 where :math:`\phi` and :math:`\lambda` are the latitude and longitude of the coordinate
 that is searched for in the grid. :math:`(E, N, U)` are the grid values in ENU-space and
 :math:`(X, Y, Z)` are the corrections converted to cartesian space.
+
+
+See also
+-----------------------------------------------------------------------------
+
+#. :ref:`Behavioural changes from version 5 to 6<differences_deformation>`
\ No newline at end of file
diff --git a/docs/source/operations/transformations/helmert.rst b/docs/source/operations/transformations/helmert.rst
index 5451c3d1..51784bdb 100644
--- a/docs/source/operations/transformations/helmert.rst
+++ b/docs/source/operations/transformations/helmert.rst
@@ -41,10 +41,7 @@ The kinematic transformations require an observation time of the coordinate, as
 as a central epoch for the transformation. The latter is usually documented
 alongside the rest of the transformation parameters for a given transformation.
 The central epoch is controlled with the parameter `t_epoch`. The observation
-time can either by stated as part of the coordinate when using PROJ's
-4D-functionality or it can be controlled in the transformation setup by the
-parameter `t_obs`. When `t_obs` is specified, all transformed coordinates are
-treated as if they have the same observation time.
+time is given as part of the coordinate when using PROJ's 4D-functionality.
 
 Examples
 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
@@ -62,16 +59,16 @@ Simplified transformations from ITRF2008/IGS08 to ETRS89 using 7 parameters:
     proj=helmert convention=coordinate_frame x=0.67678    y=0.65495   z=-0.52827
                 rx=-0.022742 ry=0.012667 rz=0.022704  s=-0.01070
 
-Transformation from `ITRF2000@2017.0`  to `ITRF93@2017.0` using 15 parameters:
+Transformation from `ITRF2000`  to `ITRF93` using 15 parameters:
 
 ::
 
-    proj=helmert convention=position_vector 
+    proj=helmert convention=position_vector
          x=0.0127     y=0.0065     z=-0.0209  s=0.00195
          dx=-0.0029   dy=-0.0002   dz=-0.0006 ds=0.00001
          rx=-0.00039  ry=0.00080   rz=-0.00114
          drx=-0.00011 dry=-0.00019 drz=0.00007
-         t_epoch=1988.0 t_obs=2017.0
+         t_epoch=1988.0
 
 Parameters
 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
@@ -178,13 +175,6 @@ Parameters
     Central epoch of transformation given in decimalyear. Only used
     spatiotemporal transformations.
 
-.. option:: +t_obs=<value>
-
-    Observation time of coordinate(s) given in decimalyear. Mostly useful
-    in 2D and 3D transformations where the observation time is not passed
-    as part of the input coordinate. Can be used to override the observation
-    time from the input coordinate.
-
 .. option:: +exact
 
     Use exact transformation equations.
diff --git a/docs/source/usage/differences.rst b/docs/source/usage/differences.rst
index 5694e4d0..d5eebe8e 100644
--- a/docs/source/usage/differences.rst
+++ b/docs/source/usage/differences.rst
@@ -65,7 +65,52 @@ Version 6.0.0
 Removal of proj_def.dat
 -----------------------
 
-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,
+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>`
+------------------------------------------------------------------
+.. _differences_deformation:
+
+
+Reversed order of operation
+...........................
+
+In the initial version of the of :ref:`deformation<deformation>` operation
+the time span between :math:`t_{obs}` and :math:`t_c` was determined 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.
+
+Removed ``+t_obs``  parameter
+.............................
+
+The ``+t_obs`` parameter was confusing for users since it effectively
+overwrote the observation time in input coordinates. To make it more clear
+what is the operation is doing, users are now required to directly specify
+the time span for which they wish to apply a given deformation. The parameter
+:option:`+dt` has been added for that purpose. The new parameter is mutually
+exclusive with :option:`+t_epoch`. :option:`+dt` is used when deformation
+for a set amount of time is needed and :option:`+t_epoch` is used (in
+conjunction with the observation time of the input coordinate) when
+deformation from a specific epoch to the observation time is needed.
\ No newline at end of file