deformation: Replace +t_obs with +dt

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
+dt has been added for that purpose. The new parameter is mutually
exclusive with +t_epoch. +dt is used when deformation
for a set amount of time is needed and +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.

2 files changed, 49 insertions, 16 deletions

diff --git a/docs/source/operations/transformations/deformation.rst b/docs/source/operations/transformations/deformation.rst
index 0a772547..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
 -------------------------------------------------------------------------------
@@ -182,3 +186,9 @@ conversion from ENU space to cartesian space is done in the 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/usage/differences.rst b/docs/source/usage/differences.rst
index b7c79f53..d5eebe8e 100644
--- a/docs/source/usage/differences.rst
+++ b/docs/source/usage/differences.rst
@@ -72,8 +72,15 @@ default ellipsoid is GRS80 (was WGS84 in previous PROJ versions).
 
 Changes to :ref:`deformation<deformation>`
 ------------------------------------------------------------------
+.. _differences_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
+
+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::
 
@@ -90,4 +97,20 @@ 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.
+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