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.

1 files changed, 34 insertions, 27 deletions

diff --git a/src/transformations/deformation.cpp b/src/transformations/deformation.cpp
index 14f34e4e..c28e1489 100644
--- a/src/transformations/deformation.cpp
+++ b/src/transformations/deformation.cpp
@@ -65,7 +65,7 @@ PROJ_HEAD(deformation, "Kinematic grid shift");
 
 namespace { // anonymous namespace
 struct pj_opaque {
-    double t_obs;
+    double dt;
     double t_epoch;
     PJ *cart;
 };
@@ -167,23 +167,20 @@ static PJ_XYZ forward_3d(PJ_LPZ lpz, PJ *P) {
     struct pj_opaque *Q = (struct pj_opaque *) P->opaque;
     PJ_COORD out, in;
     PJ_XYZ shift;
-    double dt = 0.0;
     in.lpz = lpz;
     out = in;
 
-    if (Q->t_obs != HUGE_VAL) {
-        dt = Q->t_obs - Q->t_epoch;
-    } else {
+    if (Q->dt == HUGE_VAL) {
         out = proj_coord_error(); /* in the 3D case +t_obs must be specified */
-        proj_log_debug(P, "deformation: +t_obs must be specified");
+        proj_log_debug(P, "deformation: +dt must be specified");
         return out.xyz;
     }
 
     shift = get_grid_shift(P, in.xyz);
 
-    out.xyz.x += dt * shift.x;
-    out.xyz.y += dt * shift.y;
-    out.xyz.z += dt * shift.z;
+    out.xyz.x += Q->dt * shift.x;
+    out.xyz.y += Q->dt * shift.y;
+    out.xyz.z += Q->dt * shift.z;
 
     return out.xyz;
 }
@@ -195,10 +192,10 @@ static PJ_COORD forward_4d(PJ_COORD in, PJ *P) {
     PJ_XYZ shift;
     PJ_COORD out = in;
 
-    if (Q->t_obs != HUGE_VAL) {
-            dt = Q->t_obs - Q->t_epoch;
-        } else {
-            dt = in.xyzt.t - Q->t_epoch ;
+    if (Q->dt != HUGE_VAL) {
+        dt = Q->dt;
+    } else {
+        dt = in.xyzt.t - Q->t_epoch ;
     }
 
     shift = get_grid_shift(P, in.xyz);
@@ -215,18 +212,15 @@ static PJ_COORD forward_4d(PJ_COORD in, PJ *P) {
 static PJ_LPZ reverse_3d(PJ_XYZ in, PJ *P) {
     struct pj_opaque *Q = (struct pj_opaque *) P->opaque;
     PJ_COORD out;
-    double dt = 0.0;
     out.xyz = in;
 
-    if (Q->t_obs != HUGE_VAL) {
-        dt = Q->t_obs - Q->t_epoch;
-    } else {
+    if (Q->dt == HUGE_VAL) {
         out = proj_coord_error(); /* in the 3D case +t_obs must be specified */
-        proj_log_debug(P, "deformation: +t_obs must be specified");
+        proj_log_debug(P, "deformation: +dt must be specified");
         return out.lpz;
     }
 
-    out.xyz = reverse_shift(P, in, dt);
+    out.xyz = reverse_shift(P, in, Q->dt);
 
     return out.lpz;
 }
@@ -237,8 +231,8 @@ static PJ_COORD reverse_4d(PJ_COORD in, PJ *P) {
     double dt;
 
 
-    if (Q->t_obs != HUGE_VAL) {
-            dt = Q->t_obs - Q->t_epoch;
+    if (Q->dt != HUGE_VAL) {
+            dt = Q->dt;
         } else {
             dt = in.xyzt.t - Q->t_epoch;
     }
@@ -298,16 +292,29 @@ PJ *TRANSFORMATION(deformation,1) {
         return destructor(P, PJD_ERR_FAILED_TO_LOAD_GRID);
     }
 
-    Q->t_obs = HUGE_VAL;
-    if (pj_param(P->ctx, P->params, "tt_obs").i) {
-       Q->t_obs = pj_param(P->ctx, P->params, "dt_obs").f;
+    Q->dt = HUGE_VAL;
+    if (pj_param(P->ctx, P->params, "tdt").i) {
+       Q->dt = pj_param(P->ctx, P->params, "ddt").f;
+    }
+
+    if (pj_param_exists(P->params, "t_obs")) {
+        proj_log_error(P, "deformation: +t_obs parameter is deprecated. Use +dt instead.");
+        return destructor(P, PJD_ERR_MISSING_ARGS);
     }
 
+    Q->t_epoch = HUGE_VAL;
     if (pj_param(P->ctx, P->params, "tt_epoch").i) {
         Q->t_epoch = pj_param(P->ctx, P->params, "dt_epoch").f;
-    } else {
-       proj_log_error(P, "deformation: +t_epoch parameter missing.");
-       return destructor(P, PJD_ERR_MISSING_ARGS);
+    }
+
+    if (Q->dt == HUGE_VAL && Q->t_epoch == HUGE_VAL) {
+        proj_log_error(P, "deformation: either +dt or +t_epoch needs to be set.");
+        return destructor(P, PJD_ERR_MISSING_ARGS);
+    }
+
+    if (Q->dt != HUGE_VALL && Q->t_epoch != HUGE_VALL) {
+        proj_log_error(P, "deformation: +dt or +t_epoch are mutually exclusive.");
+        return destructor(P, PJD_ERR_MUTUALLY_EXCLUSIVE_ARGS);
     }
 
     P->fwd4d = forward_4d;