import/export PROJJSON: support a interpolation_crs key to geoid_model for faithful serialization of the geoid_geog_crs parameter of proj_create_vertical_crs_ex()

3 files changed, 20 insertions, 2 deletions

diff --git a/src/iso19111/c_api.cpp b/src/iso19111/c_api.cpp
index fdfcdf7a..fd661cf8 100644
--- a/src/iso19111/c_api.cpp
+++ b/src/iso19111/c_api.cpp
@@ -3029,7 +3029,8 @@ PJ *proj_create_vertical_crs_ex(
                     ? std::dynamic_pointer_cast<CRS>(geoid_geog_crs->iso_obj)
                     : nullptr;
             const auto model(Transformation::create(
-                propsModel, vertCRSWithoutGeoid, GeographicCRS::EPSG_4979,
+                propsModel, vertCRSWithoutGeoid,
+                GeographicCRS::EPSG_4979, // arbitrarily chosen. Ignored
                 interpCRS,
                 OperationMethod::create(PropertyMap(),
                                         std::vector<OperationParameterNNPtr>()),
diff --git a/src/iso19111/crs.cpp b/src/iso19111/crs.cpp
index a55b701a..cf533bd3 100644
--- a/src/iso19111/crs.cpp
+++ b/src/iso19111/crs.cpp
@@ -2584,6 +2584,15 @@ void VerticalCRS::_exportToJSON(
         auto objectContext2(formatter->MakeObjectContext(nullptr, false));
         writer.AddObjKey("name");
         writer.Add(model->nameStr());
+
+        if (model->identifiers().empty()) {
+            const auto &interpCRS = model->interpolationCRS();
+            if (interpCRS) {
+                writer.AddObjKey("interpolation_crs");
+                interpCRS->_exportToJSON(formatter);
+            }
+        }
+
         model->formatID(formatter);
     }
 
diff --git a/src/iso19111/io.cpp b/src/iso19111/io.cpp
index a0a87f65..4d9e8633 100644
--- a/src/iso19111/io.cpp
+++ b/src/iso19111/io.cpp
@@ -5129,8 +5129,16 @@ VerticalCRSNNPtr JSONParser::buildVerticalCRS(const json &j) {
         auto propsModel = buildProperties(geoidModelJ);
         const auto dummyCRS = VerticalCRS::create(
             PropertyMap(), datum, datumEnsemble, NN_NO_CHECK(verticalCS));
+        CRSPtr interpolationCRS;
+        if (geoidModelJ.contains("interpolation_crs")) {
+            auto interpolationCRSJ =
+                getObject(geoidModelJ, "interpolation_crs");
+            interpolationCRS = buildCRS(interpolationCRSJ).as_nullable();
+        }
         const auto model(Transformation::create(
-            propsModel, dummyCRS, dummyCRS, nullptr,
+            propsModel, dummyCRS,
+            GeographicCRS::EPSG_4979, // arbitrarily chosen. Ignored,
+            interpolationCRS,
             OperationMethod::create(PropertyMap(),
                                     std::vector<OperationParameterNNPtr>()),
             {}, {}));