WKT1 import: correctly deal with missing rectified_grid_angle parameter

by setting its value from the azimuth angle.

and on export to PROJ.4 string do not emit a erroneous +gamma=0 when the
parameter it is missing.

Fixes https://lists.osgeo.org/pipermail/proj/2021-December/010475.html

2 files changed, 52 insertions, 0 deletions

diff --git a/src/iso19111/io.cpp b/src/iso19111/io.cpp
index 4dcceba2..67877206 100644
--- a/src/iso19111/io.cpp
+++ b/src/iso19111/io.cpp
@@ -3968,6 +3968,48 @@ ConversionNNPtr WKTParser::Private::buildProjectionStandard(
         }
     }
 
+    if (mapping && (mapping->epsg_code ==
+                        EPSG_CODE_METHOD_HOTINE_OBLIQUE_MERCATOR_VARIANT_A ||
+                    mapping->epsg_code ==
+                        EPSG_CODE_METHOD_HOTINE_OBLIQUE_MERCATOR_VARIANT_B)) {
+        // Special case when importing some GDAL WKT of Hotine Oblique Mercator
+        // that have a Azimuth parameter but lacks the Rectified Grid Angle.
+        // We have code in the exportToPROJString() to deal with that situation,
+        // but also adds the rectified grid angle from the azimuth on import.
+        bool foundAngleRecifiedToSkewGrid = false;
+        bool foundAzimuth = false;
+        for (size_t idx = 0; mapping->params[idx] != nullptr; ++idx) {
+            if (foundParameters[idx] &&
+                mapping->params[idx]->epsg_code ==
+                    EPSG_CODE_PARAMETER_ANGLE_RECTIFIED_TO_SKEW_GRID) {
+                foundAngleRecifiedToSkewGrid = true;
+            } else if (foundParameters[idx] &&
+                       mapping->params[idx]->epsg_code ==
+                           EPSG_CODE_PARAMETER_AZIMUTH_INITIAL_LINE) {
+                foundAzimuth = true;
+            }
+        }
+        if (!foundAngleRecifiedToSkewGrid && foundAzimuth) {
+            for (size_t idx = 0; idx < parameters.size(); ++idx) {
+                if (parameters[idx]->getEPSGCode() ==
+                    EPSG_CODE_PARAMETER_AZIMUTH_INITIAL_LINE) {
+                    PropertyMap propertiesParameter;
+                    propertiesParameter.set(
+                        Identifier::CODE_KEY,
+                        EPSG_CODE_PARAMETER_ANGLE_RECTIFIED_TO_SKEW_GRID);
+                    propertiesParameter.set(Identifier::CODESPACE_KEY,
+                                            Identifier::EPSG);
+                    propertiesParameter.set(
+                        IdentifiedObject::NAME_KEY,
+                        EPSG_NAME_PARAMETER_ANGLE_RECTIFIED_TO_SKEW_GRID);
+                    parameters.push_back(
+                        OperationParameter::create(propertiesParameter));
+                    values.push_back(values[idx]);
+                }
+            }
+        }
+    }
+
     return Conversion::create(
                PropertyMap().set(IdentifiedObject::NAME_KEY, "unnamed"),
                propertiesMethod, parameters, values)
diff --git a/src/iso19111/operation/conversion.cpp b/src/iso19111/operation/conversion.cpp
index 9927892d..77e703f3 100644
--- a/src/iso19111/operation/conversion.cpp
+++ b/src/iso19111/operation/conversion.cpp
@@ -4005,6 +4005,16 @@ void Conversion::_exportToPROJString(
                         EPSG_CODE_PARAMETER_SCALE_FACTOR_AT_NATURAL_ORIGIN) {
                         valueConverted = 1.0;
                     }
+                    if ((mapping->epsg_code ==
+                             EPSG_CODE_METHOD_HOTINE_OBLIQUE_MERCATOR_VARIANT_A ||
+                         mapping->epsg_code ==
+                             EPSG_CODE_METHOD_HOTINE_OBLIQUE_MERCATOR_VARIANT_B) &&
+                        param->epsg_code ==
+                            EPSG_CODE_PARAMETER_ANGLE_RECTIFIED_TO_SKEW_GRID) {
+                        // Do not use 0 as the default value for +gamma of
+                        // proj=omerc
+                        continue;
+                    }
                 } else if (param->unit_type ==
                            common::UnitOfMeasure::Type::ANGULAR) {
                     valueConverted =