Add method to retrieve datum transform ID from a proj string

python/core/qgscoordinatetransform.sip

@@ -281,9 +281,19 @@ Default constructor, creates an invalid QgsCoordinateTransform.
  Returns a proj string representing the specified ``datumTransformId`` datum transform ID.
 .. seealso:: :py:func:`datumTransformations()`
 .. seealso:: :py:func:`datumTransformInfo()`
+.. seealso:: :py:func:`projStringToDatumTransformId()`
  :rtype: str
 %End
 
+    static int projStringToDatumTransformId( const QString &string );
+%Docstring
+ Returns the datum transform ID corresponding to a specified proj ``string``.
+ Returns -1 if matching datum ID was not found.
+.. seealso:: :py:func:`datumTransformToProj()`
+.. versionadded:: 3.0
+ :rtype: int
+%End
+
     struct TransformInfo
     {
       int datumTransformId;

src/core/qgscoordinatetransform.cpp

@@ -816,6 +816,11 @@ QString QgsCoordinateTransform::datumTransformToProj( int datumTransform )
   return QgsCoordinateTransformPrivate::datumTransformString( datumTransform );
 }
 
+int QgsCoordinateTransform::projStringToDatumTransformId( const QString &string )
+{
+  return QgsCoordinateTransformPrivate::transformIdFromString( string );
+}
+
 QgsCoordinateTransform::TransformInfo QgsCoordinateTransform::datumTransformInfo( int datumTransform )
 {
   TransformInfo info;

src/core/qgscoordinatetransform.h

@@ -363,9 +363,18 @@ class CORE_EXPORT QgsCoordinateTransform
      * Returns a proj string representing the specified \a datumTransformId datum transform ID.
      * \see datumTransformations()
      * \see datumTransformInfo()
+     * \see projStringToDatumTransformId()
      */
     static QString datumTransformToProj( int datumTransformId );
 
+    /**
+     * Returns the datum transform ID corresponding to a specified proj \a string.
+     * Returns -1 if matching datum ID was not found.
+     * \see datumTransformToProj()
+     * \since QGIS 3.0
+     */
+    static int projStringToDatumTransformId( const QString &string );
+
     /**
      * Contains datum transform information.
      * \since QGIS 3.0

src/core/qgscoordinatetransform_p.cpp

@@ -341,6 +341,62 @@ QString QgsCoordinateTransformPrivate::datumTransformString( int datumTransform
   return transformString;
 }
 
+int QgsCoordinateTransformPrivate::transformIdFromString( const QString &string )
+{
+  sqlite3_database_unique_ptr database;
+  int openResult = database.open_v2( QgsApplication::srsDatabaseFilePath(), SQLITE_OPEN_READONLY, nullptr );
+  if ( openResult != SQLITE_OK )
+  {
+    return -1;
+  }
+
+  sqlite3_statement_unique_ptr statement;
+  QString sql = QStringLiteral( "SELECT coord_op_method_code,p1,p2,p3,p4,p5,p6,p7,coord_op_code FROM tbl_datum_transform" );
+  int prepareRes;
+  statement = database.prepare( sql, prepareRes );
+  if ( prepareRes != SQLITE_OK )
+  {
+    return -1;
+  }
+
+  while ( statement.step() == SQLITE_ROW )
+  {
+    QString transformString;
+    //coord_op_methode_code
+    int methodCode = statement.columnAsInt64( 0 );
+    if ( methodCode == 9615 ) //ntv2
+    {
+      transformString = "+nadgrids=" + statement.columnAsText( 1 );
+    }
+    else if ( methodCode == 9603 || methodCode == 9606 || methodCode == 9607 )
+    {
+      transformString += QLatin1String( "+towgs84=" );
+      double p1 = statement.columnAsDouble( 1 );
+      double p2 = statement.columnAsDouble( 2 );
+      double p3 = statement.columnAsDouble( 3 );
+      double p4 = statement.columnAsDouble( 4 );
+      double p5 = statement.columnAsDouble( 5 );
+      double p6 = statement.columnAsDouble( 6 );
+      double p7 = statement.columnAsDouble( 7 );
+      if ( methodCode == 9603 ) //3 parameter transformation
+      {
+        transformString += QStringLiteral( "%1,%2,%3" ).arg( QString::number( p1 ), QString::number( p2 ), QString::number( p3 ) );
+      }
+      else //7 parameter transformation
+      {
+        transformString += QStringLiteral( "%1,%2,%3,%4,%5,%6,%7" ).arg( QString::number( p1 ), QString::number( p2 ), QString::number( p3 ), QString::number( p4 ), QString::number( p5 ), QString::number( p6 ), QString::number( p7 ) );
+      }
+    }
+
+    if ( transformString.compare( string, Qt::CaseInsensitive ) == 0 )
+    {
+      return statement.columnAsInt64( 8 );
+    }
+  }
+
+  return -1;
+}
+
 void QgsCoordinateTransformPrivate::addNullGridShifts( QString &srcProjString, QString &destProjString,
     int sourceDatumTransform, int destinationDatumTransform ) const
 {

src/core/qgscoordinatetransform_p.h

@@ -135,7 +135,20 @@ class QgsCoordinateTransformPrivate : public QSharedData
     QReadWriteLock mProjLock;
     QMap < uintptr_t, QPair< projPJ, projPJ > > mProjProjections;
 
-    static QString datumTransformString( int datumTransform );
+    /**
+     * Returns the proj transform string corresponding to a
+     * datum transform ID.
+     * \see transformIdFromString()
+     */
+    static QString datumTransformString( int transformId );
+
+    /**
+     * Attempts to match a proj datum transform string to a datum ID.
+     * Returns -1 if datum ID was not found.
+     * \see datumTransformString()
+     * \since QGIS 3.0
+     */
+    static int transformIdFromString( const QString &string );
 
   private:
 

tests/src/python/test_qgscoordinatetransform.py

@@ -218,6 +218,19 @@ class TestQgsCoordinateTransform(unittest.TestCase):
         self.assertIn('EPSG:4326', [QgsCoordinateTransform.datumTransformInfo(t.destinationTransformId).destinationCrsAuthId for t in
                                     transforms])
 
+    def testStringToTransformId(self):
+        """
+        Test converting proj strings to corresponding datum IDs
+        """
+        self.assertEqual(QgsCoordinateTransform.projStringToDatumTransformId(''), -1)
+        self.assertEqual(QgsCoordinateTransform.projStringToDatumTransformId('not'), -1)
+        test_string = '+towgs84=-403,684,41'
+        id = QgsCoordinateTransform.projStringToDatumTransformId(test_string)
+        self.assertNotEqual(id, -1)
+        string = QgsCoordinateTransform.datumTransformToProj(id)
+        self.assertEqual(string, test_string)
+        self.assertEqual(QgsCoordinateTransform.projStringToDatumTransformId(test_string.upper()), id)
+
 
 if __name__ == '__main__':
     unittest.main()