Add API to QgsCoordinateTransformContext to prevent use of fallback

ballpark transforms for a source/dest CRS pair

python/core/auto_generated/qgscoordinatetransformcontext.sip.in

@@ -125,7 +125,7 @@ Returns ``True`` if the new transform pair was added successfully.
    Has no effect on builds based on Proj 6.0 or later, use addCoordinateOperation() instead.
 %End
 
-    bool addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString );
+    bool addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString, bool allowFallback = true );
 %Docstring
 Adds a new ``coordinateOperationProjString`` to use when projecting coordinates
 from the specified ``sourceCrs`` to the specified ``destinationCrs``.
@@ -134,6 +134,14 @@ from the specified ``sourceCrs`` to the specified ``destinationCrs``.
 string. If ``coordinateOperationProjString`` is empty, then the default Proj operation
 will be used when transforming between the coordinate reference systems.
 
+If ``allowFallback`` is ``True`` (since QGIS 3.12), then "ballpark" fallback transformations
+will be used in the case that the specified coordinate operation fails (such as when
+coordinates from outside a required grid shift file are transformed). See
+QgsCoordinateTransform.fallbackOperationOccurred() for further details. Note that if an
+existing ``sourceCrs`` and ``destinationCrs`` pair are added with a different ``allowFallback``
+value, that value will replace the existing one (i.e. each combination of ``sourceCrs`` and
+``destinationCrs`` must be unique).
+
 .. warning::
 
    coordinateOperationProjString MUST be a proj string which has been normalized for
@@ -229,6 +237,17 @@ be used.
 
 
 .. versionadded:: 3.8
+%End
+
+    bool allowFallbackTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const;
+%Docstring
+Returns ``True`` if approximate "ballpark" transforms may be used when transforming
+between a \source and ``destination`` CRS pair, in the case that the preferred
+coordinate operation fails (such as when
+coordinates from outside a required grid shift file are transformed). See
+QgsCoordinateTransform.fallbackOperationOccurred() for further details.
+
+.. versionadded:: 3.12
 %End
 
     bool mustReverseCoordinateOperation( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const;

src/core/qgscoordinatetransformcontext.cpp

@@ -80,7 +80,11 @@ QMap<QPair<QString, QString>, QString> QgsCoordinateTransformContext::coordinate
   auto res = d->mSourceDestDatumTransforms;
   res.detach();
   d->mLock.unlock();
-  return res;
+  QMap<QPair<QString, QString>, QString> results;
+  for ( auto it = res.constBegin(); it != res.constEnd(); ++it )
+    results.insert( it.key(), it.value().operation );
+
+  return results;
 #else
   return QMap<QPair<QString, QString>, QString>();
 #endif
@@ -103,18 +107,22 @@ bool QgsCoordinateTransformContext::addSourceDestinationDatumTransform( const Qg
 #endif
 }
 
-bool QgsCoordinateTransformContext::addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString )
+bool QgsCoordinateTransformContext::addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString, bool allowFallback )
 {
   if ( !sourceCrs.isValid() || !destinationCrs.isValid() )
     return false;
 #if PROJ_VERSION_MAJOR>=6
   d.detach();
   d->mLock.lockForWrite();
-  d->mSourceDestDatumTransforms.insert( qMakePair( sourceCrs.authid(), destinationCrs.authid() ), coordinateOperationProjString );
+  QgsCoordinateTransformContextPrivate::OperationDetails details;
+  details.operation = coordinateOperationProjString;
+  details.allowFallback = allowFallback;
+  d->mSourceDestDatumTransforms.insert( qMakePair( sourceCrs.authid(), destinationCrs.authid() ), details );
   d->mLock.unlock();
   return true;
 #else
   Q_UNUSED( coordinateOperationProjString )
+  Q_UNUSED( allowFallback )
   return false;
 #endif
 }
@@ -174,37 +182,54 @@ QString QgsCoordinateTransformContext::calculateCoordinateOperation( const QgsCo
   const QString destKey = destination.authid();
 
   d->mLock.lockForRead();
-  QString res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), QString() );
-  if ( res.isEmpty() )
+  QgsCoordinateTransformContextPrivate::OperationDetails res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), QgsCoordinateTransformContextPrivate::OperationDetails() );
+  if ( res.operation.isEmpty() )
   {
     // try to reverse
-    res = d->mSourceDestDatumTransforms.value( qMakePair( destKey, srcKey ), QString() );
+    res = d->mSourceDestDatumTransforms.value( qMakePair( destKey, srcKey ), QgsCoordinateTransformContextPrivate::OperationDetails() );
   }
   d->mLock.unlock();
-  return res;
+  return res.operation;
 #else
   Q_UNUSED( source )
   Q_UNUSED( destination )
   return QString();
 #endif
 }
 
+bool QgsCoordinateTransformContext::allowFallbackTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
+{
+#if PROJ_VERSION_MAJOR>=6
+  const QString srcKey = source.authid();
+  const QString destKey = destination.authid();
+
+  d->mLock.lockForRead();
+  QgsCoordinateTransformContextPrivate::OperationDetails res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), QgsCoordinateTransformContextPrivate::OperationDetails() );
+  d->mLock.unlock();
+  return res.allowFallback;
+#else
+  Q_UNUSED( source )
+  Q_UNUSED( destination )
+  return false;
+#endif
+}
+
 bool QgsCoordinateTransformContext::mustReverseCoordinateOperation( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const
 {
 #if PROJ_VERSION_MAJOR>=6
   const QString srcKey = source.authid();
   const QString destKey = destination.authid();
 
   d->mLock.lockForRead();
-  QString res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), QString() );
-  if ( !res.isEmpty() )
+  QgsCoordinateTransformContextPrivate::OperationDetails res = d->mSourceDestDatumTransforms.value( qMakePair( srcKey, destKey ), QgsCoordinateTransformContextPrivate::OperationDetails() );
+  if ( !res.operation.isEmpty() )
   {
     d->mLock.unlock();
     return false;
   }
   // see if the reverse operation is present
-  res = d->mSourceDestDatumTransforms.value( qMakePair( destKey, srcKey ), QString() );
-  if ( !res.isEmpty() )
+  res = d->mSourceDestDatumTransforms.value( qMakePair( destKey, srcKey ), QgsCoordinateTransformContextPrivate::OperationDetails() );
+  if ( !res.operation.isEmpty() )
   {
     d->mLock.unlock();
     return true;
@@ -248,6 +273,7 @@ bool QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
 
 #if PROJ_VERSION_MAJOR>=6
     const QString coordinateOp = transformElem.attribute( QStringLiteral( "coordinateOp" ) );
+    const bool allowFallback = transformElem.attribute( QStringLiteral( "allowFallback" ), QStringLiteral( "1" ) ).toInt();
 
     // try to instantiate operation, and check for missing grids
     if ( !QgsProjUtils::coordinateOperationIsAvailable( coordinateOp ) )
@@ -257,7 +283,10 @@ bool QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
       result = false;
     }
 
-    d->mSourceDestDatumTransforms.insert( qMakePair( key1, key2 ), coordinateOp );
+    QgsCoordinateTransformContextPrivate::OperationDetails deets;
+    deets.operation = coordinateOp;
+    deets.allowFallback = allowFallback;
+    d->mSourceDestDatumTransforms.insert( qMakePair( key1, key2 ), deets );
 #else
     QString value1 = transformElem.attribute( QStringLiteral( "sourceTransform" ) );
     QString value2 = transformElem.attribute( QStringLiteral( "destTransform" ) );
@@ -307,7 +336,8 @@ void QgsCoordinateTransformContext::writeXml( QDomElement &element, const QgsRea
     transformElem.setAttribute( QStringLiteral( "source" ), it.key().first );
     transformElem.setAttribute( QStringLiteral( "dest" ), it.key().second );
 #if PROJ_VERSION_MAJOR>=6
-    transformElem.setAttribute( QStringLiteral( "coordinateOp" ), it.value() );
+    transformElem.setAttribute( QStringLiteral( "coordinateOp" ), it.value().operation );
+    transformElem.setAttribute( QStringLiteral( "allowFallback" ), it.value().allowFallback ? QStringLiteral( "1" ) : QStringLiteral( "0" ) );
 #else
     Q_NOWARN_DEPRECATED_PUSH
     transformElem.setAttribute( QStringLiteral( "sourceTransform" ), it.value().sourceTransformId < 0 ? QString() : QgsDatumTransform::datumTransformToProj( it.value().sourceTransformId ) );
@@ -334,7 +364,7 @@ void QgsCoordinateTransformContext::readSettings()
 
   //collect src and dest entries that belong together
 #if PROJ_VERSION_MAJOR>=6
-  QMap< QPair< QString, QString >, QString > transforms;
+  QMap< QPair< QString, QString >, QgsCoordinateTransformContextPrivate::OperationDetails > transforms;
 #else
   QMap< QPair< QString, QString >, QPair< int, int > > transforms;
 #endif
@@ -356,7 +386,11 @@ void QgsCoordinateTransformContext::readSettings()
       }
 
       const QString proj = settings.value( *pkeyIt ).toString();
-      transforms[ qMakePair( srcAuthId, destAuthId )] = proj;
+      const bool allowFallback = settings.value( QStringLiteral( "%1//%2_allowFallback" ).arg( srcAuthId, destAuthId ) ).toBool();
+      QgsCoordinateTransformContextPrivate::OperationDetails deets;
+      deets.operation = proj;
+      deets.allowFallback = allowFallback;
+      transforms[ qMakePair( srcAuthId, destAuthId )] = deets;
     }
 #else
     if ( pkeyIt->contains( QLatin1String( "srcTransform" ) ) || pkeyIt->contains( QLatin1String( "destTransform" ) ) )
@@ -423,8 +457,10 @@ void QgsCoordinateTransformContext::writeSettings()
     const QString destAuthId = transformIt.key().second;
 
 #if PROJ_VERSION_MAJOR>=6
-    const QString proj = transformIt.value();
+    const QString proj = transformIt.value().operation;
+    const bool allowFallback = transformIt.value().allowFallback;
     settings.setValue( srcAuthId + "//" + destAuthId + "_coordinateOp", proj );
+    settings.setValue( srcAuthId + "//" + destAuthId + "_allowFallback", allowFallback );
 #else
     int sourceDatumTransform = transformIt.value().sourceTransformId;
     QString sourceDatumProj;

src/core/qgscoordinatetransformcontext.h

@@ -140,6 +140,14 @@ class CORE_EXPORT QgsCoordinateTransformContext
      * string. If \a coordinateOperationProjString is empty, then the default Proj operation
      * will be used when transforming between the coordinate reference systems.
      *
+     * If \a allowFallback is TRUE (since QGIS 3.12), then "ballpark" fallback transformations
+     * will be used in the case that the specified coordinate operation fails (such as when
+     * coordinates from outside a required grid shift file are transformed). See
+     * QgsCoordinateTransform::fallbackOperationOccurred() for further details. Note that if an
+     * existing \a sourceCrs and \a destinationCrs pair are added with a different \a allowFallback
+     * value, that value will replace the existing one (i.e. each combination of \a sourceCrs and
+     * \a destinationCrs must be unique).
+     *
      * \warning coordinateOperationProjString MUST be a proj string which has been normalized for
      * visualization, and must be constructed so that coordinates are always input and output
      * with x/y coordinate ordering. (Proj strings output by utilities such as projinfo will NOT
@@ -155,7 +163,7 @@ class CORE_EXPORT QgsCoordinateTransformContext
      *
      * \since QGIS 3.8
      */
-    bool addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString );
+    bool addCoordinateOperation( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QString &coordinateOperationProjString, bool allowFallback = true );
 
     /**
      * Removes the source to destination datum transform pair for the specified \a sourceCrs and
@@ -214,6 +222,17 @@ class CORE_EXPORT QgsCoordinateTransformContext
      */
     QString calculateCoordinateOperation( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const;
 
+    /**
+     * Returns TRUE if approximate "ballpark" transforms may be used when transforming
+     * between a \source and \a destination CRS pair, in the case that the preferred
+     * coordinate operation fails (such as when
+     * coordinates from outside a required grid shift file are transformed). See
+     * QgsCoordinateTransform::fallbackOperationOccurred() for further details.
+     *
+     * \since QGIS 3.12
+     */
+    bool allowFallbackTransform( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const;
+
     /**
      * Returns TRUE if the coordinate operation returned by calculateCoordinateOperation() for the \a source to \a destination pair
      * must be inverted.

src/core/qgscoordinatetransformcontext_p.h

@@ -59,7 +59,18 @@ class QgsCoordinateTransformContextPrivate : public QSharedData
      * Mapping for coordinate operation Proj string to use for source/destination CRS pairs.
      */
 #if PROJ_VERSION_MAJOR>=6
-    QMap< QPair< QString, QString >, QString > mSourceDestDatumTransforms;
+    class OperationDetails
+    {
+      public:
+        QString operation;
+        bool allowFallback = true;
+
+        bool operator==( const OperationDetails &other ) const
+        {
+          return operation == other.operation && allowFallback == other.allowFallback;
+        }
+    };
+    QMap< QPair< QString, QString >, OperationDetails > mSourceDestDatumTransforms;
 #else
     QMap< QPair< QString, QString >, QgsDatumTransform::TransformPair > mSourceDestDatumTransforms;
 #endif