Add mechanism to disable user-visible warnings on use of fallback tra…

…nsforms

for a particular QgsCoordinateTransform object

This can be set for transforms where we definitively know that a "ballpark"
result is acceptable, e.g. when transforming global layer extents to
a localized area-of-use CRS

Refs qgis#33929

python/core/auto_generated/qgscoordinatetransform.sip.in

@@ -350,6 +350,36 @@ coordinates.
 .. seealso:: :py:func:`coordinateOperation`
 
 .. versionadded:: 3.8
+%End
+
+    void setBallparkTransformsAreAppropriate( bool appropriate );
+%Docstring
+Sets whether approximate "ballpark" results are appropriate for this coordinate transform.
+
+When a coordinate transform is only being used to generate ballpark results then the
+``appropriate`` argument should be set to ``True``. This indicates that its perfectable
+acceptable (and even expected!) for the transform to use fallback coordinate operations
+in the case that the preferred or user-specified operation fails (such as when coordinates
+from outside of a grid shift file's extent are transformed).
+
+When ``appropriate`` is ``True``, then no warnings will be generated when the transform
+falls back to a default operation, which may introduce inaccuracies when compared to
+the default/specified coordinate operation.
+
+This should be set when a transform expects that coordinates outside of the direct
+area of use while be transformed, e.g. when transforming from a global extent to a
+CRS with a localized area of use.
+
+If ``appropriate`` is ``False`` (the default behavior), then transforms MAY STILL fallback to default operations
+when the preferred or user-specified operation fails, however whenever this occurs
+a user-visible warning will be generated.
+
+.. warning::
+
+   This setting applies to a single instance of a coordinate transform only,
+   and is not copied when a coordinate transform object is copied or assigned.
+
+.. versionadded:: 3.12
 %End
 
  int sourceDatumTransformId() const /Deprecated/;

src/core/qgscoordinatetransform.cpp

@@ -41,6 +41,7 @@
 #endif
 
 #include <sqlite3.h>
+#include <qlogging.h>
 #include <vector>
 #include <algorithm>
 
@@ -774,11 +775,14 @@ void QgsCoordinateTransform::transformCoords( int numPoints, double *x, double *
         memcpy( z, zprev.data(), sizeof( double ) * numPoints );
       }
 
-      if ( sFallbackOperationOccurredHandler )
+      if ( !mBallparkTransformsAreAppropriate && sFallbackOperationOccurredHandler )
       {
         QgsDatumTransform::TransformDetails desired = instantiatedCoordinateOperationDetails();
         QgsDatumTransform::TransformDetails used = QgsDatumTransform::transformDetailsFromPj( transform );
         sFallbackOperationOccurredHandler( d->mSourceCRS, d->mDestCRS, desired, used );
+        const QString warning = QStringLiteral( "A fallback coordinate operation was used between %1 and %2" ).arg( d->mSourceCRS.authid(),
+                                d->mDestCRS.authid() );
+        qWarning( "%s", warning.toLatin1().constData() );
       }
     }
   }
@@ -897,6 +901,11 @@ void QgsCoordinateTransform::setCoordinateOperation( const QString &operation )
   d->mShouldReverseCoordinateOperation = false;
 }
 
+void QgsCoordinateTransform::setBallparkTransformsAreAppropriate( bool appropriate )
+{
+  mBallparkTransformsAreAppropriate = appropriate;
+}
+
 const char *finder( const char *name )
 {
   QString proj;

src/core/qgscoordinatetransform.h

@@ -380,6 +380,34 @@ class CORE_EXPORT QgsCoordinateTransform
      */
     void setCoordinateOperation( const QString &operation ) const;
 
+    /**
+     * Sets whether approximate "ballpark" results are appropriate for this coordinate transform.
+     *
+     * When a coordinate transform is only being used to generate ballpark results then the
+     * \a appropriate argument should be set to TRUE. This indicates that its perfectable
+     * acceptable (and even expected!) for the transform to use fallback coordinate operations
+     * in the case that the preferred or user-specified operation fails (such as when coordinates
+     * from outside of a grid shift file's extent are transformed).
+     *
+     * When \a appropriate is TRUE, then no warnings will be generated when the transform
+     * falls back to a default operation, which may introduce inaccuracies when compared to
+     * the default/specified coordinate operation.
+     *
+     * This should be set when a transform expects that coordinates outside of the direct
+     * area of use while be transformed, e.g. when transforming from a global extent to a
+     * CRS with a localized area of use.
+     *
+     * If \a appropriate is FALSE (the default behavior), then transforms MAY STILL fallback to default operations
+     * when the preferred or user-specified operation fails, however whenever this occurs
+     * a user-visible warning will be generated.
+     *
+     * \warning This setting applies to a single instance of a coordinate transform only,
+     * and is not copied when a coordinate transform object is copied or assigned.
+     *
+     * \since QGIS 3.12
+     */
+    void setBallparkTransformsAreAppropriate( bool appropriate );
+
     /**
      * Returns the ID of the datum transform to use when projecting from the source
      * CRS.
@@ -591,6 +619,7 @@ class CORE_EXPORT QgsCoordinateTransform
 #endif
 
     mutable QString mLastError;
+    bool mBallparkTransformsAreAppropriate = false;
 
 #if PROJ_VERSION_MAJOR>=6
     bool setFromCache( const QgsCoordinateReferenceSystem &src,