Make scalebars auto selected between m/km and ft/miles when newly added

Based on current linked map scale and linked map CRS

python/core/layout/qgslayoutitemscalebar.sip

@@ -390,6 +390,16 @@ class QgsLayoutItemScaleBar: QgsLayoutItem
 .. seealso:: applyDefaultSize()
 %End
 
+    QgsUnitTypes::DistanceUnit guessUnits() const;
+%Docstring
+ Attempts to guess the most reasonable unit choice for the scalebar, given
+ the current linked map's scale.
+
+ This method also considers the linked map's CRS, in order to determine if
+ metric or imperial units are more appropriate.
+ :rtype: QgsUnitTypes.DistanceUnit
+%End
+
     void applyDefaultSize( QgsUnitTypes::DistanceUnit units = QgsUnitTypes::DistanceMeters );
 %Docstring
  Applies the default size to the scale bar (scale bar 1/5 of map item width)

src/app/layout/qgslayoutapputils.cpp

@@ -192,7 +192,7 @@ void QgsLayoutAppUtils::registerGuiForKnownItemTypes()
     if ( targetMap )
     {
       scalebar->setMap( targetMap );
-      scalebar->applyDefaultSize();
+      scalebar->applyDefaultSize( scalebar->guessUnits() );
     }
   } );
 

src/core/layout/qgslayoutitemscalebar.cpp

@@ -390,6 +390,48 @@ void QgsLayoutItemScaleBar::applyDefaultSettings()
   emit changed();
 }
 
+QgsUnitTypes::DistanceUnit QgsLayoutItemScaleBar::guessUnits() const
+{
+  if ( !mMap )
+    return QgsUnitTypes::DistanceMeters;
+
+  QgsCoordinateReferenceSystem crs = mMap->crs();
+  // start with crs units
+  QgsUnitTypes::DistanceUnit unit = crs.mapUnits();
+  if ( unit == QgsUnitTypes::DistanceDegrees || unit == QgsUnitTypes::DistanceUnknownUnit )
+  {
+    // geographic CRS, use metric units
+    unit = QgsUnitTypes::DistanceMeters;
+  }
+
+  // try to pick reasonable choice between metric / imperial units
+  double widthInSelectedUnits = mapWidth();
+  double initialUnitsPerSegment = widthInSelectedUnits / 10.0; //default scalebar width equals half the map width
+  switch ( unit )
+  {
+    case QgsUnitTypes::DistanceMeters:
+    {
+      if ( initialUnitsPerSegment > 1000.0 )
+      {
+        unit = QgsUnitTypes::DistanceKilometers;
+      }
+      break;
+    }
+    case QgsUnitTypes::DistanceFeet:
+    {
+      if ( initialUnitsPerSegment > 5419.95 )
+      {
+        unit = QgsUnitTypes::DistanceMiles;
+      }
+      break;
+    }
+    default:
+      break;
+  }
+
+  return unit;
+}
+
 void QgsLayoutItemScaleBar::applyDefaultSize( QgsUnitTypes::DistanceUnit units )
 {
   mSettings.setUnits( units );

src/core/layout/qgslayoutitemscalebar.h

@@ -379,6 +379,15 @@ class CORE_EXPORT QgsLayoutItemScaleBar: public QgsLayoutItem
      */
     void applyDefaultSettings();
 
+    /**
+     * Attempts to guess the most reasonable unit choice for the scalebar, given
+     * the current linked map's scale.
+     *
+     * This method also considers the linked map's CRS, in order to determine if
+     * metric or imperial units are more appropriate.
+     */
+    QgsUnitTypes::DistanceUnit guessUnits() const;
+
     /**
      * Applies the default size to the scale bar (scale bar 1/5 of map item width)
      * \see applyDefaultSettings()