When adding a raster layer which we know definitively contains

elevation data, and the project does not currently have any
terrain provider set, then automatically set that raster layer
as the project's terrain provider

src/app/layers/qgsapplayerhandling.cpp

@@ -17,13 +17,16 @@
 
 #include "qgsconfig.h"
 #include "qgsmaplayer.h"
+#include "qgsmaplayerelevationproperties.h"
 #include "qgsmeshlayer.h"
 #include "qgsproject.h"
+#include "qgsprojectelevationproperties.h"
 #include "qgsprojecttimesettings.h"
 #include "qgspointcloudlayer.h"
 #include "qgsmeshlayertemporalproperties.h"
 #include "qgisapp.h"
 #include "qgsmessagebar.h"
+#include "qgsterrainprovider.h"
 #ifdef HAVE_3D
 #include "qgspointcloudlayer3drenderer.h"
 #endif
@@ -65,8 +68,32 @@ void QgsAppLayerHandling::postProcessAddedLayer( QgsMapLayer *layer )
 {
   switch ( layer->type() )
   {
-    case Qgis::LayerType::Vector:
     case Qgis::LayerType::Raster:
+    {
+      bool ok = false;
+      layer->loadDefaultStyle( ok );
+      layer->loadDefaultMetadata( ok );
+
+      // if there's no (useful) terrain provider for the current project, and we know that this
+      // layer contains elevation, then automatically set it as the terrain provider
+      if ( !QgsProject::instance()->elevationProperties()->terrainProvider()
+           || ( dynamic_cast< QgsFlatTerrainProvider * >( QgsProject::instance()->elevationProperties()->terrainProvider() )
+                && QgsProject::instance()->elevationProperties()->terrainProvider()->offset() == 0
+                && QgsProject::instance()->elevationProperties()->terrainProvider()->scale() == 1 ) )
+      {
+        if ( layer->elevationProperties()->hasElevation() )
+        {
+          std::unique_ptr< QgsRasterDemTerrainProvider > terrain = std::make_unique<QgsRasterDemTerrainProvider>();
+          terrain->setLayer( qobject_cast< QgsRasterLayer *>( layer ) );
+          QgsProject::instance()->elevationProperties()->setTerrainProvider(
+            terrain.release()
+          );
+        }
+      }
+      break;
+    }
+
+    case Qgis::LayerType::Vector:
     {
       bool ok = false;
       layer->loadDefaultStyle( ok );