ekf2: Add local position ground clearance reporting

src/modules/ekf2/ekf2_main.cpp

@@ -482,10 +482,11 @@ void Ekf2::task_main()
 		// The rotation of the tangent plane vs. geographical north
 		lpos.yaw = att.yaw;
 
-		lpos.dist_bottom = -pos[2] + lpos.ref_alt; // Distance to bottom surface (ground) in meters
+		float terrain_vpos;
+		lpos.dist_bottom_valid = _ekf->get_terrain_vert_pos(&terrain_vpos);
+		lpos.dist_bottom = terrain_vpos - pos[2]; // Distance to bottom surface (ground) in meters
 		lpos.dist_bottom_rate = -vel[2]; // Distance to bottom surface (ground) change rate
 		lpos.surface_bottom_timestamp	= hrt_absolute_time(); // Time when new bottom surface found
-		lpos.dist_bottom_valid = true; // true if distance to bottom surface is valid
 
 		// TODO: uORB definition does not define what thes variables are. We have assumed them to be horizontal and vertical 1-std dev accuracy in metres
 		// TODO: Should use sqrt of filter position variances