diff --git a/ArduPlane/GCS_Mavlink.cpp b/ArduPlane/GCS_Mavlink.cpp
index 45a596bc63..d2800c3676 100644
--- a/ArduPlane/GCS_Mavlink.cpp
+++ b/ArduPlane/GCS_Mavlink.cpp
@@ -1372,43 +1372,27 @@ void GCS_MAVLINK_Plane::handle_set_position_target_global_int(const mavlink_mess
 
         mavlink_set_position_target_global_int_t pos_target;
         mavlink_msg_set_position_target_global_int_decode(&msg, &pos_target);
+
+        Location::AltFrame frame;
+        if (!mavlink_coordinate_frame_to_location_alt_frame((MAV_FRAME)pos_target.coordinate_frame, frame)) {
+            gcs().send_text(MAV_SEVERITY_WARNING, "Invalid coord frame in SET_POSTION_TARGET_GLOBAL_INT");
+            // Even though other parts of the command may be valid, reject the whole thing.
+            return;
+        }
+
         // Unexpectedly, the mask is expecting "ones" for dimensions that should
         // be IGNORNED rather than INCLUDED.  See mavlink documentation of the
         // SET_POSITION_TARGET_GLOBAL_INT message, type_mask field.
         const uint16_t alt_mask = 0b1111111111111011; // (z mask at bit 3)
             
-        bool msg_valid = true;
         AP_Mission::Mission_Command cmd = {0};
         
         if (pos_target.type_mask & alt_mask)
         {
-            cmd.content.location.alt = pos_target.alt * 100;
-            cmd.content.location.relative_alt = false;
-            cmd.content.location.terrain_alt = false;
-            switch (pos_target.coordinate_frame) 
-            {
-                case MAV_FRAME_GLOBAL:
-                case MAV_FRAME_GLOBAL_INT:
-                    break; //default to MSL altitude
-                case MAV_FRAME_GLOBAL_RELATIVE_ALT:
-                case MAV_FRAME_GLOBAL_RELATIVE_ALT_INT:
-                    cmd.content.location.relative_alt = true;
-                    break;
-                case MAV_FRAME_GLOBAL_TERRAIN_ALT:
-                case MAV_FRAME_GLOBAL_TERRAIN_ALT_INT:
-                    cmd.content.location.relative_alt = true;
-                    cmd.content.location.terrain_alt = true;
-                    break;
-                default:
-                    gcs().send_text(MAV_SEVERITY_WARNING, "Invalid coord frame in SET_POSTION_TARGET_GLOBAL_INT");
-                    msg_valid = false;
-                    break;
-            }    
-
-            if (msg_valid) {
-                handle_change_alt_request(cmd);
-            }
-        } // end if alt_mask       
+            const int32_t alt_cm = pos_target.alt * 100;
+            cmd.content.location.set_alt_cm(alt_cm, frame);
+            handle_change_alt_request(cmd);
+        }
     }
 
 MAV_RESULT GCS_MAVLINK_Plane::handle_command_do_set_mission_current(const mavlink_command_int_t &packet)