diff --git a/common/source/docs/common-compass-calibration-in-mission-planner.rst b/common/source/docs/common-compass-calibration-in-mission-planner.rst
index c31029de9f..e52c348518 100644
--- a/common/source/docs/common-compass-calibration-in-mission-planner.rst
+++ b/common/source/docs/common-compass-calibration-in-mission-planner.rst
@@ -77,7 +77,9 @@ Large Vehicle MagCal
 
 Large or heavy vehicles are impractical to rotate on all axis. This feature allows a fairly accurate calibration if GPS lock is active on the autopilot and the vehicles actual heading is known, either using a landmark reference on the Mission Planner map, or using another compass (eg cell phone) and entering the vehicles heading. 
 
-.. note:: the heading entered should be TRUE, not MAGNETIC. Using a phone's compass app will usually required adding the local declination value to the reading in order to obtain the TRUE geographic heading which should be entered. The proper orientation of the compass must also be set in order for this method to work.
+.. warning:: The proper orientation of the compass must also be set in order for this method to give a good result.  If orientation is incorrect this procedure will appear to succeed while leaving the compass calibration in a very bad state.
+
+.. note:: the heading entered should be TRUE, not MAGNETIC. Using a phone's compass app will usually required adding the local declination value to the reading in order to obtain the TRUE geographic heading which should be entered.
 
 Compass Ordering
 ================