What is n value?

An N value is also called a separation value.  It is the difference between a height calculated above mean sea level in reference to AHD and a height calculated above the mathematical surface of the ellipsoid.  When a GNSS receiver is used to determine an elevation it will usually measure the elevation of the point on land above the ellipsoid (ellipsoidal height).  A separation value (N) needs to be applied to find the correct height above/below AHD.   This might be above or below the ellipsoid.  Separation values can be obtained from Geoscience Australia’s website where latitude and longitude are entered with elevation to produce the N value. 

Some GPS receivers are set up with the AUSGeoid09 model in their memory and will therefore calculate both the ellipsoidal height and AHD height without need to find an N value.