Sometime people are referring to the same point but they get different elevations? How is that possible? This confusion can often be explained by the fact that both persons measured the points elevation in reference to a different elevation reference. This article present some background on this topic.

The shape of our earth is not perfectly spherical. So when we try to approximate the shape of it, we have to find a appropriate model. A commonly known ellipsoid is WGS84, which is also used in GPS. Another important ellipsoid is the GRS80 ellipsoid which is used for measuring coordinates in the United States and Canada. 

The geoid is a more advanced reference surface which is used to approximate the shape of the Earth and to measure elevations.

Essentially there are two main categories of elevation references:

• Orthometric elevations
• Ellipsoidal elevations

The Ellipsoidal Height is the vertical distance from a location on the Earth’s Surface to the ellipsoid (ochre surface in the illustration). This is the value returned by the GPS receiver. 

The Orthometric Height (or Geodetic Height) of that same point of the Earth Surface is the distance to the geoid (blue surface in the illustration). Because the earth geoid is set a the level of the average sea level it is often called the elevation at Mean Sea Level (MSL). 

The Undulation is the difference brtween the ellipsoidal height and the orthomethric height on that location. The values of the undulation for specific countries and regoins are stored in files known as Geoid Models. These models are calculated based on gravity readings on land and omplex math.

Credit: https://support.virtual-surveyor.com/en/support/solutions/articles/1000261346