Dear Mr/Mrs Geodesy Engineers

I have a quite deep question here,

AIP data locations are determined by geographic coordinates /X & Y axis/ and vertical values /Z axis/ via Geodesy science.

ICAO adopted EGM-96 is standard for Vertical Reference System.

Question is raised regarding to Annex-15, "......When a geoid model other than the EGM-96 model is used, a description of the model used, including the parameters required for height transformation between the model and EGM-96, shall be provided in the Aeronautical Information Publication (AIP)."

In Mongolia, We are not using EGM-96 for Vertical Reference System. We are using Baltic sea level value /1940-1970/ in Air Navigation.

Question is: How to figure out the parameters required for height transformation between any model and EGM-96 for AIP publication?

In my thinking the parameters must contain following values: delta X, delta Y, delta Z, X rotation, Y rotation, Z rotation, Scale... etc. Is this correct?


2 Answers 2


If you have a geoid model, that is often a set of equations to convert between gravity-related heights (elevations) and ellipsoidal heights. It's often modeled as a grid file. There are both versions for EGM96. A possible workflow would be:

  1. Convert Mongolian geoid/Baltic-based gravity-related heights to ellipsoidal heights. The geographic (latitude-longitude) coordinates will be the same as used by the geoid model.
  2. Optional. Convert between the geographic coordinate reference system (CRS) to WGS 84. This step can affect the ellipsoidal heights.
  3. Use the EGM96 geoid model to convert from ellipsoidal heights to geoidal heights.

Edit: I missed your last paragraph when I wrote my answer. Those parameters are for a 3D Cartesian transformation that usually converts between two geographic coordinate reference systems. It can convert heights but only ellipsoidal heights so you would use it in step 2.


You would need to know what the relationship between the Baltic model and the ellipsoid heights area. Then you could create a transformation equation for the z values to give e height. You would then convert to your new vertical datum with its conversion equation.

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