Tip # 81 : How the geoid affects Height and Elevation computations

By Sudam Fernando posted 08-02-2019 14:02


TBC’s computation engine is always looking to compute grid coordinates (N,E,Elevation) and global coordinates (Lat, Long, Ellipsoidal Heights). But let’s imagine a scenario where you are measuring the coordinates of a point using both GNSS measurements and total station observations. Which method does TBC use to compute the points elevation and height ? 



We know that GNSS observations give us ellipsoidal heights measured from the reference ellipsoid. To be able to convert these heights into elevations (Orthometric Heights) we must subtract the geoid undulation (Geoid Height) from the elevation taken from GNSS observations. 




Let us look at the point quality hierarchy used in TBC : 


  1. Network adjusted 
  2. Control Quality 
  3. Survey Quality
  4. Survey Geoid Quality 
  5. Mapping Quality 
  6. Mapping Geoid Quality 
  7. Unknown Geoid 
  8. Unknown



In our example, our measurements are measured with survey quality. Hence, 

  • Total station observations produce:
    • Survey quality elevations
    • Survey geoid quality heights
  • GNSS observations produce:
    • Survey quality heights
    • Survey geoid quality elevations


Why? Because the geoid reduces the quality when converting from heights to elevations and vice versa. 



This means our GNSS observations has survey quality heights but the total station observations has survey geoid quality heights. 


So, to answer our original question, TBC uses the elevations from the Total Station observations and Heights from GNSS observation to keep the highest quality. 


For more information on TBC's computation engine, please check out our Power hour about Integrating multiple types of data here