FYI here is a brief chat GPS explanation of what @bedoali8607 is referring to: The difference between orthometric (or ortho) elevation and ellipsoid elevation is related to how elevation or altitude is measured and referenced. Understanding this difference is crucial when assessing the accuracy of GPS altitude measurements. Orthometric Elevation: Orthometric elevation, often referred to as ortho height, is the elevation above or below the geoid, which is the true shape of the Earth's surface when considering variations in gravity. It is the measure of true elevation relative to mean sea level or a local reference surface like a tidal benchmark. It provides a practical measure of elevation that is essential for many applications, such as civil engineering, construction, and geodetic surveying. Orthometric heights can vary from place to place due to variations in the geoid. Ellipsoid Elevation: Ellipsoid elevation, also known as GPS height or ellipsoidal height, is the elevation measured relative to a simplified mathematical model of the Earth's surface, the reference ellipsoid. GPS receivers typically provide altitude measurements relative to this ellipsoid, which is a smooth and consistent mathematical shape that simplifies calculations. Ellipsoid elevations are constant across the entire Earth's surface and are not directly related to mean sea level. The difference between orthometric and ellipsoidal elevations, often referred to as the geoid separation or geoid undulation, is the key factor to consider when assessing GPS altitude accuracy. This difference arises from the fact that GPS receivers provide altitude measurements relative to the ellipsoid, while many practical applications require elevations referenced to the geoid. To ensure accurate altitude measurements, especially in applications where orthometric heights are critical, you should apply the appropriate geoid separation correction to GPS-derived ellipsoidal heights. This correction accounts for the difference between the ellipsoid and the geoid and provides you with the orthometric height, which is the true elevation relative to mean sea level or another local reference. Using trig beacons as a reference point can help you assess the accuracy of GPS altitude measurements by comparing the GPS-derived ellipsoidal height with the known orthometric height. If the two values match closely after applying the geoid separation correction, it indicates that the GPS altitude accuracy is reliable for your specific application.
