Tech Chat | What’s the difference between geodetic height, normal height and ortho-height?

In our daily surveying tasks, elevation is important in construction projects and other applications. In the elevation system, the most commonly used are orthometric height, normal height and geodetic height. What exactly do they refer to and what is the difference between them?

In this blog, we will introduce how these three elevations are defined and their differences and main applications.

What is Elevation?

Elevation is simply the distance from a point to the datum surface, and different elevations correspond to different datum surfaces (geoid, quasi-geoid, ellipsoid, etc.).

Different Datum Surface

Reference ellipsoid: A reference ellipsoid is a mathematically defined surface that approximates the geoid, which is close to the size and shape of the earth. For example, WGS-84 is a commonly used reference ellipsoid in daily surveying.

Geoid: The geoid is the shape of the ocean surface under the influence of the Earth's gravitational pull (both gravity and the Earth's rotation), extending into the interior of the continents.

Quasi-geoid: The quasi-geoid refers to the closed curve formed by measuring the normal height of the end point from the ground point along the normal gravity line. Strictly speaking, it is not a level surface, but close to the level surface, only for the calculation of the auxiliary surface.

Since the geoid and quasi-geoid are irregular, in daily RTK surveys and other applications, we often use the ellipsoid to calculate coordinates. However, in order to better fit the actual terrain, the geoid will be introduced to convert the elevation after the calculation.

Geodetic Height, Normal Height and Ortho-height

Geodetic height (H): The distance from a point on the earth to the reference ellipsoid along the normal direction passing through the point, such as height measured by the GNSS system. 

Orthometric height: The distance from a point on the earth to the geoid along the vertical direction of the point, also known as altitude, absolute elevation. 

Normal height: The distance from a point on the earth to a quasi-geoid along a vertical line passing through the point.

Relationship:

Geodetic height = normal height + N 

Geoid difference (N): the elevation difference between the geoid and the reference ellipsoid 

Geodetic height = normal height + ξ 

Elevation anomaly (ξ): the elevation difference between quasi-geoid and reference ellipsoid.

Nowadays, we often use GNSS RTK receivers for field surveying, and what we get is the geodetic height of the survey points. But sometimes due to project requirements, we need normal height that is more suitable for the local terrain, how can we easily convert it? We will introduce related content in the next blog.

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