Orthometric Height Definition
Orthometric height represents the vertical distance from a point on the Earth's surface to the geoid, measured perpendicular to the direction of gravity. This measurement is fundamental in surveying and geodesy, as it accounts for the actual gravitational pull on objects rather than geometric distance from a mathematical reference surface.
Unlike ellipsoidal height, which measures distance from the WGS84 ellipsoid, orthometric height provides a practical measure of elevation that reflects how water flows and how gravity affects physical objects. The relationship between these two measurements is expressed through the geoid undulation (N), where: Ellipsoidal Height = Orthometric Height + Geoid Undulation.
Technical Details and Calculation
Geoid and Gravity Relationship
The geoid is an equipotential surface defined by mean sea level extended beneath continents. Orthometric heights are measured along plumb lines—the paths that gravity actually follows. This makes orthometric height the most physically meaningful elevation measurement for engineering and civil works.
Calculating orthometric height requires:
Difference from Ellipsoidal Height
Ellipsoidal height measures distance from the WGS84 reference ellipsoid, while orthometric height measures from the geoid. The difference can vary significantly by location—sometimes exceeding 100 meters. In mountainous regions and over ocean areas, geoid undulation variations are substantial, making the distinction critical for accurate surveying work.
Surveying Applications
Engineering Projects
Orthometric heights are essential for:
Geodetic Networks
National datum systems and control networks primarily use orthometric heights for vertical reference. Surveyors establishing benchmarks and control points must convert GNSS ellipsoidal heights to orthometric heights using local geoid models for consistency with national standards.
Related Surveying Instruments
[GNSS Receivers](/instruments/gnss-receiver) provide ellipsoidal heights that must be converted to orthometric values using appropriate geoid models. [Total Stations](/instruments/total-station) measure relative heights that can be adjusted to orthometric references through known control points. Modern instruments from manufacturers like [Leica Geosystems](/companies/leica-geosystems) include software for automatic conversion between height systems.
Practical Examples
Bridge Construction
When constructing a bridge across a valley, engineers need orthometric heights to ensure proper water flow under the structure and correct slope calculations. GNSS measurements providing ellipsoidal heights must be converted using the regional geoid model before determining drainage and foundation elevations.
Cadastral Surveying
In property boundary surveys, orthometric heights establish land boundaries and property elevations. These heights ensure that boundary markers reflect true vertical positions and that property descriptions match the gravitational reality of the landscape.
Flood Management
Flood risk assessment and water management systems rely exclusively on orthometric heights. Water naturally flows along equipotential surfaces defined by gravity, making orthometric heights the only meaningful reference for flood modeling and prevention infrastructure.
Modern Standards and Best Practices
Professional surveyors must:
The evolution from classical leveling to GNSS-based orthometric height determination has improved accuracy and efficiency, though both methods remain important in modern surveying practice.
Conclusion
Orthometric height remains the most physically relevant elevation measurement in surveying, providing the vertical reference system that gravity and water actually follow. Understanding the distinction between orthometric and ellipsoidal heights, combined with proper geoid modeling, ensures accurate results for all surveying applications.