‹ back to GIS Glossary

Geoid Modeling

Definition

Geoid modeling is the creation of a mathematical surface that represents the Earth's gravitational equipotential that best approximates mean sea level. The geoid is irregular because gravity varies with mass distribution. A geoid model provides separations between the reference ellipsoid and the geoid, often called geoid undulations, which are needed to convert GPS ellipsoidal heights to orthometric heights used on engineering drawings and flood maps. Modern models blend terrestrial gravity, GNSS and leveling, satellite missions, and digital terrain to achieve centimeter level accuracy over many regions. Choosing the correct model and epoch is critical for consistent vertical datums.

Application

Surveyors and engineers use geoid models to compute elevations for construction, hydrology, and transportation design. National mapping agencies publish official geoid models that define vertical datums for legal and scientific work. Flood studies, dam monitoring, and subsidence detection rely on accurate conversions between ellipsoidal and orthometric heights. GIS teams embed geoid models in transformation pipelines so 3D data aligns vertically across projects and time.

FAQ

What is geoid modeling and why is it essential for converting GPS heights to elevations used in design?

Geoid modeling provides the link between the smooth reference ellipsoid used by GNSS and the physical mean sea level surface. GPS reports ellipsoidal heights, but engineers need orthometric heights. A geoid model supplies the separation required for conversion, which makes vertical coordinates comparable across surveys and compatible with hydraulic and structural calculations.

How are modern geoid models built and how do you apply them correctly in GIS workflows?

Agencies integrate gravity observations, GNSS and leveling networks, satellite gravity fields, and terrain corrections to solve for the geoid over a region. In GIS or survey software, select the official model and correct epoch, then transform ellipsoidal heights to orthometric before analysis. Document the model name and version in metadata so others can reproduce the numbers.

What pitfalls occur when the wrong geoid model or epoch is used and how can you avoid them?

Using an outdated or incorrect model can shift elevations by several decimeters, enough to break design tolerances or flood studies. Confusing quasi geoid with geoid can also introduce bias. Avoid errors by following national guidance, controlling versions in templates, and running spot checks against known benchmarks before publishing results.

Where does geoid modeling provide clear value for infrastructure and public safety?

Accurate geoid use prevents elevation mismatches between contractors, reduces change orders on highways and bridges, and ensures floodplain maps align with true water levels. Water utilities depend on consistent elevations for pressure modeling. Emergency managers need reliable vertical references for storm surge scenarios and evacuation planning.