The geoid is defined as:
Correct Answer: B. Zero elevation
📚 Detailed Explanation: Geoid Is the Surface of Zero Elevation
Why B (Surface of zero elevation) is correct: The geoid is the equipotential surface of the Earth's gravity field that corresponds to mean sea level (MSL). It is defined as the surface where the Reduced Level (RL) = 0. All elevations in surveying are measured above or below the geoid.
Geoid vs. Other Reference Surfaces
| Surface | Shape | Definition | Use in Surveying |
|---|---|---|---|
| Geoid | Irregular (lumpy) equipotential surface | Surface of zero gravitational potential; corresponds to MSL everywhere; RL = 0 on this surface | Reference for all elevation (height) measurements |
| Reference Ellipsoid (Spheroid) | Smooth mathematical ellipsoid of revolution | Best-fit geometric approximation of Earth's shape | Reference for horizontal (x, y) positions; GPS uses WGS-84 ellipsoid |
| Level surface | Any equipotential surface (not just geoid) | A surface at constant gravitational potential; perpendicular to gravity everywhere | The geoid is one particular level surface (the one at MSL) |
| Horizontal plane | Flat plane, tangent to geoid at one point | Local approximation; not a global surface | Used for small-area surveys where Earth curvature is negligible |
- Geoid = the equipotential surface coinciding with MSL; by definition, elevation = 0 on it.
- Geoid is irregular — it does not coincide perfectly with either the sea surface or the ellipsoid.
- Geoidal undulation = vertical separation between geoid and reference ellipsoid (up to ±100 m globally).