Geoid versus Ellipsoid: The Basic Rundown!

Ellipsoids: A (Slightly) More Precise Representation of Earth's Surface Geoids: Unveiling the Less Appealing Reality of Our Planet Maintaining Data Consistency through the Application of Vertical Datums

In any worksite survey where vertical measurements are crucial, achieving precise local elevation calculations is imperative.

While it might be apparent for quarries and mining sites dealing with metrics like pit depths and shelf slopes, the significance remains equally high for landfills, where staying informed about cell volumes and remaining airspace is crucial for future planning.

Even in civil construction with smaller vertical measurements like cut/fill remainders and stockpile heights, having the most accurate local elevation data is essential for achieving precision. This not only ensures accuracy but also promotes smarter and more cost-efficient work.

To provide insight into how Spacesium utilizes coordinate reference systems and the knowledge of geodesy to deliver highly accurate vertical measurements, we aim to demystify two often perplexing concepts: ellipsoids and geoids.

Ellipsoids: a More Accurate Earth Model

Let's clear up a common misconception about our planet: it's not perfectly spherical. (And for the record, it's definitely not flat either.)

In reality, the Earth's shape is more like an ellipsoid or a spheroid. Unlike perfect spheres, ellipsoids are round and smooth but not symmetrical in all directions. Because the Earth's equator is about 42 miles (67 kilometers) longer than its meridians, it's not a perfect sphere.

Scientists have created various ellipsoidal models of the Earth over time, with the WGS84 being the most well-known. This model serves as the basis for the coordinate reference system used in GPS data, defining points on Earth through degrees of latitude and longitude.

Ellipsoidal models are especially useful for measuring distances across the Earth's surface when the difference between miles and kilometers matters, such as plotting flight paths or tracking continental drift over long periods.

Geoids: The Earth's Bumpy Reality

Adding more complexity, the Earth's surface isn't smooth like those idealized ellipsoidal models. Due to variations in density, gravity pulls or pushes the Earth's surface in different places, making our planet look more like a lumpy potato than an egg.

Models that represent this lumpy potato are called geoids. The surface of a geoid indicates a Mean Sea Level (MSL), essentially what the ocean's surface would look like without factors like tides and winds. The only influence on the MSL's shape is the Earth's gravity field.

Unlike ellipsoidal models, geoid models are more locally based. For example, surveyors in the United States currently use the North American Vertical Datum of 1988 (NAVD88), which is set to be replaced in 2022 by a newer GPS-derived model.

How We Keep Your Data Consistent with Vertical Datums

Vertical datums, like ellipsoid and geoid models, are crucial reference points for surveyors, helping determine elevations (both positive and negative). There are two types: tidal and geodetic datums. We'll focus on geodetic datums, as tidal datums relate to the ocean-land interface and are less relevant for most surveyors.

In surveying, even small measurements matter, so it's vital to stick to the same geodetic datums throughout a project. Switching models midway can lead to data discrepancies. If you have different coordinate reference systems and datums in your data sets, like a topographical survey and a design file, you'll need to transform one to match the other for accurate alignment.

Propeller offers a user-friendly coordinates converter for this purpose, aiding in establishing local grids specific to a site.

When converting elevation data, consider three types of height:

Ellipsoid height (h): The difference between the ellipsoid and a point on the Earth's surface. GPS data references the ellipsoid and needs transformation to match the more accurate geoid.

Geoid height (N): The offset value between the reference geoid and ellipsoid models.

Orthometric height (H): The distance between a point on Earth's surface and the geoid, representing Mean Sea Level. This is the height you care about when elevation data is described as "X feet above (or below) sea level."

To ensure consistent orthometric heights across your site, Spacesium uses your chosen datums and a simple formula: H = h – N. It's that straightforward! At Spacesium, we take pride in providing the most accurate drone survey data possible.

Feel free to reach out if you have any further questions. We have a complete guide to transform your heights! info@spacesium.com