Geoids vs. Ellipsoids: What’s the Difference?

For any worksite survey in which vertical measurement plays a significant role, being able to accurately calculate local elevation is critical. 

For quarries and mining sites, which deal with metrics like pit depths and shelf slopes, this may seem fairly obvious. The same goes for landfills, where staying up-to-date with cell volumes and remaining airspace is important for planning future work. 

But even for earthwork on civil construction sites, where vertical measurements are smaller in scale (cut/fill remainders, stockpile heights), having the best local elevation data is key to accuracy, and thus smarter, more cost-efficient work.  

visualization of elevation data for a landfill

To help you understand how Propeller uses coordinate reference systems and the science of geodesy to deliver highly accurate vertical measurements, we want to shed some light on a couple often confusing concepts: ellipsoids and geoids

Ellipsoids: a (slightly) more accurate model of the Earth

To start, let’s dispel with some conventional wisdom about our planet: it’s not spherical. (Don’t worry; your conspiracy theorist friends who say it’s flat aren’t right either.)

More accurately, the Earth is an ellipsoid, sometimes referred to as a spheroid. While ellipsoids are round and smooth like spheres, they are not symmetrical when divided in all directions. Because the circumference of the Earth’s equator is about 42mi (67km) longer than its meridians, the planet cannot be described as a perfect sphere.

Scientists have developed several ellipsoidal models of the Earth over the years, the most well-known being the one that serves as the basis for the WGS84 coordinate reference system. 

WGS84 is a geographic coordinate reference system, meaning it contextualizes a point on a 3D surface—in this case, the Earth—using degrees of latitude and longitude. If you’ve ever used GPS data, the coordinates were derived using WGS84.  

By themselves, ellipsoidal models are primarily used to measure distances across the surface of the Earth, and when miles and kilometers, rather than inches and centimeters, make the difference. Think charting a flight path or tracking continental drift over millennia.

Geoids: the uglier truth about our planet

Complicating things further, the Earth is not actually smooth like these idealized ellipsoidal models. Because the density of the planet is inconsistent, gravitational forces push out or pull in the surface in different places, resulting in an Earth that resembles a lumpy potato more so than an egg.

Models that approximate this lumpy potato we call home are called geoids. The surface of a geoid represents a Mean Sea Level (MSL), or a conjecture of the ocean’s surface if tides, wind, and some other factors that affect its movements didn’t exist. The only factor that affects the MSL’s shape is the earth’s gravitational field.

depiction of geoids

Unlike ellipsoidal models, geoids are locally based—or, at least, more local than the entire surface of the Earth. For instance, surveyors in the United States currently use the North American Vertical Datum of 1988 (NAVD88). 

That is, they will for the next couple of years. The National Geodetic Society is set to replace the NAVD88 in 2022 with a newer model derived using GPS, rather than physical survey marks like the current model does.

How we use vertical datums to keep your data consistent 

Both ellipsoid and geoid models (of which there are many) are examples of vertical datums. For surveyors, vertical datums serve as reference points from which elevation (positive altitudes and negative depressions) can be determined. 

There are actually two types of vertical datums: tidal and geodetic datums. For our purposes, let’s ignore tidal datums, which concern the interface between the ocean and land, and are therefore less applicable to most surveyors. 

Fractions of an inch matter in surveying, which is why it’s critical that surveyors use the same geodetic datums throughout the lifecycle of a project. Switching ellipsoid or geoid models midstream causes data discrepancies. 

If you have datasets that use different coordinate reference systems and datums (such as a topographical survey and a design file) you need to transform one to match the other, otherwise the measurements won’t line up. 

Propeller has developed an easy-to-use coordinates converter to help with this. It’s also a helpful tool for establishing local grids, or arbitrarily defined coordinate reference systems that are specific to one site.

diagram of vertical datums

When it comes to converting elevation data, there are three types of height to keep in mind:

  1. Ellipsoid height (h) is the difference between the ellipsoid and a point on the Earth’s surface. It is also called the geodetic height (not to be confused with geodetic datums). If you have coordinates that were captured with a GPS receiver, the elevation data reference the ellipsoid, meaning it has to be transformed to match the more accurate geoid instead.
  1. Geoid height (N) is the offset value between the reference geoid and the ellipsoid models.
  1. Orthometric height (H)—AKA the one you really care about— is the distance between a point on the Earth’s surface and the geoid. As we already discussed, the geoid represents Mean Sea Level. When you hear elevation data described as “X feet above (or below) sea level, that’s referring to orthometric height.

To deliver you consistent orthometric heights across your site, we use your chosen datums and this simple formula: H = h – N. Simple, right?

Propeller prides ourselves on our ability to provide the most accurate drone survey data possible. Talk to a member of our team today about getting Propeller on your sites.

Related Resources:

Understanding Coordinate Systems and Map Projections

Using Local Grid Coordinates on Surveys with Propeller

How Propeller Handles The World’s Many Coordinate Reference Systems

The New Survey Grade Accuracy Tool: Propeller PPK to AeroPoints

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