Have you ever heard the term Height Above Ellipsoid or HAE? It is a form of measurement common in GPS circles, but perhaps unknown for most lay people. Most people just use GPS for a horizontal location – standard latitude and longitude, or an x/y. That’s what we’re used to seeing on the maps on our phones. Yet tour GPS receiver might also display a height measurement, which sometimes appears in HAE. Understanding this vertical location measurement requires a little more information about how GPS systems produce their data.
The “ellipsoid” part of HAE refers to a mathematical model of the earth. (Remember that the Earth is not a sphere – it’s actually flatter at the poles, hence “ellipsoid”.) GPS scientists created these reference ellipsoids to represent a pristinely smooth version of the Earth’s surface. The ellipsoids used by GPS devices vary – most currently use a model called the WGS84 ellipsoid.
When GPS satellites transmit signals down to a receiver, the raw data they produce is your height above or below that theoretical ellipsoid. This is the HAE measurement.
Of course, the Earth isn’t a perfectly smooth ellipsoid. It has all sorts of geographical features. Even if you tried to measure height using sea level as a baseline, that wouldn’t work – the height of the world’s oceans varies quite a bit. To account for geographical features and allow conversion into more user friendly height measurements, GPS scientists defined a dataset known as geoid.
In a way, geoid is the opposite of that theoretical ellipsoid. It’s a hyperlocal survey of height measurements and a bunch of other factors, including geographical formations, the Earth’s gravity, and mean sea level (MSL), all of which are used to formulate height measurements for every location in a given area. Geoid datasets are maintained by individual countries, and are constantly updated with new survey data and research findings. For every location on earth, there is a geoid measurement which will tell you what geographical features are on that spot.
Neither the Height Above Ellipsoid (HAE) nor geoid are particularly useful for non-GPS types, however. In isolation, they don’t have a lot of value – they need context to be truly useful. It’s only when you convert HAE into what’s called an orthometric height by combining it with geoid measurements that you can tell how high up you actually are.
If you take your height above the theoretical shape of the earth (HAE) and subtract the geoid height, you’ll get the orthometric height. This is what your device displays.
How NextNav calculates HAE
Like the GPS satellites which stay in the same geosynchronous orbits, NextNav’s terrestrial sensors are in fixed, surveyed locations. This allows NextNav to calculate the location of a device against that same theoretical ellipsoid with the same degree of accuracy that GPS would.
In addition to Height Above Ellipsoid, NextNav also calculates a different metric we call Height Above Terrain. Understanding this data – and how it can be used – is important in developing z-axis, or vertical location data, which is what our NextNav Pinnacle service does.
Learn more about NextNav Pinnacle, our vertical location service.