The easy explanation is this - by using the geoid and modern technology, we can determine elevations faster and cheaper, whether far apart or close together, in any kind of weather, day or night, 365 days a year. As result, for example, construction projects can be completed faster and for less cost - so, we all benefit.

Levelling versus GPS

Levelling has been used to determine elevations for hundreds of years. It can be very accurate, but it's also takes a long time and needs several people to do the job. Poor weather and rough terrain cause more problems. This makes levelling expensive. However, until recently, it was the only truly reliable method.

Today, the Global Positioning System (GPS) can be used to quickly and easily determine very accurate positions, including elevations. We don't need to worry about the distance from the last point where we got an elevation, the type of the terrain or whether or not the weather is bad.

That is the good news. So what about the "not so good" news...

Challenge to Using GPS Heights

The first challenge - GPS heights are not linked in any way to mean sea level! GPS heights are referenced to a mathematical model that does not physically exist. This model, the ellipsoid, does not agree with mean sea level. That means the height of a point determined from GPS is not the same as its sea level elevation as determined by levelling.

The second challenge - mean sea level is not a constant surface. There are variations in the surface as a result of number of different factors including variations in the earth's gravitational field. Water flows down hill right? It seeks it's own level. Therefore differences in the gravitational field will affect the way water flows which in turn affects the mean sea level surface.

So, how are we supposed to use this surface as a reference?

That is where the geoid comes in...