Tutorial: The Shape of Earth and Reference Ellipsoids

### Elevation and the Ellipsoid

### The Shape of the Earth: An Introduction to the Ellipsoid

### The GPS Receiver and the Reference Ellipsoid Model

### Models and Their Usefulness in Science

#### Accuracy and Using Averages to Increase Accuracy

GPS receivers use a smoothed model of sea level to calculate elevation. The model is called a ** reference ellipsoid**. This means that the elevation reading from the GPS receiver is not the vertical distance from true sea level, but rather the distance from the

The Earth is not a perfect sphere, but is instead more like a slightly squashed sphere called an ** ellipsoid**. An

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A GPS receiver gives latitude, longitude, and elevation. Let’s focus specifically on how a GPS receiver calculates elevation. The information required is:

- The
of at least four satellites in space*location* - The current
*time* - The
for the entire Earth*reference ellipsoidal shape of sea level*

There are many different ** reference ellipsoids**, but all GPS receivers use the same one, and it is called the

- The distance from the North Pole to the Center
- The distance from any point on the Equator to the Center

Notice that the **Semi Major Axis (a)** is LONGER than the **Semi Minor Axis (b)**, Physically, this means the distance from side to side on the Earth is LONGER than from top to bottom, just like the squashed basketball. The lengths shown in the image define the **exact** shape of this ellipsoid, and are used by the GPS receiver to help it calculate elevation.

So, if a GPS receiver reports an elevation of 1600 meters, what that means is that you are 1600 meters above the reference ellipsoid.

The ** reference ellipsoid** is a simplified model of the world around us. It is a smoothed mathematical representation of Earth’s sea level surface and ignores the effects of tides, seasonal currents, and waves. On land, the reference ellipsoid forms the mathematical surface that would be taken by sea level if the land areas were crisscrossed by canals. Why, then, would a model be useful in science? In the case presented here, a model (the

You may now be wondering, “If the GPS receiver is using a ** model** that only approximates sea level to determine elevation, what effect does the shape of the

Since a GPS receiver uses the ** reference ellipsoid**, and not true sea level, to calculate elevation, then the elevation measurement is sometimes less accurate. We say “sometimes,” because there are times when the

How can you increase accuracy? Let's say you took GPS measurements for a study site and you have taken 15 readings, 1 each minute for 15 minutes. As you did this exercise, the GPS readings probably seemed to wander, and so you used an average of all 15 measurements to get the position. This averaging increased the accuracy of your GPS coordinates from 100 meters to 30 meters.

Now we know the GPS receiver uses a ** model** to figure out your elevation. To learn more about what using this

Last modified: Tuesday, 08-May-2018 03:02:05 UTC

2018