Mean Sea Level

Mean sea level is the average level of the ocean surface halfway between the highest and lowest levels recorded. We use mean sea level as a plane upon which we can reference or describe the heights of features on, above or below the ground.

Elevations

The height of a feature above mean sea level is called an elevation. We use the elevation of features in our everyday lives for example to build roads and in all types of construction. Also, since water flows downhill, we need to know where the water will flow, for instance when draining water away from your house or for sewer systems and water mains. Well, you get the idea.

Elevations are measured by one of two methods, either by using a survey technique called "levelling" or by using a Global Positioning System (GPS) receiver.