GIS Files 2: Geographical data

2.3: Position matters (3)

The third dimension: height

Height can be expressed in two ways:

The first, and by far the most obvious, is to measure heights from sea level. However, this can be difficult as the sea level is irregular and constantly changing, making measurements of height inland both complex and expensive.

The second was created specifically because of these drawbacks. Scientists invented a more regular surface called an ellipsoid, which approximates to sea level. However, because the shape of sea level is complex, hundreds of different ellipsoids are required depending on the area of earth being modeled. Within Great Britain the ellipsoid of choice is know as Airy 1830 and this is used for the National Grid projection.

Global, regional and national systems

Many national mapping agencies have defined local referencing systems to meet their needs. In Britain, for example, we have the National Grid. These are perfectly acceptable when working within a country, but problems exist in drawing up multinational data unless there is cross-border convergence. In Europe local coordinate projections are often referred to the European Terrestrial Reference System 1989 (EtrS89) because it is fixed at a point in time and well defined.

Probably the most common global coordinate system in use today is the GPS-based World Geodetic System 1984 (WGS84). This is fixed to points on the earth's surface which move over time because of changes in the earth's crust. A significant global projection system is Universal Transverse Mercator (UTM). This is a defined set of projections that cover the whole world and allow countries to share spatial data more easily.

Once the digital data has been georeferenced, you can display the information in an infinite number of ways. See section 2.4 – What does GIS data look like?

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