Caliper Mapping and Transportation Glossary

DEFINITION

What is an Ellipsoid?

Ever since it was discovered that the Earth was round, spherical globes have been used to represent the Earth. Unfortunately, the Earth is not really a perfect sphere; it bulges out around the equator and its poles are slightly flattened. A more accurate, but still imperfect, represen­tation of the Earth’s shape is an ellipsoid, which is an ellipse rotated around its shorter axis.

Different reference ellipsoids have been used for surveying various regions of the world, such as the Clarke 1866 ellipsoid for North America and the Airy ellipsoid for Great Britain. Until recently, the ellipsoids were often chosen to give a "best fit" for a particular region, with little concern for how well they represented the remainder of the Earth. This minimized the local positional errors but led to the creation of incompatible maps that are difficult to stitch together into a worldwide geographical dataset.

In the later part of the twentieth century, new geocentric ellipsoids (e.g. GRS80, WGS72, and WGS84) were defined to make worldwide mapping feasible. The new ellipsoids are centered at the Earth’s center and attempt to minimize the overall distortion for the entire planet. The resulting model is suitable for worldwide mapping, but creates slight positional errors in certain regions.

What is an ellipsoid: image of sphere and exaggerated ellipsoid

Mapping GIS Software

Maptitude Mapping Software gives you all of the tools, maps, and data you need to analyze and understand how geography affects you and your business. Many of the coordinate systems supported by Maptitude specify a particular ellipsoid in their defini­tion. For example, the NAD27 State Planes use the Clarke 1866 ellipsoid and the NAD83 State Planes use the GRS80 ellipsoid. General worldwide coordinate systems such as Universal Transverse Merca­tor (UTM) and Gauss-Krüger are defined independent of the ellipsoid. For these coordinate systems, Maptitude lets you choose the proper ellipsoid.

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