Ellipsoids

For maps of the Earth where accuracy is not of particular concern, we can safely assume that the Earth is perfectly spherical in shape. However, the Earth is actually somewhat ellipsoidal (or egg-shaped), approximately 1/300 wider than it is tall, assuming a vertical orientation with respect to the axis of rotation. This shape needs to be taken into account to produce maps of any significant accuracy.

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This diagram shows the difference between an

ellipsoid (solid line) and a sphere (dashed line).

While an ellipsoid is a closer approximation of the Earth's shape than a sphere, the Earth's surface is not uniform in curvature, so any ellipsoidal representation of the Earth is still only an approximation. This being the case, cartographers have historically used a number of slightly different ellipsoidal representations in attempts to produce more accurate maps of different regions of the Earth.

You can define the ellipsoids with Map | Change Projection .

To access the ellipsoid options:

Click on one of the Predefined systems in the Select a Projected Coordinate System list.
Click the Modify button.
In the Define Coordinate System dialog, select one of the predefined or <custom> datum.
Set the Ellipsoid options and click OK in both dialogs.

Ellipsoid Definition

You can choose an ellipsoid type from the Ellipsoid drop-down list. If you select <custom>, set the next parameters in the Ellipsoid section. The Semimajor Axis should be set in meters. Next, set either the Semiminor Axis in meters or the Inverse Flattening (1/f) ratio value. If you do not understand ellipsoids and datum definitions, please use the defaults. For more information on these subjects, see the references provided.