Customize menu > Preferences > Preferences Settings dialog > GCS panel > Global Coordinate System group > Define > Click Define or Modify. > Non-Geodetic > Define > Ellipsoid Manger > Define
The Define Ellipsoid dialog enables you to create an ellipsoid.
Code—Specifies a code. The code must be unique within a category and across all coordinate systems, must comprise of 23 characters or less, and must not contain spaces. Only the following characters are valid: A-Z (upper or lower case), 0-9, dash, and underscore.
Description—Specifies a description. This description, not the code, appears in all list boxes, so be sure it is specific and descriptive.
Calculation Method—Specifies a calculation method, which determines which two of the four options on the right of this group are enabled.
Equatorial Radius—Specifies the equatorial radius in meters.
Polar Radius—Specifies the polar radius in meters.
Flattening Ratio—Specifies the denominator of the flattening ratio. That is, if the flattening ration is 1/x, specifies x.
Eccentricity—Specifies eccentricity as a real number.
OK—Closes the dialog and saves any changes you’ve made.
Cancel—Closes the dialog without saving any changes you have made.