Select Ellipsoid

For many maps it is assumed that the Earth is a sphere. However, because of the Earth’s rotation, the shape of the Earth is not a perfect sphere. Actually, the Earth is slightly flattened towards the poles: the equatorial axis (line from the center to the equator) is longer than the polar axis. The Earth's shape can better be represented by an ellipsoid. Over the years, various different ellipsoids are calculated. Variations in calculated ellipsoids are due to the irregularities in the surface of the Earth. The choice of the ellipsoid which fits best a certain region of the Earth surface to be mapped, depends on the surface curvature and undulations in that region. Hence every country has its own 'best fit' ellipsoid.

For large scale topographic maps such an ellipsoid must be chosen. For a number of projections available in ILWIS, an ellipsoid can be selected. Spherical projection algorithms are provided for certain less current projections and projections for global atlas maps. In addition, with small scale maps, the flattening is negligible and a spherical model is preferred.

This dialog box appears:

• when you click the Ellipsoid button in the Edit Coordinate System Projection dialog box, or
• when you click the Ellipsoid button in the Edit Coordinate System LatLon dialog box.

Dialog box options:

Next dialog boxes:

• When you clicked the Datum button in the Edit Coordinate System Projection dialog box, that dialog box will reappear; you will need to fill out the remaining projection parameters.
• When you clicked the Datum button in the Edit Coordinate System LatLon dialog box, that dialog box will reappear.

Additional information

Available ellipsoids:

Available ellipsoids are listed in the ellips.def file in the ILWIS system directory.

The ellipsoids, are sometimes called spheroids (shapes that are generated by revolving an ellips around its minor axis). A spheroid by this definition has two different semi-axes a and b, where a is the radius of the equator circle and b is the half axis of rotation (b < a).

The flattening f and eccentricity e of the ellipsoid are defined by respectively:

f = (a - b) / (a)

e * e = (a * a - b * b) / (a * a)

It follows that the ellipsoid shape is also completely defined by a and f or by a and e. You are free to define your own pair of ellipsoid constants a and f. However, most (about 20) ellipsoid shapes are predefined and selectable in ILWIS. They are stored in the text file ellips.def. The file lists the shape of the defined ellipsoids, expressed in the length of the equatorial axis (a) in meters and the inverse flattening (1/f).

If no ellipsoid is known, the choice by default is a sphere with a = b = 6371007.0 m. A sphere with this radius has an area equal to that of the WGS84 ellipsoid.

See also:

ILWIS objects : coordinate systems

Edit Coordinate System Projection (dialog box)

Select Projection (dialog box)

Select Datum (dialog box)

Edit Coordinate System LatLon (dialog box)