el2cgv_c

 Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input Detailed_Output Parameters Exceptions Files Particulars Examples Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

```   void el2cgv_c ( ConstSpiceEllipse   * ellipse,
SpiceDouble           center,
SpiceDouble           smajor,
SpiceDouble           sminor  )

```

#### Abstract

```
Convert a CSPICE ellipse to a center vector and two generating
vectors.  The selected generating vectors are semi-axes of the
ellipse.
```

```
ELLIPSES
```

```
ELLIPSE
GEOMETRY

```

#### Brief_I/O

```
Variable  I/O  Description
--------  ---  --------------------------------------------------
ellipse    I   A CSPICE ellipse.
center,
smajor,
sminor     O   Center and semi-axes of ellipse.
```

#### Detailed_Input

```
ellipse        is a CSPICE ellipse.
```

#### Detailed_Output

```
center,
smajor,
sminor         are, respectively, a center vector, a semi-major
axis vector, and a semi-minor axis vector that
generate the input ellipse.  This ellipse is the
set of points

center + cos(theta) smajor + sin(theta) sminor

where theta ranges over the interval (-pi, pi].
```

```
None.
```

```
Error free.
```

```
None.
```

#### Particulars

```
CSPICE ellipses serve to simplify calling sequences and reduce
the chance for error in declaring and describing argument lists
involving ellipses.

The set of ellipse conversion routines is

cgv2el_c ( Center and generating vectors to ellipse )
el2cgv_c ( Ellipse to center and generating vectors )

A word about the output of this routine:   the semi-major axis of
an ellipse is a vector of largest possible magnitude in the set

cos(theta) vec1  +  sin(theta) vec2,

where theta is in the interval (-pi, pi].  There are two such
vectors; they are additive inverses of each other. The semi-minor
axis is an analogous vector of smallest possible magnitude.  The
semi-major and semi-minor axes are orthogonal to each other.  If
smajor and sminor are choices of semi-major and semi-minor axes,
then the input ellipse can also be represented as the set of
points

center + cos(theta) smajor + sin(theta) sminor

where theta ranges over the interval (-pi, pi].

```

#### Examples

```
1)  Find the semi-axes of the limb of an ellipsoid.

#include "SpiceUsr.h"
.
.
.
/.
Our viewing location is viewpt.  The radii of the
ellipsoid are a, b, and c.
./
edlimb_c ( a, b, c, viewpt, &limb );

el2cgv_c ( &limb, center, smajor, sminor );
```

```
None.
```

```
None.
```

#### Author_and_Institution

```
N.J. Bachman   (JPL)
```

#### Version

```
-CSPICE Version 1.0.0, 12-JUN-1999 (NJB)
```

#### Index_Entries

```
ellipse to center and generating vectors
```
`Wed Apr  5 17:54:34 2017`