# STC-S系统描述的椭圆

GaussClumps是采用STC-S来描述椭圆的形状 关于STS-S可参考http://www.ivoa.net/Documents/Notes/STC-S/

<pos>中心位置
<posangle> 代表的是y轴正方向逆时针旋转到与长轴重合的角度（180≤posangle360）

4.4.1.2.5 Position Angles
Position angle definitions can be a great source of confusion. To reduce
ambiguity, we will attach a Reference attribute to position angles that may have
the values “North”, “X” (default), or “Y”. If the reference is “North”, position angles
will be counted from north through east; this should only be used with coordinate
systems where “North” has meaning. If the reference is “X” (the default), the
position angle will be counted from the positive first coordinate axis through the
positive second coordinate axis. If the reference is “Y”, the position angle will be
counted from the positive second coordinate axis through the positive first
coordinate axis. These definitions are independent of the handedness of the
coordinate system. For spherical systems, the longitude is considered the first
axis in this context, and latitude the second. Note that in most (though not all)
cases “North” is equivalent to “Y”.

4.5.1.3 Ellipse
The Ellipse (2-dimensional) is similar to the Circle but has, in addition, a minor
radius and a position angle. Position angles are measured following the definition
in Section 4.4.1.2.5 and refer to the first axis. The definition of an ellipse in a
Cartesian coordinate system is unambiguous, but this is not the case for
spherical coordinates. In a spherical coordinate system the ellipse shall be
defined as the intersection of an elliptical cone with the unit sphere, where the
axes and position angle describe the geometry of the cone.

ARD Descriptions描述的椭圆

ELLIPSE- A 2-dimensional ellipse. The argument listshould contain 5 values; the first pair give the user coordinates ofthe centre of the ellipse, the second pair give the half-lengths of thetwo axes of the ellipse (in user coordinates), and the fifth value givesthe angle (in degrees) from the first axis of the user coordinate systemto the first axis of the ellipse (positive rotation is from the first tothe second user axis). This keyword can only be used in 2-dimensional ARDdescriptions.

#   ARD description generated by ARDGEN
COFRAME(SKY,SYSTEM=FK5,EQUINOX=2000.0,EPOCH=2000.0)
ELLIPSE( 16:28:34.6, -24:13:41, 0:34:57, 0:09:35, -118.6131 )

#   ARD description generated by ARDGEN
COFRAME(SKY,SYSTEM=FK5,EQUINOX=2000.0,EPOCH=2000.0)
ELLIPSE( 16:26:40.4, -24:12:01, 0:33:33, 0:10:52, -62.57536 )

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