Hi Jeff,
>
>Hi colleagues!
>
>We agreed to substitute BOX for RECTANGLE. It was closer to STC. I
>thought ELLIPSE was agreed to as well, but I've no qualms about omitting
>it. However, it would be good to understand the various arguments around
>the two constructs, and I need some help elucidating Box.
>
>On Wed, 2008-05-28 at 19:28 +0200, Francois Ochsenbein wrote:
>> 1. for a BOX made of great circles, the length of a side
>> is not identical to the corresponding width or height:
>> -- the width is the length of the great circle oriented
>> E-W which includes the center of the box
>> -- the side is of the box is generally smaller.
>> For instance, if the width and height are 90 degrees,
>> the side is only cos^-1(1/3) =~ 70.5 degrees.
>> And if the width is 180deg, the NS sides have zero length
>> (is not a box any more...)
>
>I can change the definition of box to...
>
><box> ::=
> BOX <left_paren> <coord_sys>
> <comma> <coordinates>
> <comma> <numeric_value_expression>
> <comma> <numeric_value_expression>
> <right_paren>
>
>without mentioning sides. (I still think some intermediate definition
>would make it more readable, like <size1> and <size2>).
>
>STC defines box as :
>
>"A Box is a special case of a Polygon, defined purely for convenience.
>It is specified by a center position and size (in both coordinates)
>defining a cross centered on the center position and with arms
>extending, parallel to the coordinate axes at the center position, for
>half the respective sizes on either side. The box’s sides are line
>segments or great circles intersecting the arms of the cross in its end
>points at right angles with the arms."
>
>If I appended this to the definition, or some paraphrase and place a
>reference to STC, would that allay Francois' points. Is it sufficient?
==> I think both definitions agree -- "width" and "height" are other
words for what Arnold names "arms" . My point was just that the "side" of a box is generally understood as the line joining 2 corners -- and the spherical distance of such a side differs from the spherical distance measured along the "arm". Therefore I would agree if you make a reference to STC. BUT could you please add the units (arguments in degrees?) and the limitations (each <numeric_value_expression> must not be between 0 and 180degrees) and maybe the remark that the box is reduced to 2 sides when any of the arguments is 180deg (when both arguments are 180deg, it's an hemisphere)
>
>Ellipse...
>
>On Wed, 2008-05-28 at 07:32 -0400, Alex Szalay wrote:
>> I do not believe that ELLIPSE is a well defined concept. It only exists in
>> the tangent plane. Given a center, minor axis, major axis, this covers
>> Different pixels in different projections, thus defining it as it is
>> meaningless. Then we need also the WCS of the projection, like those of a
>> FITS image. Then ELLIPSE becomes well defined WITHIN THAT CONTEXT.
>>
>> Or, we can conceivably define it as a cone whose aspect ratio is warped, but
>> then it is not an ellipse on a TAN projection in general. So, something must
>> be sacrificed. I do not think this has been thought through, so I would like
>> to vote against using ELLIPSE altogether in a generic sense until we
>> clarified these issues.
>
>I defined ellipse as
>
><ellipse> ::=
> ELLIPSE <left_paren> <coord_sys>
> <comma> <coordinates>
> <comma> <radius>
> <comma> <minor_radius>
> <comma> <position_angle>
> <right_paren>
>
>
>In its textual spec, STC defines ellipse as
>
>"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."
>
>So I assume the problem with my definition was the absence of rigour
>with respect to <position_angle>. Is that correct?
>
>Well, I'll drop Ellipse from the diagrams unless someone shouts really
>loud. It can wait for another day. And I'll improve Box, but please...
>give me feedback.
>
===> ELLIPSE is important, and it would be a pity to ignore it.
Again the spherical ellipse is a well-defined figure -- I don't understand the STC terms of ambiguity neither Alex's terms of a non-well defined concept. Unlss I missed anything ?
The definition as
ELLIPSE <left_paren> <coord_sys>
<comma> <coordinates>
<comma> <major_radius>
<comma> <minor_radius>
<comma> <position_angle>
Merci, Francois
Francois Ochsenbein ------ Observatoire Astronomique de Strasbourg 11, rue de l'Universite F-67000 STRASBOURG Phone: +33-(0)390 24 24 29 Email: francois-at-astro.u-strasbg.fr (France) Fax: +33-(0)390 24 24 32 ================================================================================Received on 2008-05-30Z15:47:12