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?
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.
Regards
Jeff
-- Jeff Lusted tel: +44 (0)116 252 3581 Astrogrid Project mob: +44 (0)7973 492290 Dept Physics & Astronomy email: jl99-at-star.le.ac.uk University of Leicester web: http://www.astrogrid.orgReceived on 2008-05-30Z09:38:00