(r4) STC-1_33-Erratum-1 < IVOA

IVOA Web>IvoaDataModel>STC>STC-1_33-Errata>STC-1_33-Erratum-1 (revision 4) (raw view)~~EditAttach~~

---+ STC-1.33 Erratum 1:

Author: DM WG

Date last changed: 2018-11-16

<span style="background-color: transparent;">Date accepted:</span>
---++ Rationale

This erratum is to correct an error reported at the IVOA interop in Victoria, BC: May 2018

   * <a target="_blank" href="http://wiki.ivoa.net/internal/IVOA/InterOpMayy2018DAL/ivoa_201805_micol_polygons.pdf">POLYGON with different orientations issue</a>

<div title="Page 6">

cite:

<em>The formula in Section 4.5.1.4 (pg 30: Region.Polygon) states<br />"The summation is over determinants of matrices formed by the position vectors xi of successive vertices; xn+1 = x1. In spherical space (left-handed coordinates) the area is:</em><em><br />A = - SUM[ &alpha;(i) ] &ndash; (n-2) *pi</em><em>&alpha;i are the polygon&rsquo;s angles at the vertices. Reverse the sign for right-handed coordinates."<br /></em><em>but.<br />A = (RA,DEC)[0] = (90, 0)</em><em><br />B = (RA,DEC)[1] = (0, 0)</em>
<div title="Page 6"> _C = (RA,DEC)[2] = (0, 90)_ </div>

_The three angles of the polygon are all 90 deg, or pi/2 rad by construction. A in the above case is: -3 * pi/2 &ndash;pi = -5/2 pi_ <em>One expects an area of pi/2.<br />The correct formula is:</em><em><br />A = <strong>+</strong>SUM[ &alpha;(i) ] &ndash; (n-2) *pi</em>
</div>

---++ Erratum Content
<div title="Page 6">
---++++ Original Wording
   * 4.5.1.4 page 30 <br />A = - SUM[ &alpha;(i) ] &ndash; (n-2) *pi
</div>
---++++ New Wording
   * 4.5.1.4 page 30 <br />A = <span style="color: crimson;" class="WYSIWYG_COLOR">+</span> SUM[ &alpha;(i) ] &ndash; (n-2) *pi
---++ Impact Assessment

Software using the wrong formula must be updated
---++ Notes

   * We need to confirm that the example has considered the 'left-handed' vs 'right-handed' qualifiers from the text.

Topic revision: r4 - 2018-12-20 - LaurentMichel

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