STC-1.33 Erratum 1:

Author: DM WG

Date last changed: 2018-11-16

Date accepted:

Rationale

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

cite:

The formula in Section 4.5.1.4 (pg 30: Region.Polygon) states
"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:

A = - SUM[ α(i) ] – (n-2) *pi
αi are the polygon’s angles at the vertices. Reverse the sign for right-handed coordinates."but.
A = (RA,DEC)[0] = (90, 0)

B = (RA,DEC)[1] = (0, 0)
C = (RA,DEC)[2] = (0, 90)

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 –pi = -5/2 pi
One expects an area of pi/2.
The correct formula is:

A = +SUM[ α(i) ] – (n-2) *pi

Erratum Content

Original Wording

  • 4.5.1.4 page 30
    A = - SUM[ α(i) ] – (n-2) *pi

New Wording

  • 4.5.1.4 page 30
    A = + SUM[ α(i) ] – (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.
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Topic revision: r3 - 2018-12-19 - LaurentMichel
 
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