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 2018Erratum Content
The formula in Section 4.5.1.4 (pg 30: Region.Polygon) states:
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
"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
Impact Assessment
Notes
- We need to confirm that the example has considered the 'left-handed' vs 'right-handed' qualifiers from the text.
Topic revision: r2 - 2018-11-18 - MarkCresitelloDittmar
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