TWiki> IVOA Web>IvoaDataModel>STC>STC-1_33-Errata>STC-1_33-Erratum-1 (2018-11-18, MarkCresitelloDittmar) EditAttach

Author: DM WG

Date last changed: 2018-11-16

Date accepted:

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

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 x*i* of successive vertices; x*n+1* = x*1*. In spherical space (left-handed coordinates) the area is:

A = - SUM[ α(i) ] – (n-2) *pi

α*i* 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

- 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: r2 - 2018-11-18 - MarkCresitelloDittmar

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