Difference: STC-1_33-Erratum-1 (1 vs. 2)

Revision 22018-11-18 - MarkCresitelloDittmar

 
META TOPICPARENT name="STC-1_33-Errata"

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

Added:
>
>
 

Erratum Content

Changed:
<
<		The formula in 4.5.1.4 states:
>
>		The formula in Section 4.5.1.4 (pg 30: Region.Polygon) states:
Added:
>
>
 
Changed:
<
<		 “A = - SUM[ A(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:
 
Changed:
<
<		but:
A = (RA,DEC)[0] = (90, 0)
>
>		A = - SUM[ α(i) ] – (n-2) *pi
 
Changed:
<
<		 B = (RA,DEC)[1] = (0, 0)
>
>
 
Changed:
<
<		 C = (RA,DEC)[2] = (0, 90)
>
>		αi are the polygon’s angles at the vertices. Reverse the sign for right-handed coordinates."
 
Changed:
<
<		The three angles of the polygon are all 90 deg, or pi/2 rad by construction.
>
>
 
Changed:
<
<		A in the above case is: -3 * pi/2 –pi = -5/2 pi
>
>		but..
 
Changed:
<
<		One expects an area of pi/2.
>
>
A = (RA,DEC)[0] = (90, 0)
 
Added:
>
>		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:
Changed:
<
<		 A = +SUM[ A(i) ] – (n-2) *pi
>
>		A = +SUM[ α(i) ] – (n-2) *pi
 
Added:
>
>
 

Impact Assessment

Notes

Added:
>
>
  • We need to confirm that the example has considered the 'left-handed' vs 'right-handed' qualifiers from the text.
 

Revision 12018-11-16 - MarkCresitelloDittmar

 
META TOPICPARENT name="STC-1_33-Errata"

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

Erratum Content

The formula in 4.5.1.4 states:

“A = - SUM[ A(i) ] – (n-2) *pi“

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[ A(i) ] – (n-2) *pi

Impact Assessment

Notes

View topic | History: r7 < r6 < r5 < r4 | More topic actions...
 
This site is powered by the TWiki collaboration platform Powered by Perl This site is powered by the TWiki collaboration platformCopyright © 2008-2025 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback