Re: [Qgis-user] about some ellipsoïdal distances : I fully agree . just may be better to use symbol rf (or something else) for inverse flattening ...

Sat Apr 7 03:39:17 PDT 2012

You're right. I was right thinking about it yesterday while committing the
changes.
*f *is misleading, and it should be substituded by *rf* in my formula.

Thanks for having reported your measurements. They helped a lot to figure
out the bug ;)
giovanni

2012/4/6 blavet <didier.blavet at ird.fr>

> **
> *Good evening*
>
> Of course thank you again i fully agree with what you found which is
> mathematically correct ,
>  it was not easy to find, and is essential for debugging the calculation
> of ellipsoîdal distances
>
> I just had a quick look at the code found at
> http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540
>
> i'm not an informatician so i can just say few words
>
> inverse flattening seems correctly defined in line 148 as
> mInvFlattening = mSemiMajor / ( mSemiMajor - mSemiMinor )
>
> therefater the bug that you found could be in line 153 (and may be in
> some other lines of the code, i cannot say)
>
> Therafter, it is just a detail (and a convention) but
> just to try to help to avoid a potential confusion in the future code
> maintenance. ,
> about the letter "f" used in your message about "inverse flattening" i
> think that it would may be  better to use another letter than f  for
> designation of inverse flattening ... I say this   just because  in line
> 550 of the code found today at
> http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540the letter f was already assigned as f = 1/mInvFlattening, indicating that
> letter f in this code is already reserved for ... flattening)
>
> So of course and again what you said is perfecly correct from a
> mathematical point of view (as long as you say, by convention,  that f =
> inverse flattening) ,
> Therefore the demonstration  would may be a bit clearer just with
> replacing this letter f by some other symbol or combination of symbols for
> the designation of inverse flattening
> (as this f is already used for something else in the code).
> Would it be possible for instance to use the symbolic combination *rf*,
> used in proj4  (for "reciprocal flattening" which is, as far as i know,  in
> english, equivalent to inverse flattening)
>
> then this would give, from what you found this morning , and with exactly
> the same meaning that what you said :
>
> "
> *In Qgis the semi-minor axis of the ellipsoid is calculated with:
>      b = a - (rf/a)
>      where
>      b = semi-minor axis
>      a = semi-major axis
>      rf =  inverse flattening (= "reciprocal flattening" = a/(a-b)  )
>
>      while it should be:
>      b = a - (a/rf)*
> "
>
> therefore, this rf symbol would also fit with the +rf proj4 parameter
> defined as
> "+rf       reciprocal of the ellipsoid flattening term (e.g. 298)"
> (also in proj4 +f is used for flattening
> +f         Flattening of the ellipsoid (often presented as an inverse,
> e.g. 1/298)
>
> (by the way , I guess, however, that the adjective reciprocal for
> "reciprocal numbers" can have also some other meanings for the pure
> mathematicians ..
> ... Therefore it would also be possible to use another symbol for inverse
> flattening than rf  (but preferentially not f (:-))
>
> *Thank you again
>
> Best regards
> *
> Le 06/04/2012 14:15, G. Allegri a écrit :
>
> Not at all.
> The change has already been committed to the development version.
>
>  giovanni
>
> 2012/4/6 blavet <didier.blavet at ird.fr>
>
>>  Thank you very much for this very fast and efficient diagnosis
>>
>> Best regards
>>
>> Didier Blavet
>>
>> Le 06/04/2012 12:01, G. Allegri a écrit :
>>
>> Probably I've found the bug.
>> In Qgis the semi-minor axis of the ellipsoid is calculated with:
>>
>>  b = a - (f/a)
>>
>>  where
>> b = semi-minor axis
>> a = semi-majot axis
>> f = inverse flattening
>>
>>  while it should be:
>>
>>  b = a - (a/f)
>>
>>
>>  In Qgis the WGS84 semi-minor axis is 6378136,xxx
>> while it should be 6356752.xxx
>>
>>  giovanni
>>
>>
>>
>>
>>
>> 2012/4/6 G. Allegri <giohappy at gmail.com>
>>
>>> AFAICS, the Qgis formula [1] to calculate distances on ellipssoid is
>>> Vincenty's.
>>> I've reproduced it for the first set of points and I can confirm tha
>>> QGis gives 540.9973919726176 m
>>>
>>>  The online Vincenty calculator [2] gives 540.045 m
>>>
>>>  I suppose it's a rounding issue.
>>> I will investigate it more...
>>>
>>>  giovanni
>>>
>>>
>>>  [1]
>>> http://trac.osgeo.org/qgis/browser/trunk/qgis/src/core/qgsdistancearea.cpp#L540
>>>
>>> [2] http://www.movable-type.co.uk/scripts/latlong-vincenty.html
>>>
>>>  2012/4/6 blavet <didier.blavet at ird.fr>
>>>
>>>>  Dear QGis Developpers and Users
>>>>
>>>>
>>>>
>>>> _______________________________________________________________________________________
>>>>
>>>> *Didier B**lavet*
>>>>
>>>> Institut de Recherche pour le Développement - IRD (http://www.ird.fr)
>>>>
>>>> UMR Eco&Sols - Ecologie Fonctionnelle & Biogéochimie des Sols &
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>>>>
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>>
>> --
>>
>>
>> _______________________________________________________________________________________
>>
>> *Didier B**lavet*
>>
>> Institut de Recherche pour le Développement - IRD (http://www.ird.fr)
>>
>> UMR Eco&Sols - Ecologie Fonctionnelle & Biogéochimie des Sols &
>> Agroécosystèmes
>>
>> (Montpellier SupAgro-CIRAD-INRA-IRD) (
>> http://www.montpellier.inra.fr/ecosols)
>>
>> Bâtiment 12, 2 place Viala, F-34060 Montpellier Cedex 2 , France
>>
>> Tel bureau : +33 (0)4 99 61 21 33
>>
>> Secrétariat : +33 (0)4 99 61 21 01 / Fax : +33 (0)4 99 61 21 19
>>
>> Courriel : didier.blavet at ird.fr
>>
>>
>> _______________________________________________________________________________________
>>
>
>
> --
>
>
> _______________________________________________________________________________________
>
> *Didier B**lavet*
>
> Institut de Recherche pour le Développement - IRD (http://www.ird.fr)
>
> UMR Eco&Sols - Ecologie Fonctionnelle & Biogéochimie des Sols &
> Agroécosystèmes
>
> (Montpellier SupAgro-CIRAD-INRA-IRD) (
> http://www.montpellier.inra.fr/ecosols)
>
> Bâtiment 12, 2 place Viala, F-34060 Montpellier Cedex 2 , France
>
> Tel bureau : +33 (0)4 99 61 21 33
>
> Secrétariat : +33 (0)4 99 61 21 01 / Fax : +33 (0)4 99 61 21 19
>
> Courriel : didier.blavet at ird.fr
>
>
> _______________________________________________________________________________________
>
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