[Proj] Any access to geodetic functions ??

[Mikael Rittri]

If 1 percent accuracy is enough, I think you can use spherical formulas 
with a fixed Earth radius. You can find good formulas in the Aviation Formulary 
of Ed Williams, http://williams.best.vwh.net/avform.htm . 

For the fixed Earth radius, I would choose the average of the 
    c   = radius of curvature at the poles, 
b^2 / a = radius of curvature in a meridian plane at the equator,
since these are the extreme values for the local radius of 
curvature of the earth ellipsoid.

If your coordinates are given in WGS84, then 
    c   = 6 399 593.626 m, 
b^2 / a = 6 335 439.327 m, 
(see http://home.online.no/~sigurdhu/WGS84_Eng.html ) 
so their average is 6 367 516.477 m. 
The maximal error for distance calculation should then be less than 0.51 percent.

When computing the azimuth between two points by the spherical formulas, 
I think the maximal error on WGS84 will be 0.2 degrees, at least if 
the points are not too far away (less than 1000 km apart, say).
The error should be maximal near the equator, for azimuths near northeast etc. 

I am not sure about the spherical errors for the forward geodetic problem:
point positioning given initial point, distance and azimuth. 

Best regards,
--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
SWEDEN
Tel: +46-31-775 57 37
Mob: +46-703-60 34 07 
mikael.rittri at carmenta.com
www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Benoît Andrieu
Sent: den 12 november 2008 10:07
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Any access to geodetic functions ??

Thank you so much for your answer, Frank.

So, this leads to some questions (that are somehow related...) :
- how are computed distance calculation between two points by softwares like Postgis for example ?
- is the best way to compute distance calculation to use UTM systems ? I have to compute distances that could be very long (hundreds of kilometers), strong accuracy is not required but I don't want to have more than 1% accuracy error.
- in what way are those functions distinct from the proj4 ? Am I wrong to use them ? Are there others functions in proj4 that would allow me to do the same things that I can do with those functions (distance calculation between two points and point positionning given initial point, distance and azimuth) ?

Thanks again.

Best regards,

Benoît Andrieu
bea at ixsea.com
benoit.andrieu at gmail.com

----- Original Message -----
From: "Frank Warmerdam" <warmerdam at pobox.com>
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
Sent: Monday, November 10, 2008 10:59 PM
Subject: Re: [Proj] Any access to geodetic functions ??


> Benoît Andrieu wrote:
>> Hi all,
>>  We are using Proj4 in our softwares and we are using too Mapserver 
>> (FWTools compilation).
>> Actually, we are using custom built dlls for Proj4 : proj.dll and 
>> geod.dll. Those builds are based on the Proj4 4.5.0 version.
>> Why we have custom builds is that there was at the time no access to 
>> geodetic functions (and I need them... ^^).
>>  From what I remember, this is the kind of messages that made me make 
>> custom builds :
>> http://osdir.com/ml/gis.proj-4.devel/2006-12/msg00006.html
>>  We are actually switching from FWTools 1.3.2 to FWTools 2.2.6 which 
>> should embed Proj4 4.6.1.
>> So if I want my softwares to be synchronized with this version I would 
>> have to recompile a Proj4 4.6.1 to have access to the geodetic functions 
>> 'except' if they are already exported.
>>  So my question is : are the geodetic functions now exported ? (please 
>> answer yes ...)
>> And if affirmative, in which dlls will I be able to find them ? (coz I 
>> can't find them actually)
>
> Benoît,
>
> Currently the geodesic functions are not exported from proj.dll, nor
> (I think) are they even built into the DLL.   I'm a bit hesitant to treat
> them as part of the support PROJ.4 API since they are quite distinct
> from the core of PROJ.4.  The geod_* functions also depend on global
> variables which is not a pattern I'm keen on setting in stone.
>
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, 
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
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