[Proj] Stereo 1970 (EPSG 31700)

Thu Oct 7 02:13:10 EST 2010

Hello Noel,

> The least-squares adjustment of 7 transformation parameters 
> (3 translations, 3 rotations and a scale) for an area as 
> small as the country of Romania is badly ill-conditioned. 

Now I am confused.

What you say is that it is a difficult statistical problem
to derive a 7-parameter transformation from observations in
a small area. But if someone I can trust has already derived
a 7-parameter transformation, then there is no numerical 
problems when using it, is there? 

> In these cases use a 3-parameter transformation instead.  
> Or a Molodensky-Badekas transformation ... as has become 
> common in some smaller countries (e.g. Luxembourg, Venezuela) 
> ... if you can find (or derive) one.

Well, but if a Molodensky-Badekas transformation is okay, 
then there is always a 7-parameter transformation that gives
the same result.  For all I know, the Romanian authorities
may have started out by deriving a Molodensky-Badekas 
transformation, and then decided to publish it as the 
equivalent 7-parameter transformation.  If that is what
they did, there would be no reason to suspect that their
transformation is less accurate than the 3-parameter 
transformations, is there? 

(An example: the datum shift EPSG:4829, "S-JTSK to ETRS89 (3)",
 is a Molodensky-Badekas transformation, and the datum 
 shift EPSG:4827, "S-JTSK to ETRS89 (4)", is the same 
 transformation expressed as a 7-parameter transformation.)

Best regards,

Mikael Rittri
Carmenta AB
Sweden
www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Noel Zinn (cc)
Sent: den 6 oktober 2010 16:47
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Stereo 1970 (EPSG 31700)

Although the third of the three EPSG transformations cited herein has the
(Romanian) National Agency for Cadastre and Land Registration as its source (and is, therefore, authoritative) and it has more parameters (7 versus 3 for the Shell and NIMA transformations), it should not be regarded as more accurate.  The least-squares adjustment of 7 transformation parameters (3 translations, 3 rotations and a scale) for an area as small as the country of Romania is badly ill-conditioned.  The 7 parameters are highly correlated and, therefore, are driven by small survey errors, not physical reality. 
Simply stated, the translations pull in one direction and the rotations in another.  Australia is big enough, but not Romania.  Not even Germany.  In these cases use a 3-parameter transformation instead.  Or a Molodensky-Badekas transformation ... as has become common in some smaller countries (e.g. Luxembourg, Venezuela) ... if you can find (or derive) one.

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

--------------------------------------------------
From: "Mikael Rittri" <Mikael.Rittri at carmenta.com>
Sent: Wednesday, October 06, 2010 2:38 AM
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
Subject: Re: [Proj] Stereo 1970 (EPSG 31700)

> Hello, Thibaut.
>
> For the record, www.epsg-registry.org gives three datum shifts from 
> Pulkovo 1942(58) to WGS84 that are valid in Romania.
>
> Code Accuracy Source Proj.4 syntax
> 15496 10 meters Shell SIEP +towgs84=44.107,-116.147,-54.648
> 15497 7 meters NIMA TR8350.2 +towgs84=28,-121,-77
> 15995 3 meters www.ancpi.ro (*)
> +towgs84=2.329,-147.042,-92.08,-0.309,0.325,0.497,5.69
> (*) National Agency for Cadastre and Land Registration
>
> The third one seems to be more accurate than the two you mentioned. Of 
> course, this may not matter if your main concern is to get ArcMap and 
> Qgis to agree.
>
> Actually, it is possible that the Shell datum shift is more accurate 
> than the NIMA one, since accuracies that EPSG quotes from different 
> sources are not comparable.  I think accuracies from NIMA are usually 
> one-sigma (about 67 percent confidence), but I don't know about Shell.  
> But my impression is that the ANCPI datum shift is the most accurate.
>
> By the way, I noticed that the code EPSG:31700 is deprecated, because 
> it used the the geodetic datum Dealul Piscului 1970, EPSG:6317.  This 
> was deprecated in September 2008 because
>
>   "Datum does not exist but is an alias for S-42 in Romania" (Change 
> ID 2008.011).
>
> And S-42 is (in this context) the same as Pulkovo 1942(58).
> So, EPSG:31700 has been replaced by EPSG:3844, "Pulkovo 1942(58) / 
> Stereo70".
>
> Best regards,
>
> Mikael Rittri
> Carmenta AB
> Sweden
> www.carmenta.com
>
> ________________________________
>
> From: proj-bounces at lists.maptools.org 
> [mailto:proj-bounces at lists.maptools.org] On Behalf Of Thibaut Gheysen
> Sent: den 4 oktober 2010 17:24
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] Stereo 1970 (EPSG 31700)
>
>
> I checked in Arcmap, there are two "geographic coordinate system 
> transformations" for stereo 1970 to wgs84. I used the first with those 
> parameters : +towgs84=44.107,-116.147,-54.648,0,0,0,0. The suggested
> proj.4 definition correspond to the second "geographic coordinate 
> system transformations" in ArcMap.
>
> All is fine now.
>
> Thanks for your help,
> Thibaut.
>
>
> 2010/10/4 Thibaut Gheysen <gheysen.t at gmail.com>
>
>
> Thanks Frank.
> I use QGis 1.5 from osgeo4w (I don't know exactly which version of 
> Proj.4 is used).
> I just try with your suggested definition. It's better but I always 
> have a deviation of about 5 meters.
>
> Thibaut.
>
>
> 2010/10/4 Frank Warmerdam <warmerdam at pobox.com>
>
>
> Thibaut Gheysen wrote:
> > Hi,
> >
> > I have a problem in QGIS with the Stereo 1970 coordinate system 
> > (EPSG 31700).
> > I have a shapefile in the WGS 84 coordinate system (from a GPS device).
> > I projected this shapefile in Stereo 1970 using ArcMap.
> > In ArcMap, the source and projected points are in the same place. In 
> > Qgis, I have a deviation of about 100 meters when displaying this 2 
> > shapefiles and activating the "on the fly projection" (stereo.jpg). 
> > This deviation is the same that the one I obtain in ArcMap without 
> > "geographic coordinate system transformations".
>
>
> Thibault,
>
> I'm not sure exactly what version of PROJ.4 you are using, but after 
> my "datum shift identification upgrade" the EPSG dictionary for proj 
> now has this as the suggested definition of 31700:
>
>
> +proj=sterea +lat_0=46 +lon_0=25 +k=0.99975 +x_0=500000 +y_0=500000 
> +ellps=krass +towgs84=28,-121,-77,0,0,0,0 +units=m +no_defs
>
> Perhaps you could try with this?
>
> 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 
> <http://pobox.com/%7Ewarmerdam>
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
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