[Proj] Stereo 1970 (EPSG 31700)

[Mikael Rittri]

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So, EPSG:31700 has been replaced by EPSG:3844, "Pulkovo 1942(58) / Stereo70".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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