[Proj] Offset when converting coordinates from ESRI:31700 to EPSG:4326

Tue Apr 26 09:54:56 EST 2011

Hello Adrian,

I agree with Jean-Claude and José, but I would like to add
something:

The datum Dealul Piscului 1970 used to have EPSG code 6317,
but it was deprecated by EPSG in September 2008. Reason:

    "Datum does not exist but is an alias for S-42 in Romania."

The replacement datum has code 6179, and is actually called
Pulkovo 1942(58), not S-42. 

For the same reason, the projected CRS 

    Dealul Piscului 1970 / Stereo 70 (EPSG code 31700) 

has been deprecated and replaced by 

    Pulkovo 1942(58) / Stereo70 (EPSG code 3844)

(although both "S-42 / Stereo 70" and "Dealul Piscului 1970/ Stereo 70" are aliases.)

Anyway, with this background knowledge, I found a better
(I think) datum shift to use: 

   +towgs84=2.329,-147.042,-92.08,-0.309,0.325,0.497,5.69  

which EPSG calls "Pulkovo 1942(58) to WGS 84 (19)", with code 15995.
The horizontal accuracy is supposed to be 1.5 to 3 meters.
(I have negated the three rotations to fit the cs2cs rotation sign convention.)

Warning: I haven't tried this on any real data. 

Best regards,
Mikael Rittri
Carmenta
Sweden 
http://www.carmenta.com

________________________________

From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Jose Gonçalves
Sent: den 22 april 2011 12:50
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Offset when converting coordinates from ESRI:31700 to EPSG:4326

Adrian

The problem is that datum transformation is not being done. The coordinate system definition esri:31700 only considers the projection and not a datum transformation (e.g. +towgs84=...). There are different sets of datum shift parameters for datum Dealul_Piscului_1970. In the ArcGIS documentation I found the following:

dX=44.107 dY=-116.150 dZ=-54.648
dX=28.000 dY=-121.000 dZ=-77.000

You must include one of them it in the cs2cs command:

cs2cs +init=esri:31700 +towgs84=28,-121,-77 +to +init=epsg:4326 -f '%0.6f'

Regards

José A. Gonçalves

2011/4/22 Adrian Popa <adrian_gh.popa at romtelecom.ro>

	Hello everyone,

	I have a problem when converting coordinates from ESRI:31700 (Stereo 70)
	to WGS84 (EPSG:4326). The coordinates I get from the conversion have an
	offset from the original coordinate of about 50m. Here's an example:

	Original coordinates (Stereo 70):  587486.770, 326878.452
	Converted coordinates (WGS84): 26.099032,44.436575
	Real coordinates (converted from Stereo 70 with Global Mapper and
	checked with a GPS unit): 26.097414, 44.436335

	For the conversion I'm using cs2cs Rel. 4.7.1, 23 September 2009 on
	Ubuntu 10.04.

	My ESRI/EPSG definitions are as follows:

	EPSG:
	# Dealul Piscului 1970/ Stereo 70 (deprecated)
	<31700> +proj=sterea +lat_0=46 +lon_0=25 +k=0.99975 +x_0=500000
	+y_0=500000 +ellps=krass +units=m +no_defs <>
	# WGS 84
	<4326> +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs <>

	ESRI:
	# Dealul Piscului 1970/ Stereo 70
	<31700> +proj=stere +lat_0=46 +lon_0=25 +k=0.999750 +x_0=500000
	+y_0=500000 +ellps=krass +units=m +no_defs  no_defs <>

	The command line I'm using is:
	$ echo 587486.770 326878.452 | cs2cs +init=esri:31700 +to
	+init=epsg:4326 -f '%0.6f'
	26.099032    44.436575 0.000000
	$ echo 587486.770 326878.452 | cs2cs +init=epsg:31700 +to
	+init=epsg:4326 -f '%0.6f'
	26.098934    44.436629 0.000000

	I've read that there is a small difference between stere and sterea
	projections, and sterea should give a better approximation, but I've
	tried with both projections and I still get the ~50m offset.

	Can anybody clue me in as to what I'm doing wrong? I must be missing
	something, but I don't know what (I'm somewhat of a noob when it comes
	to map projections).

	P.S. The definition for Stereo 70 that I received with my data looks
	consistent with the definition inside proj4:

	PROJCS["Stereo_70",GEOGCS["GCS_Dealul_Piscului_1970",DATUM["D_Dealul_Piscului_1970",SPHEROID["Krasovsky_1940",6378245,298.3]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Double_Stereographic"],PARAMETER["False_Easting",500000],PARAMETER["False_Northing",500000],PARAMETER["Longitude_Of_Origin",25],PARAMETER["Scale_Factor",0.99975],PARAMETER["Latitude_Of_Origin",46],UNIT["Meter",1]]

	Thanks,
	Adrian
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