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

[Jose Gonçalves]

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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