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

[Mikael Rittri]

Hello Adrian.

First a minor point: sterea is better than stere
(at least for Romania, the Netherlands and Canada)
as you mentioned in your first post, so I think
using +init=epsg:31700 is better than +init=esri:31700, 
since only the epsg definition gives sterea, also 
according to you first post.  (In the Netherlands,
I think the difference between stere and sterea can
be up to 7 or 8 meters.)

Second, you have added a +towgs84 parameter, but 
omitted the +to and what comes after.  However, 
the +towgs84 is not a replacement for the +to etc., 
just additional information. 
(In fact, a +towgs84 parameter is often needed on 
 both sides of the +to separator. I'm surprised
 that cs2cs gives any output at all without any 
 +to separator.)

I tried the following:

C:\Program Files\FWTools2.4.7>echo 587486.770 326878.452 | cs2cs +init=epsg:31700 +towgs84=2.329,-147.042,-92.08,-0.309,0.325,0.497,5.69 +to +init=epsg:4326 -f '%0.6f'
'26.097437'     '44.436336' '36.350887'

You wrote:

> Real coordinates (converted from Stereo 70 with Global Mapper 
> and checked with a GPS unit): 26.097414, 44.436335

So the error is now 1.8 meters for this example 
(measured on http://geographiclib.sourceforge.net/cgi-bin/Geod),
which is what one could expect (since the horizontal accuracy
of this datum shift is supposed to be 1.5 to 3 meters).

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com


-----Original Message-----
From: Adrian Popa [mailto:adrian_gh.popa at romtelecom.ro] 
Sent: den 2 maj 2011 10:03
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Offset when converting coordinates from ESRI:31700 to EPSG:4326

Hello everyone,

First of all - thank you all for the information provided (and sorry I didn't reply sooner).

Second of all - I still must be doing something wrong - the coordinates I get are the same in all cases (the same as EPSG 4326):

adrianp at frost:~$ echo 587486.770 326878.452 | cs2cs +init=esri:31700 +to
+init=epsg:4326 -f '%0.6f'
26.099032    44.436575 0.000000
adrianp at frost:~$ echo 587486.770 326878.452 | cs2cs +init=esri:31700
+towgs84=28,-121,-77 -f '%0.6f'
26.099032    44.436575 0.000000
adrianp at frost:~$ echo 587486.770 326878.452 | cs2cs +init=esri:31700
+towgs84=44.107,-116.150,-54.648 -f '%0.6f'
26.099032    44.436575 0.000000
adrianp at frost:~$ echo 587486.770 326878.452 | cs2cs +init=esri:31700
+towgs84=-5.20,-126.75,-97.08,0.3160,0.4470,0.1340,0.99999  -f '%0.6f'
26.099032    44.436575 0.000000
adrianp at frost:~$ echo 587486.770 326878.452 | cs2cs +init=esri:31700
+towgs84=2.329,-147.042,-92.08,-0.309,0.325,0.497,5.69   -f '%0.6f'
26.099032    44.436575 0.000000

I'm sorry, but I'm a complete noob to proj and I do not understand the full theory behind projections, datums and all the rest - so I hope my questions are not too irritating :)

So - any clue to what I'm doing wrong?
Thank you,

Adrian


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