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List:       postgis-users
Subject:    Re: [postgis-users] Problems with the Transformation of Coordinates
From:       "Suhr, Ralf" <Ralf.Suhr () itc-halle ! de>
Date:       2010-02-10 9:20:05
Message-ID: 305F51E63512244D853C549502A6132439278D () VVVMAIL ! vvv ! vvv-konzern ! net
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Hi,

I see some errors.

Your coordinates in EPSG:4267 are on the right ellipsoid (clrk66) and can direktly \
project to EPSG:3796 without the need for any towgs84 parameter. If you have \
coordinates in EPSG:4326 you need 2 Transformations. Otherwise your result in \
EPSG:3796 will based on the wgs84 ellipsoid. 

Try ST_Transform(ST_Transform(wgs84_geom, 4267), 3796)

Gr Ralf

-----Ursprüngliche Nachricht-----
Von: postgis-users-bounces@postgis.refractions.net \
[mailto:postgis-users-bounces@postgis.refractions.net] Im Auftrag von Pablo Velazco \
                Villares
Gesendet: Mittwoch, 10. Februar 2010 04:40
An: postgis-users@postgis.refractions.net
Betreff: [postgis-users] Problems with the Transformation of Coordinates forCuba

Hello I am entering recently in the list, greetings to the colleagues.  

The process of migration of our space databases and applications GIS to platforms of \
Free Software, implies in occasions the necessity to repair in aspects that in the \
software proprietors, they were we quite transparent. The case that occupies me is a \
palpable example of what I comment them.     The situation is the following one. I am \
from Cuba and our CRS in the EPSG they are the following ones: 3795: North Cuba, \
3796: South Cuba, both for the work with plane coordinates, and the 4267 NAD27, for \
the case of the geographical ones. The CRS 4267 are the base for the CRS 3795 and \
3796 and he has as DATUM the "Nad 27 Continental for US."   
In the case of the operations of CONVERSION OF COORDINATED: of 3795 at 3796 or 3796 \
at 4267 (in this case of plane to geographical) we achieve extremely precise results \
in the conversion, but it is not the case, when we carry out operations of \
TRANSFORMATION that imply the step the transformation of a DATUM to other, for \
example:  of NAD27 to WGS84.    The problem is related, with the fact that the \
specific Datum doesn't exist for the area of Cuba and the local CRS they are \
inheriting the parameters of transformation of another region. The problem begins him \
to perceive when superimposing space layers as: GPS or Images of Goolgle (both in \
WGS84), have more than enough layers in local CRS.  

To exemplify the actions that we have carried out, we take like case of study the \
function Postgis:  ST_Transform and we carry out consultations like these, for the \
transformation tests. Select Astext(ST_Transform(ST_SetSRID ('point(-76.670120 \
20.388508) ':: geometry, 4267),4326));   Previously we carry out the creation of a \
New SRID (4267001) in the chart: spatial_ref_sys of Postgis, with: proj4txt:  \
"+proj=longlat +ellps=clrk66 +towgs84=2.478, 149.752, 197.726, -0.526356, -0.497970, \
0.500831, 0.685238 +no_defs ".   

As you it can appreciate, we eliminate the parameter +datum=NAD27, and we incorporate \
those (7) transformation parameters to WGS84 of our country for the method of \
Bursa-Wolfe. For what we have studied on the topic, we lack the treatment of the \
parameter +nadgrids, that which we have not been able to implement, because we lack \
information on the form of configuring an appropriate grill to our territory. In the \
Database of EPSG, the CRS 4267, they have registered the Variant: 88, the one which \
this associated to the transformation parameters to WGS84, before exposed, but we \
have not been able to give to this a practical sense that facilitates us to work with \
a NAD 27 for Cuba, as we had it in Mapinfo, for example:  "Longitude / Latitude (NAD \
27 for Cuba_CNGPS) ", 1, 9999, 7, 2.478, 149.752, 197.726, -0.526356, -0.497970, \
0.500831, 0.685238, 0  

So that they can help us, I include a group of test data:  
Point1 in NAD27 for Cuba    
Long: -76 º 38' 28.0824" or in DD: -76.64100612    
Lat: 20 º 22' 16.19" or in DD:  20.371090164    
Point1 in having Transformed correctly to WGS84  
Long: -76 º 38' 27.171" or in DD: -76.6408814349  
Lat: 20 º 22' 18.192" or in DD:  20.37172032737  
  
Point in plane coordinates "South Cuba" 3796  
X: 520065.10m Y:190891.5m  
  
When carrying out the transformation operations in postgis (with the parameters \
before mentioned), we obtained the following erroneous results:   Point1 become to \
                WGS84 with postgis DD:  
Long: -76.6406516  
Lat: 20.3718218  
Transformed Point1 of WGS84 to South Cuba 3796 (plane coordinates):  
X: 520043.06m Y:190910.53m  

It is evident the great difference between the prospective results and those really \
reached ones.   Excuse the extensive of the mail, but I didn't have another form of \
trying to make me to understand and to treat such a complicated problem.   I request \
them collaboration, in the solution of this situation, their ideas will be of an \
useful helps for us.   Greetings Mckensen
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