[Gvsig_english] gvSIG and OSGB36 (EPSG:27700)

[Juan Lucas Dominguez Rubio]

Hello, Nick,
 
When you do a conversion between EPSG:4326 <--> EPSG:27700 you have to choose the "EPSG transformation" option in the combo box and gvSIG will automatically list 5 or 6 possible options: one for England, one for England+Wales, etc. I have used that particular conversion several times without a problem. That gives you an accuracy of about 2 metres (somethimes 1 metre, sometimes 3 metres, etc)
 
You can also use NTv2 grids for the UK available here (we can assume that ETRS89 and WGS84 are equivalent, if I'm not wrong):
 
http://lists.maptools.org/pipermail/proj/2006-November/002658.html
 
With those grids I think the accuracy will be a few inches, perhaps 1 inch, perhaps 3 inches, etc.
 
Regards,
Juan Lucas Domínguez Rubio
---
Prodevelop SL, Valencia (España)
Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
http://www.prodevelop.es <http://www.prodevelop.es/> 
---

________________________________

De: gvsig_internacional-bounces at listserv.gva.es en nombre de Nick Hopton
Enviado el: jue 28/07/2011 13:03
Para: gvsig_internacional at listserv.gva.es
Asunto: [Gvsig_english] gvSIG and OSGB36 (EPSG:27700)



I have recently installed gvSIG Version 1.11.0 (Build 1305) running
under Ubuntu 11.04 (amd64). On checking the Proj4
parameter string that gvSIG uses for EPSG:27700 I found that it was
thus:

PROJ4 string: +proj=tmerc +lat_0=49.0 +lon_0=-2.0 +k=0.9996012717
+x_0=400000.0 +y_0=-100000.0 +ellps=airy +units=m

This string does not include the '+datum=OSGB36' parameter and as a
result the datum shift between OBSG36 and WGS84 will not be applied.
If I recall correctly, the shift between the two datums is something
like 112 metres at Greenwich.

This was a known problem with older versions of Proj4.

Is there any way to edit the parameter string that gvSIG?

Regards,
Nick.



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