[QGIS-trac] [Quantum GIS] #1558: WGS84 conversion to EPSG:27700 in error by greater than 100 metres

[Quantum GIS]

#1558: WGS84 conversion to EPSG:27700 in error by greater than 100 metres
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   Reporter:  nhopton                           |              Owner:  nobody
       Type:  bug                               |             Status:  new   
   Priority:  major: does not work as expected  |          Milestone:        
  Component:  Projection Support                |            Version:  HEAD  
   Keywords:                                    |   Platform_version:        
   Platform:  Windows                           |           Must_fix:  No    
Status_info:  0                                 |  
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 OSGB (EPSG: 27700) CRS Problem. Version 1.0.0 (II)

 When reprojecting a WGS84 shape file to EPSG:27700 (Ordnance Survey of
 Great Britain) on the fly in QGIS I noticed errors of around 125 metres in
 the conversion. This is quite serious for users in the UK. Error in
 conversion of this magnitude are usually the result of the conversion not
 taking into account the fact the WGS84 datum and the OSGB36 datum are not
 one and the same thing (but the conversion does use the +datum=osgb36
 parameter, I don't understand).

 For the time being I've defined my own OS CRS using the following
 parameters:

 +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601 +x_0=400000 +y_0=-100000
 +ellps=airy
 +towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m
 +no_defs no_defs <>

 Which converts from WGS84 to EPSG:27700 good to a metre or so. I've had
 this problem with other software that uses Proj.4.

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Ticket URL: <https://trac.osgeo.org/qgis/ticket/1558>
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