[QGIS-trac] Re: [Quantum GIS] #2890: Georeference's plugin can work with changes of projections

[Quantum GIS]

#2890: Georeference's plugin can work with changes of projections
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   Reporter:  lmotta            |              Owner:  mmassing     
       Type:  patch             |             Status:  assigned     
   Priority:  minor: annoyance  |          Milestone:  Version 1.6.0
  Component:  C++ Plugins       |            Version:  Trunk        
   Keywords:  Georeferencer     |   Platform_version:  1.6          
   Platform:  All               |           Must_fix:  No           
Status_info:  0                 |  
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Comment(by mmassing):

 Thanks Luiz for these excellent additions. I have commited parts of your
 changes in r13959, r13960 and r13962.

 I am not including the reprojection code at the moment, because it has
 some issues which I'd like to resolve first. The main problem is that a
 user can easily run into cases which
 result in destructive transformation of GCP coordinates or which work in a
 counter-intuitive
 fashion. I think with the current implementation it would be best to only
 let the user reproject
 GCPs explicitly (e.g. by  selecting it as a function from the menu, or by
 enabling it
 explicitly), and not trying to  guess from/to which SRS to reproject to
 avoid any confusion or
 unintentional reprojection.


 1) Does only work with EPSG projections (i.e. does not work with user-
 defined projections, or +proj4 based projections without EPSG code).
 Entering a non-EPSG projeciton trips up the automatic reprojection
 completely.

 2) Automatic reprojection may not be desired for a variety of reasons,
 e.g. because reprojection
 may be associated with a loss of precision.

 3) The transform may fail (e.g. because some coordinates can not be
 represented in the new
 coordinate system), resulting in information loss - this can happen as the
 result of a seemingly
 unrelated user-action (e.g., opening a new project, or changing the
 projects transformation settings in QGis)

 4) The reprojection makes assumptions about the destination SRS which may
 not be valid.
 In particular, it may become impossible to even open a file if the
 projection (as set in the
 gis project settings) results in an invalid transformation of the GCPs.

 These problems could maybe be addressed cleanly by storing all GCP
 destination coordinates in a
 fixed geographic reference frame (e.g. WGS84), and deriving the
 reprojected coordinates only
 when needed (e.g. when displaying them to the user, or when fitting the
 parametrized
 transform). This would avoid SRS ambiguities, curb precision loss and make
 it possible to recover from failed reprojection attempts.

-- 
Ticket URL: <https://trac.osgeo.org/qgis/ticket/2890#comment:9>
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