[GRASS-dev] Re: [GRASS GIS] #1300: WxGUI measure tool gives wrong results

[GRASS GIS]

#1300: WxGUI measure tool gives wrong results
--------------------------+-------------------------------------------------
 Reporter:  marisn        |       Owner:  martinl            
     Type:  defect        |      Status:  assigned           
 Priority:  blocker       |   Milestone:  6.4.1              
Component:  wxGUI         |     Version:  svn-releasebranch64
 Keywords:  measure tool  |    Platform:  MSWindows Vista    
      Cpu:  Unspecified   |  
--------------------------+-------------------------------------------------

Comment(by hamish):

 Replying to [comment:10 martinl]:
 > backported in  r45891

 cheers

 > > lat/lon is still reporting meaningless results, in grass7 it should
 use
 > > m.measure to do the calc so geodesic distance is automatically used if
 needed.
 >
 > probably we should warn user about that. Would be
 >
 {{{
 > Warning: Measuring reports meaningless results for LL projection
 (geodetic
 > distance ignored). Check future versions.
 }}}
 >
 > be OK?

 sounds fine. I'd consider the additional step of disabling the tool after
 showing that message. Maybe with "Geodesic distance is not yet supported
 by this tool, try d.measure or m.measure.", or something. (apple usability
 guide: an error message should say what the trouble was, and give a hint
 about where the user should look next/for more info)

 veering off-topic a little, but along the same reporting seemingly valid
 (but in fact broken) numbers to the user, I've been trying to follow how
 the mapscale setting in the drop down menu on the bottom bar of the Map
 Display window does its job. Does .ppm[0], .ppm[1] refer to image
 width,height? Map scale is fundamentally unknowable if you don't know the
 correct pixels-per-mm ratio for the user's particular monitor.
 X11,OSX,Window's reports for that have in the past not been very good,
 maybe with modern versions it has improved? (being optimistic..)
 We get away with it in ps.map because we know the exact size of the paper,
 and PostScript exists in that reference frame.
 GIMP provides a calibration tool (Prefs->Display->Calibrate) where the
 user can press a ruler up to their screen and type in the distance covered
 by x,y pixel bars, then hope the user doesn't change screen resolution.


 Hamish

 ps- At some point I'd like to add the v.in.geodesic addon to trunk, but
 I'd like feedback on options & assumptions made before finalizing it and
 porting it to python, or turning it into a new C module reusing proj4's
 `geod` code if that is preferred.

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Ticket URL: <http://trac.osgeo.org/grass/ticket/1300#comment:11>
GRASS GIS <http://grass.osgeo.org>