[GRASS-dev] Re: [GRASS GIS] #182: v.segment: offset units support

[GRASS GIS]

#182: v.segment: offset units support
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  Reporter:  martinl  |       Owner:  grass-dev at lists.osgeo.org
      Type:  task     |      Status:  new                      
  Priority:  minor    |   Milestone:  7.0.0                    
 Component:  default  |     Version:  svn-trunk                
Resolution:           |    Keywords:  vector, LL               
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Comment (by hamish):

 ''If'' it is wanted, G_distance() includes code to automatically convert
 map units to meters- for Lat/Lon geodesic (great circle) distance is
 automatically used.


 lib/gis/distance.c
 {{{
 /**
  * \fn double G_distance (double e1, double n1, double e2, double n2)
  *
  * \brief Returns distance in meters.
  *
  * This routine computes the distance, in meters, from
  * <b>x1</b>,<b>y1</b> to <b>x2</b>,<b>y2</b>. If the projection is
  * latitude-longitude, this distance is measured along the geodesic. Two
  * routines perform geodesic distance calculations.
  *
  * \param[in] x1
  * \param[in] y1
  * \param[in] x2
  * \param[in] y2
  * \return double
  */
 }}}


 this is used by d.measure, swig/python/m.distance, ...

 also check what r.buffer does, it goes to a lot of trouble with units and
 projections and seems to end up doing the right thing.

 I think importantly for the vector modules Vect_line_length() just uses
 hypot(dx,dy) so will only work properly in projected locations. For
 Lat/Lon there is Vect_line_geodesic_length(). (and I assume ''area''
 equivalents) I do not know if it is better to do `if(G_projection() ==
 PROJECTION_LL)` in each module or automatically take care of that in the
 library like for G_distance().

 There is also the small matter of the vector library not wrapping at 180
 longitude, while libgis generally does; but I won't hijack the ticket. :)


 Hamish

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Ticket URL: <http://trac.osgeo.org/grass/ticket/182#comment:2>
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