[GRASS-dev] [GRASS GIS] #3680: [PATCH] r.object.geometry: add output of mean coordinates of objects - difference to r.mapcalc/r.univar output

[GRASS GIS]

#3680: [PATCH] r.object.geometry: add output of mean coordinates of objects -
difference to r.mapcalc/r.univar output
--------------------------+------------------------------------------------
  Reporter:  mlennert     |      Owner:  grass-dev@…
      Type:  enhancement  |     Status:  new
  Priority:  normal       |  Milestone:
 Component:  Addons       |    Version:  unspecified
Resolution:               |   Keywords:  r.object.geometry cell coordinates
       CPU:  Unspecified  |   Platform:  Unspecified
--------------------------+------------------------------------------------

Comment (by mlennert):

 Replying to [comment:1 marisn]:
 > Comments in the code of r.mapcalc
 (https://trac.osgeo.org/grass/browser/grass/trunk/raster/r.mapcalc/xcoor.c#L27):
 >
 > {{{
 > x() easting at center of column
 > y() northing at center of row
 > }}}
 >
 > And the code itself:
 >
 > {{{
 > x = Rast_col_to_easting(0.5, &current_region2);
 > y = Rast_row_to_northing(current_row + 0.5, &current_region2);
 > }}}

 Yes, because in the programmer's manual, it says:


 {{{
 /*!
   250  * \brief Column to easting.
   251  *
   252  * Converts a <i>col</i> relative to a <i>window</i> to an easting.
   253  *
   254  * <b>Note:</b> <i>col</i> is a <i>double</i>:
   255  *  - col+0.0 will return the easting for the western edge of the
 column.
   256  *  - col+0.5 will return the easting for the center of the column.
   257  *  - col+1.0 will return the easting for the eastern edge of the
 column.
   258  *
   259  * \param col column number
   260  * \param[in] window pointer to Cell_head
   261  *
   262  * \return east coordinate
   263  */
   264 double Rast_col_to_easting(double col, const struct Cell_head
 *window)
   265 {
   266     return window->west + col * window->ew_res;
   267 }
 }}}

 And in the code of r.mapcalc, you can see


 {{{
 27          x = Rast_col_to_easting(0.5, &current_region2);
 28
 29          for (i = 0; i < columns; i++) {
 30              res[i] = x;
 31              x += current_region2.ew_res;
 32          }
 }}}

 i.e. the first x is calculated for col = 0.5 and all others for
 respectively one ew_res to the east, so always col + 0.5.

 That is why I'm surprised to not get the same answer.

 Maybe it is due more to r.univar than to r.mapcalc ?

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/3680#comment:2>
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