[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 mmetz):

 Replying to [ticket:3680 mlennert]:
 > For some applications it can be useful to obtain the mean coordinates of
 raster objects. I would like to add this functionality to
 r.object.geometry. For this I use the attached patch.
 >
 > However, when I use this to calculate mean coordinates of objects, I get
 one unit of resolution difference between these results and the results of
 the following r.mapcalc/r.univar call:
 >
 >
 > {{{
 > g.region rast=MyMap
 > r.mapcalc "x = x()"
 > r.univar -t x zones=MyMap
 > }}}
 >
 > Here's some output for comparison:
 >
 >
 > {{{
 > r.mapcalc/r.univar      r.object.geometry
 >
 > zone|mean               cat|mean_x
 > 1|270347.103514056      1|270347.353514
 > 2|270355.499712833      2|270355.749713
 > 3|270243.128030303      3|270243.378030
 > 4|270270.053380443      4|270270.303380
 > 5|270280.90239726       5|270281.152397
 > 6|270277.778301887      6|270278.028302
 > }}}
 >
 > The result of r.object.geometry is always shifted one resolution (0.25)
 to the East.

 the reason is that r.object.geometry pads the current region with one
 column to the west and east, and the module iterates over columns with

 {{{
         for (col = 1; col <= ncols; col++) {
 ...
 }}}

 that also means that 0.5 needs to be subtracted:

 {{{
                 obj_geos[cur - min].x += Rast_col_to_easting(col - 0.5,
 &cellhd);
 }}}

 to get the real center for each column in the current region.

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/3680#comment:3>
GRASS GIS <https://grass.osgeo.org>