#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
Keywords: r.object.geometry cell coordinates | CPU: Unspecified
Platform: Unspecified |
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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:
Is the error in my patch, or maybe in the r.mapcalc x() function ?
Any objections to applying this patch ? It changes the output of the
module, but it add the additional values at the end, so unless some
scripts rely on fd being the last value, this should not cause too much
trouble.
#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, ¤t_region2);
> y = Rast_row_to_northing(current_row + 0.5, ¤t_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, ¤t_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 ?
#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
Replying to [comment:3 mmetz]:
> Replying to [ticket:3680 mlennert]:
> >
> > 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
Right, yes, I should have remembered that (or identified it in the code).
Sorry.