[GRASS-dev] v.surf.bspline density and distance estimates

Markus Metz markus.metz.giswork at gmail.com
Mon Jul 24 15:02:49 PDT 2017


On Mon, Jul 24, 2017 at 6:30 PM, Paulo van Breugel <p.vanbreugel at gmail.com>
wrote:
>
>
>
> On 6/25/17 11:09 PM, Markus Metz wrote:
>
>
>
> On Sat, Jun 24, 2017 at 11:23 AM, Paulo van Breugel <
p.vanbreugel at gmail.com> wrote:
> >
> >
> > On 6/16/17 3:20 PM, Markus Metz wrote:
> >
> >
> >
> > On Fri, Jun 16, 2017 at 11:28 AM, Paulo van Breugel <
p.vanbreugel at gmail.com> wrote:
> > >
> > > Hi devs, can anybody tell how v.surf.bspline computes the point
density and mean distance.
> >
> > point density is the number of points divided by the size of the
bounding box of the points
> > mean distance is the square root of the size of the bounding box
divided by the number of points
> >
> > > And what is the unit, pixels, as also used for length spline steps or
meters?
> >
> > the unit is map unit, you can then figure out a reasonable resolution
(in map units) for the current region
> >
> >
> > I think it would be good to add your explanation to the manual page. I
was trying to come up with a text, but then I realized there is still some
ambiguity left.
> >
> > According to the manual page, the ns_step and ew_step parameters are in
pixels (which I would interpret as 'number of pixels'). However, the
example of 'Basic interpolation and raster output with a longer spline
step' in the manual page talks about mapping units.
> >
> > v.surf.bspline input=point_vector raster=interpolate_surface ew_step=25
ns_step=25
> > A bilinear spline interpolation will be done with a spline step length
of 25 map units.
> >
> > So if the region's resolution is 100 meter, is the ns_step and ew_step
2500 meter (25 pixels) or 25 meters? The former would be more conform
common practice in grass I think.
>
> The unit of ns_step and ew_step is map units, but that does not really
make sense, the unit of the spline steps should rather be pixels. However,
the unit of the spline step options can not be changed because it would
break compatibility.
>
>
> In the manual page, in the first paragraph of the Notes, may I suggest to
change the text:
>
> The length of each spline step is defined by ew_step for the east-west
direction and ns_step for the north-south direction. Step is defined in
number of pixels.
>
> to something along the lines:
>
> The length (in mapping units) of each spline step is defined by ew_step
for the east-west direction and ns_step for the north-south direction.

Done in trunk r71312 for all affected lidar modules.

Markus M
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