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

[Markus Metz]

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.

Markus M
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