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

[Paulo van Breugel]

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 <mailto: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 <mailto: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.

>
> Markus M

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