[GRASS-dev] r.fill.gaps porting

Vaclav Petras wenzeslaus at gmail.com
Mon Apr 17 12:26:52 PDT 2017


On Mon, Apr 17, 2017 at 2:58 PM, Benjamin Ducke <benducke at fastmail.fm>
wrote:

> > I've added a figure which shows the r.slope.aspect products from the
> > surface without and with -p. You can see that the smoothing gives
> > significantly different results and probably better ones. I had the same
> > experience in past when trying to patch result of r.neighbors with the
> > original raster. Perhaps using smaller distance as you say or greater
> > power would help, but I haven't tested that yet.
>
> I find the results for "curvature" most interesting.
> In the unsmoothed LiDAR data, that metric is basically useless.
>
>
It is the same for profile and tangential curvature. At certain zoom level,
you can see, with a lot of imagination, some pattern, but comparing to the
more expected result from the smoothed data, it is just noise.


> If I understand correctly, then slope, aspect
> and curvature are all computed within a small
> neighbourhood of cells.


Right, that's what r.slope.aspect does. You can avoid that by using
r.param.scale, but for another analysis, it would matter (depending on the
implementation/method).


> So if LiDAR data is
> also noisy by nature, then it is not surprising
> that these metrics are all distorted by strongly
> fluctuating local means.
>
>
Yes, there is noise. Usually, you either interpolate from points or you do
binning at much courser res averaging the noise out.


> Please try experimenting with the power parameter,
> if you have the time. In theory, this should allow
> you to interpolate over larger distances with less
> of a low-pass effect (or the other way around,
> depending on what you want to achieve).
>

I have this on my todo list.


>
> >
> > I managed to commit just the figure its caption. If somebody gets to it,
> > please extent the section.
>
> I will update the HTML page, including a more
> prominent mentioning of the default smoothing.
> I also found some small typos that need fixing.


Thanks. I just added a section, high res image and some basic text.
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