v.surf.idw: very incorrect results

[hamish]

Hi,

I have noticed with some data very bad results with v.surf.idw. The r.surf.idw and r.surf.idw2 modules do ok with the same data after v.to.rast of the starting points. But the v.surf.idw results are not realistic both spatially and numerically.

In my real data I was seeing bad slivering between sample sites, but can recreate other little-relation to source data weirdness with synthetic input.

commands to make the attached screenshot in grass6:

g.region -d    # spearfish
# from addons:
r.surf.volcano out=gauss method=gaussian kurtosis=0.5 peak=5000
r.random in=gauss vect=gauss_rand_500 n=500
v.surf.idw in=gauss_rand_500 out=gauss_rand_500.vidw column=value

plot:

d.mon x0
d.resize w=666 h=1006
d.font Vera
d.split.frame frames=2
#
d.frame uno
MAP=gauss
r.colors "$MAP" col=bcyr
d.rast "$MAP"
d.vect gauss_rand_500 icon=basic/point fcolor=none color=60:60:60
d.legend "$MAP" at=5,60,2,5
echo "$MAP" | d.text color=black size=4
#
d.frame dos
MAP=gauss_rand_500.vidw
r.colors "$MAP" col=bcyr
d.rast "$MAP"
d.vect gauss_rand_500 icon=basic/point fcolor=none color=60:60:60
d.legend "$MAP" at=5,60,2,5
echo "$MAP" | d.text color=black size=4

fwiw I also notice that the module does not scale well to large region sizes (at all).

thanks, Hamish

[hamish]

w/ annotations

[hamish]

for comparison, the working r.surf.idw* methods: }}} v.to.rast in=gauss_rand_500 out=gauss_rand_500.pts column=value r.surf.idw in=gauss_rand_500.pts out=gauss_rand_500.ridw1 r.surf.idw2 in=gauss_rand_500.pts out=gauss_rand_500.ridw2 }}}

[hamish]

Hi,

some more screenshots showing results of r.plane* + v.random + v.surf.idw, including some derivative maps from r.slope.aspect (devbr6). The 1000 vector sampling points created by v.random are shown as black dots.

[*] trend-plane created by the r.cog addon for spearfish's elevation.dem

this example is not completely wrong (as is the case in the earlier attached screenshot!), just a net of overlain noise on top. So there could be two bugs here.

• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfidw.png
• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfidw.slope.png
• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfidw.pcurv.png
• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfidw.tcurv.png

note the strands of noise in the NW corner in the curvature maps, and the clump of it the center-right of the lower edge in the slope map. Try to match them with the r.surf.idw slope map below.

---

for comparison, same-data slope map from v.surf.rst and r.surf.idw modules.

• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfrst.slope.png
• ​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/rsurfidw.slope.png

r.surf.idw2 was very similar, but not identical, to r.surf.idw; so not shown. As expected, v.surf.rst is much smoother throughout although it suffers from edge-effects and stitching of the quadtree segmentation around the edges. This suggests the quadtree vector map should be created and the outer row of segments from it discarded.

Hamish

[hamish]

Replying to hamish:

​http://bambi.otago.ac.nz/hamish/grass/bugs/v.surf.idw/vsurfrst.slope.png

...

As expected, v.surf.rst is much smoother throughout although it suffers from edge-effects and stitching of the quadtree segmentation around the edges. This suggests the quadtree vector map should be created and the outer row of segments from it discarded.

funny that the artifacts only show up in the horizontal but not the vertical splits..

Hamish

[neteler]

Continued in #2820

[mlennert]

Closing this ticket as #2820 seems to have dealt with it. If the issue is still alive, please post to #2820.