#2820: v.surf.idw results seem seriously wrong and don't match r.surf.idw results
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Reporter: lrntct | Owner: grass-dev@…
Type: defect | Status: new
Priority: blocker | Milestone: 7.0.3
Component: Vector | Version: 7.0.1
Resolution: | Keywords: v.surf.idw r.surf.idw interpolation
CPU: x86-64 | Platform: Linux
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Comment (by mlennert):
Investigating further. Adding the following patch for diagnosis (in
addition to the above sqrt patch):
{{{
Index: main.c
--- main.c (révision 67166)
+++ main.c (copie de travail)
@@ -386,6 +386,7 @@
sum1 = 0.0;
sum2 = 0.0;
for (n = 0; n < nsearch; n++) {
+ fprintf(stdout, "%f,%f,%f,%f\n", east, north,
list[n].dist, list[n].z);
if ((dist = list[n].dist)) {
sum1 += list[n].z / pow(dist, p);
sum2 += 1.0 / pow(dist, p);
}}}
and then running the following commands (i.e. creating one vector point
and getting distance and annual precipitation values from the 12 nearest
stations to that point, as well as the same values for the equivalent
pixel in both runs of v.surf.idw (without and with -n flag):
{{{
g.region vect=nc_state res=500 -ap
echo "567250|214750" | v.in.ascii in=- out=center --o
v.distance -ap from=center to=precip_30ynormals up=dist,to_attr
to_col=annual dmax=64000 --q | tail -n -12
v.surf.idw precip_30ynormals col=annual out=vsurfidw --o > list.csv
v.surf.idw -n precip_30ynormals col=annual out=vsurfidw_noindex --o >
list.csv
grep "567250.000000,214750" list.csv > list_center.csv
grep "567250.000000,214750" list_noindex.csv > list_center_noindex.csv
}}}
and putting together the results of the v.distance, v.surf.idw and
v.surf.idw -n runs, I get (results ordered by increasing distance):
* v.distance
{{{
distance value
8400,5853273955 1224,28
33001,5791065737 1183,64
33543,2163702733 1183,64
39603,9217121153 1234,44
41429,8528851821 1158,24
41667,8156183992 1206,5
41677,4928801831 1143
42993,6439600215 1209,04
55288,6854942564 1170,94
58543,7865733219 1216,66
60471,6142897 1219,2
62763,095429678 1094,74
}}}
* v.surf.idw
{{{
distance value
80047,835648 1191,26
90264,15377 1183,64
91013,285596 1158,24
99493,013889 1219,2
120304,207816 1120,14
171007,225542 1358,9
183102,228906 1440,18
184179,357051 1170,94
194397,039723 1234,44
200403,915046 1163,32
230189,059243 1546,86
231312,997387 1259,84
}}}
* v.surf.idw -n
{{{
8400,585327 1224,28
33001,579107 1183,64
33543,21637 1183,64
39603,921712 1234,44
41429,852885 1158,24
41667,815618 1206,5
41677,49288 1143
42993,64396 1209,04
55288,685494 1170,94
58543,786573 1216,66
60471,61429 1219,2
62763,09543 1094,74
}}}
In other words, v.surf.idw -n gives the exact same values as v.distance,
but v.surf.idw without the flag gives a completely different set of
distances. So, I have the feeling that the issue is in the
[https://trac.osgeo.org/grass/browser/grass/trunk/vector/v.surf.idw/main.c#L458
calculate_distances()] function or possibly in the creation of the
[https://trac.osgeo.org/grass/browser/grass/trunk/vector/v.surf.idw/main.c#L187
shortlist of cells to look in] in main.c, but I haven't been able to
pinpoint the culprit, yet.
Any help is more than welcome.
--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2820#comment:13>
GRASS GIS <https://grass.osgeo.org>
