[GRASSLIST:7759] RE: cross validation on v.srf.rst. Does it work?

[Tom Colson]

Just for grins..try smooth=.01 or .02. My first suspicion would be that
there is too much smoothing. How many points are in the rainfall data
set...and what is the average distance between points? 


Thomas Colson
North Carolina State University
Department of Forestry and Environmental Resources
(919) 673 8023
tom_colson at ncsu.edu

Calendar:
www4.ncsu.edu/~tpcolson
 

-----Original Message-----
From: owner-GRASSLIST at baylor.edu [mailto:owner-GRASSLIST at baylor.edu] On
Behalf Of Jaime Carrera
Sent: Tuesday, August 02, 2005 8:57 AM
To: grasslist at baylor.edu
Subject: [GRASSLIST:7758] cross validation on v.srf.rst. Does it work?

Hi all,

I'm trying to use the cross validation option on both v.srf.rst and
v.vol.rst. However I'm getting some strange results for the slovakia
rainfall data set. 
1) I ran v.srf.rst as follows: 
GRASS 6.0.0 (slovakia3d):~ > v.surf.rst input=rain3d
zcolumn=int_1 cvdev=cv1 -v mask=mask at PERMANENT

Which wrote a table with difference values on Postgres.

2) Then I created another table without the gauging station #122 (which I
called rain3d_2) and interpolated using the same parameters as above:
GRASS 6.0.0 (slovakia3d):~ > v.surf.rst input=rain3d_2 zcolumn=rain
mask=mask at PERMANENT elev=vsurf1_wo122

AFAIK cross validation removes one point at a time, interpolates using the
remainder points and computes the difference between the interpolated value
at a certain location (i.e. station 122) and the real value (record at
station 122).  Accordingly if I query the raster map vsurf1_wo122 at the
coordinates of station
122 and substract the interpolated value to the real value I should get the
value that was written to the cross validation table for station 122, so I
query the
map:
GRASS 6.0.0 (slovakia3d):~ > r.what input=vsurf1_wo122
east_north=4416904,5414029
4416904|5414029||1057.9642333984

The real value at station 122 is 798 mm; according to these values the
difference is: 1057.96-798 = 259.96.
The value that was written by the v.srf.rst module on the cross-validation
table is -5.31.

Can someone tell me what's going on here?

Cheers,

Jaime



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