[GRASS-user] does v.surf.idw raster surface pass through input points?

[Luigi Ponti]

Hello,

I am trying to fill missing data in daily temperatures via v.surf.idw 
interpolation. I have imported daily temperatures via v.in.ascii to a 
latlong location, with missing values indicated by "", as suggested by 
Hamish:
http://www.intevation.de/rt/webrt?serial_num=2763&display=History

Then I have v.surf.idw interpolated the vector point values, both in the 
latlong import location and in a Lambert Conformal Conic (LCC) location, 
uploading raster values at positions of vector points to the attribute 
table via v.db.addcol & v.what.rast.

Following is an excerpt of the attribute table I have obtained, where 
TEMP is the daily temperature at a particular LON and LAT, while SURF1 
and SURF2 are values uploaded from interpolated raster in the latlong 
and LCC locations, respectively (I have added the MISSING flag to 
indicate a missing temperature value, which is a "" in the real table, 
for clarity):

CAT | LON | LAT | TEMP | SURF1 | SURF2
1 | 8.283300 | 40.633300 | 14.600000 | 6.585142 | 7.339586
2 | 16.314700 | 40.283100 | MISSING | 11.720935 | 11.169486
3 | 11.850000 | 43.466700 | 12.000000 | 8.832849 | 9.011710

In the help, I read that v.surf.idw generates the interpolated surface 
from the data points.

Would this mean that the interpolated raster pass exactly through the 
vector input points (i.e., uploaded values should equal original input 
values), contrary to v.surf.rst where the smooth paramenter controls the 
deviation between the given points and the resulting surface?

Thanks and regards,

Luigi