Hello Luigi
On Wed, 13 Feb 2008, Luigi Ponti wrote:
Paul Kelly wrote:
On Wed, 13 Feb 2008, Luigi Ponti wrote:
I started using v.surf.idw (for irregularly spaced points) in grass6.0
with no -n flag, then in 6.1 I noticed that the output raster changed from
what it used to be (i.e., some stair-like artifacts appeared) and it was
restored to the original look (i.e., a much smoother look) by adding the
-n flag. By looking at your PhD thesis excerpt, it looks like the -n flag
should only make the interpolation faster
It might do, for a small number of points. It certainly uses less memory -
that is clear. But for a large number (I'm talking many hundreds of
thousands) the overhead of implementing the search radius method is well
outweighed by the time saving in not having to search through hundreds of
thousands of points for every cell in the output raster.
How many points are typically in the maps you used it with? I really can't
see any changes between 6.0 and 6.1 that would have affected the output -
it's quite intriguing - you can look at the WebCVS yourself:
http://freegis.org/cgi-bin/viewcvs.cgi/grass6/vector/v.surf.idw/main.c and
see if you can see anything.
Paul
True: the code did not change much from 6.0 to 6.1. Maybe I just happened to
find a peculiar combination of data that revealed the possible output
difference with and without the -n flag (I use v.surf.idw in a script to
routinely map output of ecological models). Typically, the input vector has
108 input points and the raster output is 1207 rows by 1350 columns. Is this
considered a small number of points?
Yes, I would consider that very small. As you can see in the thesis
excerpt, the size of dataset I had in mind when I implemented the new
algorithm was hundreds of thousands of points.
Did you avoid replying also to the list on purpose (this is just for me to
understand mailing list etiquette -- I have limited experience)?
Oops - for some reason I thought you had replied to me off-list, and so I
sent my reply off-list as well. Sorry for the confusion. Back on-list now.
Paul
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