[GRASS-user] Re: strange behaviour of v.surf.idw

[Paul Kelly]

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