[STATSGRASS] questions regarding GRASS/gstat

[Thomas Adams]

Edzer,

Thank you for your response. The models I have tried are: Spherical, 
Exponential, and Power, nothing else. Poizot Emmanuel, in his response, 
said that in his experience, one must not have points outside the mask 
area. I have not tried this as yet. When I do I let both you and Pouzot 
know what happened.

Regards,
Tom


Edzer J. Pebesma wrote:

>
>
> Thomas Adams wrote:
>
>> I have been attempting to use gstat within GRASS and have had success 
>> up to a point. I am attempting to estimate precipitation over a large 
>> river basin (~450000 sq km) from monthly (not mean monthly) point 
>> values. I'm working with a Lambert Conic Conformal grid region with a 
>> ~5 km grid spacing (the x & y spacings are slightly different). My 
>> orginal dataset has nearly 500 data points, but some were too close 
>> together (actually coincident points with different IDs), so I wrote 
>> a program to eliminate points that were too close, <7.5 km. I have 
>> increased this threshold several times, to 50 km and still get a 
>> gstat error when I attempt the ordinary kriging, which says that I 
>> have a "matrix library error: singular matrix".
>>
>> I must be doing something wrong since my once nearly 500 station 
>> locations is now ~100! One other note, is that gstat reports that 
>> some of my stations lie outside my mask region (the river basin 
>> boundary), but I don't see that this should be a problem, is it? 
>
>
> Once you end up with a singular covariance matrix, it is pretty hard
> (and gstat does not attempt) to find out the reason why it occurs.
> Possible reasons are (as you said): duplicate observations -- gstat
> can catch them by taking their average, using something like:
>
> data(rainfall): 'rainfall', ... , average=1;
>
> other reasons may be the variogram model you use: the Gaussian
> model often leads to singular models, and some authors (M. Stein)
> argue not to use it at all; other models, such as linear-with-sill
> should be avoided in 2D or 3D situations, but gstat does not check
> this.
>
> Let me know if this didn't help.
> -- 
> Edzer
>

-- 
Thomas E Adams
National Weather Service
Ohio River Forecast Center
1901 South State Route 134
Wilmington, OH 45177

EMAIL:	thomas.adams at noaa.gov

VOICE:	937-383-0528
FAX:	937-383-0033