Doubts about kriging
On Thursday 02 October 2008, GEMA FERNANDEZ-AVILES CALDERON wrote:
Dear list, I have two problem and I would like to know you opinion. 1. I am kriging 6 environmental variables, SO2, NOx, O3, CO, NO, PM10, to built and enviromental index in R. I have standardized the observed values and I am implementing in geoR the function likfit to obtain the parameter lambda (Box-Cox transformation) to see if the standarized variables are normal. But I have error using the likfit function. It could be that the error arise because the standarized variables contain negative values? 2. I have values of the 6 variables at 25 locations but I want to use a linear combination of the six variables to deal with only a variable. Finally, I want to interpolate this only variable in several sites where I have not observed data. What do you think is better? - To interpolate each one of the six variables and then elaborate the linear combination. - Or to elaborate a linear combination with the observed values and finally krige the linear combination. Thanks in advance, Gema Fern?ndez-Avil?s Universidad de Castilla-La Mancha. (Spain) [[alternative HTML version deleted]]
Hi, Dodging your questions a bit, but I have some concerns and/or issues for your consideration: 1. 25 locations is very small for kriging, unless you have some other means to develop your spatial covariance models. Perhaps you mean that the 6 variables are collocated at only 25 locations, but you have other sites with fewer than all 6? If not - if all you have are 25 sites with six variables each - I wouldn't attempt kriging in any form. 2. Actually I think 25 is pretty small for the multivariate analysis you propose as well. 3. With respect to your point 2, I would develop the linear combination with the real data, not on a bunch of interpolated 'data'. Standard methods of combining the variables will assume that the observations are independent; if they aren't then the model will be flawed, so be cautious of that. Hope this helps, Ashton
Ashton Shortridge Associate Professor ashton at msu.edu Dept of Geography http://www.msu.edu/~ashton 235 Geography Building ph (517) 432-3561 Michigan State University fx (517) 432-1671