Re: [GRASS-user] Grass Script- Implementing a Quadratic Mahalanobis Distance
R has a command to calculate the Mahalanobis distance [1], so I believe it's just a matter of getting the grass data into R in the correct way and using the already R built in tools in R. Unfortunately, this is as far as I can help, not being an expert in R myself Cheers Daniel [1] - http://stat.ethz.ch/R-manual/R-devel/library/stats/html/mahalanobis.html On Fri, Oct 15, 2010 at 8:45 AM, Nikos Alexandris nikos.alexand...@felis.uni-freiburg.de wrote: ( Pedro, I kindly ask you to keep the discussion on the list - it's very interesting (to me at least) and of wider interest (I guess) as a small challenge for grass. I am sure others can help here. Also, accept my apologies for re-structuring the mail to make it easy to follow for other readers. ) Pedro wrote: I need to implement a Quadratic Mahaloanobis Distance by using GRASS. This is based on medium vector and covariance matrix. 1- As anyone implemented anything similar? 2- If not, Is there any GRASS script that accesses medium vector and Covariance matrix in its processing? Nikos: Not sure at all but my guess is that it's a (simple) job for grass if you know the formula. I am not familiar with the term medium vector. But there is r.covar to get the covariance (or correlation) matrix based on raster maps. Pedro: Medium vector is a vector/array in which each element is the average of that class in each image [average in image1; average in image2, ...] The problem of implemeting this in a script is that it required matrix calculation. I mean, for each pixel in a class, I will have to multiplicate byt the covariance matriz and this vector. I don't have a quick answer on how to do it within grass using r.mapcalc. But I am convinced that using grass + R is a solution. Also, is the following matlab (open source?) code of any use? http://www.hackchina.com/en/r/129852/mahalanobis.m__html I've read on the net efforts trying to link R and matlab. If the latter is practically doable, then you can even avoid writing the code (again). So, I imagine something like: 1. import data in grass and prepare/process etc. 2. load data of interest in R 3. calculate the (qudratic) mahalanobis distances of interest (within R with R code/ with matlab code ?) 4. export results (or even maps) in grass back again Cheers, Nikos ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Grass Script- Implementing a Quadratic Mahalanobis Distance
Daniel Victoria wrote: R has a command to calculate the Mahalanobis distance [1], so I believe it's just a matter of getting the grass data into R in the correct way and using the already R built in tools in R. Unfortunately, this is as far as I can help, not being an expert in R myself Sorry that I did not take the time to search better within R. So, it seems to be very easy then using the R function mahalanobis(). As Daniel notes, it's just a matter of correctly loading the data in R. @Pedro: If you have difficulties to get started with R, check the wiki [2]. There are examples. Myself I calculated separability measures this way (grass raster samples - R - separability measures - exported as csv). Cheers, Nikos [1] - http://stat.ethz.ch/R-manual/R-devel/library/stats/html/mahalanobis.html [2] http://grass.osgeo.org/wiki/R_statistics ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Grass Script- Implementing a Quadratic Mahalanobis Distance
( Pedro, I kindly ask you to keep the discussion on the list - it's very interesting (to me at least) and of wider interest (I guess) as a small challenge for grass. I am sure others can help here. Also, accept my apologies for re-structuring the mail to make it easy to follow for other readers. ) Pedro wrote: I need to implement a Quadratic Mahaloanobis Distance by using GRASS. This is based on medium vector and covariance matrix. 1- As anyone implemented anything similar? 2- If not, Is there any GRASS script that accesses medium vector and Covariance matrix in its processing? Nikos: Not sure at all but my guess is that it's a (simple) job for grass if you know the formula. I am not familiar with the term medium vector. But there is r.covar to get the covariance (or correlation) matrix based on raster maps. Pedro: Medium vector is a vector/array in which each element is the average of that class in each image [average in image1; average in image2, ...] The problem of implemeting this in a script is that it required matrix calculation. I mean, for each pixel in a class, I will have to multiplicate byt the covariance matriz and this vector. I don't have a quick answer on how to do it within grass using r.mapcalc. But I am convinced that using grass + R is a solution. Also, is the following matlab (open source?) code of any use? http://www.hackchina.com/en/r/129852/mahalanobis.m__html I've read on the net efforts trying to link R and matlab. If the latter is practically doable, then you can even avoid writing the code (again). So, I imagine something like: 1. import data in grass and prepare/process etc. 2. load data of interest in R 3. calculate the (qudratic) mahalanobis distances of interest (within R with R code/ with matlab code ?) 4. export results (or even maps) in grass back again Cheers, Nikos ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Grass Script- Implementing a Quadratic Mahalanobis Distance
On Monday 11 of October 2010 19:21:54 Pedro Roma wrote: Greetings I need to implement a Quadratic Mahaloanobis Distance by using GRASS. This is based on medium vector and covariance matrix. 1- As anyone implemented anything similar? 2- If not, Is there any GRASS script that accesses medium vector and Covariance matrix in its processing? Thanks PEdro Hi Pedro! Not sure at all but my guess is that it's a (simple) job for grass if you know the formula. I am not familiar with the term medium vector. But there is r.covar to get the covariance (or correlation) matrix based on raster maps. Nikos ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] Grass Script- Implementing a Quadratic Mahalanobis Distance
Greetings I need to implement a Quadratic Mahaloanobis Distance by using GRASS. This is based on medium vector and covariance matrix. 1- As anyone implemented anything similar? 2- If not, Is there any GRASS script that accesses medium vector and Covariance matrix in its processing? Thanks PEdro ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
