[R-sig-Geo] creating irregularly-shaped grids for kriging

[Pierre Roudier]

Hi again, and apologises to the list for the posting problem,

I am using a workaround for this using inout() and getpoly() (or
locator()) on agricultural fields that have irregular boundaries:

1- Make your regular grid using expand.grid. Then you got your
rectangular grid under a variable called grd.

2a- If you already have the coordinates of the boundaries of the
polygon, have a look to ?inout from the splancs package to sort the
coordinates of your grid.

bound <- read.csv('myboundary.txt',header=TRUE)
grd <- grd[inout(grd,bound),]

2b- If you want to enter manually those boundaries, plot the
coordinates of your grid and enter the polygon using getpoly() from
the same package :

plot(grd,pch='+')
bound <- getpoly()
grd <- grd[inout(grd,bound),]

Here is a simple example that works on my box:

require(splancs)
x <- seq(0,100,by=2)
y <- seq(0,100,by=5)
grd <- expand.grid(x=x,y=y)
plot(grd,pch='+')
bound <- getpoly()
grd.irregular <- grd[inout(grd,bound),]
plot(grd,pch='+')
points(grd.irregular,pch=16,col='red')

Then you can convert it to a sp object:
names(grd.irregular)[1] <- 'x'
names(grd.irregular)[2] <- 'y'
coordinates(grd.irregular) <- c('x','y')
gridded(grd.irregular) <- TRUE

All the best,

Pierre

2009/7/20, Pierre Roudier <pierre.roudier at gmail.com>:
> Hi Craig,
>
> I have a workaround for this using the getpoly() inout()
>
> 2009/7/20, Scanlan, Craig <craig.scanlan at agric.wa.gov.au>:
>> Hi all,
>>
>> I need to krige data that is irregularly spaced within irregularly
>> shaped areas (agricultural fields).
>>
>> At present, I am creating a regular grid for newdata in
>> krige(data,newdata....) by
>>
>> x.range <- as.integer(range(a2 at coords[,1]))
>> y.range <- as.integer(range(a2 at coords[,2]))
>> grd <- expand.grid(x=seq(from=x.range[1], to=x.range[2], by=10),
>> y=seq(from=y.range[1], to=y.range[2], by=10) )
>>
>> I have not been able to find a method to trim the newdata grid to the
>> same shape as the original data - is this possible?
>>
>> The task I am trying to achieve is the same as creating meuse.grid from
>> meuse data.
>>
>> Can anyone help?
>>
>> Regards,
>>
>> Craig.
>>
>>
>>
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