Well, it seems you didnt go too far ;-)
Lets say that our point is in the center of the raster
>point <-c(5,5)
# compute the distances from this point to each
cell of the raster (the object named "cosa")
> mg <- expand.grid(x=seq(0,10,length=500), y=seq(0,10,length=500))
> cosa <- crossdist(point[1],point[2],mg$x,mg$y)
# See how many cells are at *exactly* 2 units distance from the point
>plot(M)
> points(mg[cosa==2,],pch=19, col=2)
# none in my computer !
# See how many cells are in a ring of width 0.02
units at 2 units distance from the point
> points(mg[cosa<2.01 & cosa>1.99,],pch=19, col=2)
# how many?
> sum(cosa<2.01 & cosa>1.99)
# get their values
> M[mg[cosa<2.01 & cosa>1.99,]]
Hope this helps,
Marcelino
At 02:25 13/07/2012, Lorenzo Cattarino wrote:
Hi R users,
I would like to be able to calculate how many
cells of a raster, and their values (1 or 0),
lie at a distance r from a particular cell in
the raster. In other words, I need to calculate
the number and values of the cells lying on a
ring with radius r and centred in x0 and y0.
This is as far I got:
require(spatstat)
# Build an example image of 0's and 1's:
M <- im(matrix(sample(0:1,500*500,TRUE),nrow=500),xcol=seq(0,10,length=500),
yrow=seq(0,10,length=500))
Thanks
Lorenzo
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___________________________________________________
Marcelino de la Cruz Rot
Departamento de Biología y Geología
Escuela Superior de Ciencias Experimentales y Tecnología
Universidad Rey Juan Carlos
c/ Tulipán, s/n.
E-28933 Móstoles (Madrid)
España
Fax: +34 91 664 7490
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