Am 10.02.2012 08:29, schrieb Edzer Pebesma:
On 02/10/2012 08:00 AM, Tom Gottfried wrote:
Edzer,
Am 09.02.2012 17:58, schrieb Edzer Pebesma:
On 02/09/2012 05:02 PM, Etienne B. Racine wrote:
It seems that sampleRandom isn't so random.
# try this :
library(raster)
r<- raster(matrix(1:25, 5, 5))
sampleRandom(r, 25)
## 1 6 11 16 21 2 7 12 17 22 3 8 13 18 23 4 9 14 19 24 5 10 15
sampleRandom(r, 25)
## 1 6 11 16 21 2 7 12 17 22 3 8 13 18 23 4 9 14 19 24 5 10 15
I haven't seen any documentation about this behavior.
Isn't the more important question whether you saw documentation that is
in conflict with this behaviour?
taking that to the extremes: undocumented software always works perfectly.
And is useless, too -- no, these kind of questions are often highly
useful because they improve software or the documentation, and both are
valuable activities.
From the base::sample analogy, I would have expected a random
permutation, so IMO it would make sense to document this is not the case
(the function is not called sample, anyway -- question are: what would
the random permutation be good for? and would it still perform well when
the image is huge?)
(By the way I've heard that's the way ESRI did it with their
"Geostatistical Analyst")
You might be mixing up things here or have outdated information; the
author of Geostatistical Analyst published in 2011 (at ESRI press) a
nearly 1000 page e-book "Spatial Statistical Data Analysis for GIS
users", which obviously addresses ArcGIS, but also R and WinBugs for
areas where ArcGIS does not offer solutions. He is on this list, too.
thanks, just ordered from the library to have a look and spread the word.
Tom
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