[R-sig-Geo] A few more questions about Adehabitat!?

[Sander Oom]

Dear Clement,

I have made enormous progress, but I am still struggling with a couple 
of things, hence the following questions:

- How can I determine the area of the polygons in an object of class
'asc'? The 'ararea' function only works on objects of class 'area', and
I can not convert 'asc' to 'area'. I would prefer a list of areas for 
each polygon, which I can than total to the whole home range area.

- Would it be possible to create a function to extract the extent (x and
y limits) of an 'asc' or 'kasc' object, such that I can use the result
as an argument in the 'subsetmap' function to set xl and yl?

- Mean('asc object') returns a single value, as I was expecting, but
var('asc object') returns a matrix and sd('asc object') returns a
vector. I must be missing something! Also mean() gives NA if part of the 
grid is NA. See example below!

- Is it possible to do calculations with an 'asc' object? Such as 
multiplying each cell with a value? I read a grid from file and would 
like to change the cell values. I would also like to change certain cell 
values to NA,before doing the calculations as above. The same would be 
possible if I could get the map out of the 'asc' object and put it in a 
matrix of data frame.

Thanks in advance,

Sander.

Sample code for reading in an ascii grid:

 > (file1 <-  paste(system.file(package = "adehabitat"),
+                "ascfiles/elevation.asc", sep = "/"))
[1] "/usr/lib/R/library/adehabitat/ascfiles/elevation.asc"
 > xl <- c(701000, 702000)
 > yl <- c(3160000, 3161000)
 >
 > el <- import.asc(file1)
 > image(el)
 > el
Raster map of class "asc":
Cell size:  100
Number of rows:  121
Number of columns:  111
Type:  numeric
 > mean(el)
[1] NA
 > elsub<-subsetmap(el, xlim = xl, ylim = yl)
 > image(elsub)
 > mean(elsub)
[1] 265.281
 > rr <- lapply(el, function(x) x*1000)
 > mean(rr)
[1] NA
Warning message:
argument is not numeric or logical: returning NA in: mean.default(rr)
 > var(elsub)
             [,1]  [,2]      [,3]       [,4]       [,5]        [,6] 
  [,7]
  [1,] 456.818182 397.1 345.58182 304.172727 298.054545 179.9727273 
170.61818
  [2,] 397.100000 348.6 304.80000 269.300000 262.400000 158.6000000 
146.90000
  [3,] 345.581818 304.8 270.01818 241.027273 235.945455 140.8272727 
132.18182
  [4,] 304.172727 269.3 241.02727 224.490909 219.818182 127.3909091 
118.17273
  [5,] 298.054545 262.4 235.94545 219.818182 217.963636 125.4181818 
117.65455
  [6,] 179.972727 158.6 140.82727 127.390909 125.418182  80.6909091 
71.17273
  [7,] 170.618182 146.9 132.18182 118.172727 117.654545  71.1727273 
80.01818
  [8,] 110.600000  93.6  84.10000  70.400000  69.400000  45.8000000 
61.70000
  [9,]  38.972727  26.8  23.52727   7.490909   7.318182  16.7909091 
49.07273
[10,]  -8.809091 -14.1 -10.89091 -26.536364 -26.027273  -0.6363636  38.39091
[11,] -70.254545 -73.4 -66.44545 -81.718182 -79.963636 -30.0181818  18.94545
        [,8]       [,9]       [,10]     [,11]
  [1,] 110.6  38.972727  -8.8090909 -70.25455
  [2,]  93.6  26.800000 -14.1000000 -73.40000
  [3,]  84.1  23.527273 -10.8909091 -66.44545
  [4,]  70.4   7.490909 -26.5363636 -81.71818
  [5,]  69.4   7.318182 -26.0272727 -79.96364
  [6,]  45.8  16.790909  -0.6363636 -30.01818
  [7,]  61.7  49.072727  38.3909091  18.94545
  [8,]  59.8  73.000000  75.7000000  74.10000
  [9,]  73.0 136.290909 161.5636364 191.18182
[10,]  75.7 161.563636 201.8545455 242.62727
[11,]  74.1 191.181818 242.6272727 315.56364
 >
-- 
--------------------------------------------
Dr. Sander P. Oom
Animal, Plant and Environmental Sciences,
University of the Witwatersrand
Private Bag 3, Wits 2050, South Africa
Tel (work)      +27 (0)11 717 64 04
Tel (home)      +27 (0)18 297 44 51
Fax             +27 (0)18 299 24 64
Email   sander at oomvanlieshout.net
Web     www.oomvanlieshout.net/sander