[R-sig-Geo] Converting image to asc

[Kévin Morelle]

Dear list members,

I'm facing the following error when I want to convert an image object 
(obtained from the density function of the SpatStat package) into a asc 
object using the im2asc function of adehabitat:

mais.asc <- im2asc(mais.dens)
Erreur dans im2asc(mais.dens) :
   the grid cellsize should be identical for both X and Y directions.

I previously delimited my study area by a polygon, could it be the 
external grid cells (cut by that polygon) that would give this error 
(because leading to non-square cells...)?
Below, my code more detailed.

Thanks for any help,
Regards,

Kevin

# Maize fied for the whole region
-------------------------------------------
mais <- readShapePoly("D:/Carto/Landscape/Mais_2009.shp")

#  Limiting to my study area (W.sp)
-----------------------------------------------
mais.ze <- crop (mais,W.sp)

# Deriving density of maize field throught the density function
------------------------------------------------------------------------------------
#1. find centroid of maize field

mais.ps <- SpatialPolygons2PolySet(mais.ze)
mais.centroids <- calcCentroid(mais.ps, rollup=1)
mais.df <- as.data.frame(mais.centroids)

#2. convert to point pattern class+apply density function

mais.ppp <- ppp (mais.df$X, mais.df$Y, window = W2)
mais.dens <- density(mais.ppp, sigma=1000)

#3. convert mais.dens to asc for ENFA anaysis (in adehabitat)

mais.asc <- as.asc(mais.mx)
=>Erreur dans im2asc(mais.dens) :
   the grid cellsize should be identical for both X and Y directions.

-- 
Morelle Kevin
Université de Liège - Gembloux Agro-Bio Tech
Unité de Gestion des Ressources forestières et des Milieux naturels
--------
University of Liège - Gembloux Agro-Bio Tech
Unit of Forest and Nature Management
--------
2, Passage des Déportés
B-5030  GEMBLOUX
BELGIQUE

Tél : + 32 (0)81 62.22.14
Fax : + 32 (0)81 62.23.01
E-mail : kevin.morelle at ulg.ac.be
Web : http://www.fsagx.ac.be/gf/