[R] importing ascii grids (for gstat)

[Andy Bunn]

If you have exported the grid from Arc using the asciigrid command then
you can read it in with scan or read.table. You can tell R to skip the
six lines of header info and to convert -9999 to NA e.g., 

$ snep.tmin <- read.table(file = "tmin.asc", sep = " ", na.strings =
"-9999", skip = 6)

Check the number of rows and columns to make sure it matches your data
(in Windows, Arc puts a space before the line return at the end of a row
making the resulting R object have one too many columns.)

If that happens, then remove it:

$ snep.tmin <- snep.tmin[,-ncol(snep.tmin)]

(If there is a work around for the read.table command that somebody else
uses then I'd love to hear it.)

For gstat, it looks like it would be helpful to put the grid into a
vector and attach the coordinate information in a data frame? If so,
from the header information take the lower left corner and make your
coordinate columns and join it to the grid data. e.g.,

$ xLLcorner <- -1855500
$ yLLcorner <-  -944500
$ cellsize <- 1000
$ 
$ xURcorner <- xLLcorner + (cellsize * (ncol(snep.tmin) - 1))
$ xLRcorner <- xURcorner
$ xULcorner <- xLLcorner
$ 
$ yULcorner <- yLLcorner + (cellsize * (nrow(snep.tmin) - 1))
$ yURcorner <- yULcorner
$ yLRcorner <- yLLcorner
$ 
$ coords <- expand.grid(y = seq(yULcorner, yLRcorner, by = -1000),
+                       x = seq(xULcorner, xLRcorner, by = 1000))
$ 
$ tmin.frame <- data.frame(coords, tmin = as.vector(c(snep.tmin,
recursive = T))) $ 
$

Watch your signs depends on the coordinate system. From there you can
krige or whatever easily.

HTH, Andy



$ version
         _              
platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    1              
minor    8.1            
year     2003           
month    11             
day      21             
language R