As you said, readGDAL will give you a SpatialGridDataFrame object, which
behaves much like a normal data.framet.  Your best bet is to run it through
names to see what data are available to work with:

names(map)

Like a regular data.frame this will give you the bands or data read in from
your ASCII grid, on which you can pull subsets or index like you would a
normal vector (e.g. pretend your map object has "band1" as its only data,
you can pull a subset of the first 10 grid values like this:

map$band1[1:10]

I hope that gets you closer to solving the rest of your question,

Forrest Stevens
Department of Geography
Land Use and Environmental Change Institute
University of Florida

2011/8/26 Michał Kwieciński <jamesbond6@gmail.com>

> Hi,
>
> I am having difficulty in reading data for further use that is in
> ASCII grid format (it's the UN population data from
> http://sedac.ciesin.columbia.edu/gpw). I used readGDAL function which
> leaves me with SpatialGridDataFrame class object, but apart from
> creating simple image with
>
> map = readGDAL("eur.asc", region.dim=c(889,1050), output.dim=c(889,1050))
> image(map, col=topo.colors(256, alpha=1))
>
> , I have no clue how to operate on this object. All I can see it
> contains x and y coordinates, but any head() or summary() does not
> provide me with information on how to refer to the actual data that
> ASCII grid contains. And yet the image can be generated properly, so
> the data obviously is somewhere there! I can see I theoretically
> should find something under map["z"], but all I have is an error:
>
> Error in `[.data.frame`(x@data, , i, drop = FALSE) :
>  undefined columns selected
>
>
>
> Or entire problem may be stated in terms of question: how do I set
> colours for map like that (
> http://sedac.ciesin.columbia.edu/gpw/maps/poldens.pdf )? (let's say I
> want to manually set intervals for colouring the variable that is in
> the grid file)
>
> This is not the goal of my work with this data, but I believe the
> solution do this problem has very much in common with general
> operations on data from ascii grid file. Disregarding the need to
> create beautiful maps, I can always try to use read.table (oh, wait,
> the 5 MB bit freezes my R) and then use this data for other complex
> spatial calculations that require also other data to be combined with
> data from grid, but right now my problem is just as specified above.
>
>
> Or maybe I should have used something else than readGDAL?
>
>
> I am using R2.9.2, as any later version leaves me with blank output
> instead of the image I want. Don't know the reason for it, but I
> thought I will stay with the version that works instead of unnecessary
> troubleshooting. I also wrote a program in C that extracted the europe
> part of the world data, so that my ASCII file is 5 MB instead of 117
> MB, but it appears to be OK, so I don't think the problem lies here.
>
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