R help - Jul 2008 - Are centre coordinates or upper left corners used of x, y for SpatialPixels?

Dear all,

I'm working with satellite images in R and plotting them via the code
below.
I was wondering whether coordinates (spatial["x"],
spatial["y"]) are
used as centre coordinates of the pixels in the GRID? In this script;
spatial["x"] & spatial["y"] are the centre coordinates
of the satellite
image pixels. I'm asking this because some software packages use the
upper left corner of a pixel as a reference.

require(sp)
  # --> Create GRID and image in R
   S <- SpatialPoints(cbind(spatial["x"], spatial["y"]),
proj4string proj)
    SP <- SpatialPixels(S, proj4string = proj, tolerance = 5e-07)
    GRID <- SpatialPixelsDataFrame(points = SP , tolerance = 5e-07, data
= spatial[columnname])
    proj4string(GRID) <- proj
    gridded(GRID)
    
    nr <- 30
    zaxis <- c(0.7, 1)
    plot(as(GRID, "Spatial"), axes=T)
    title(main = paste(" Day ",dy[i,1], " Year ",dy[i,2],
sep=""))
    image(GRID, zlim=zaxis, col=rainbow(nr,start=0, end=2/6),add=T)

Thanks for your help,
Regards,
Jan

Ps. Is this good approach to plot satellite images as spatial grids in
R? is rgdal an better approach?

Hi Jan,

In response to your ps. I think using rgdal is the best way to go. If 
your data is for example in a GeoTIFF file, reading it is as simple as:

GRID = readGDAL("your_file.tif")

Saves you some hassel and a number of lines of code.

cheers and hth,
Paul

Jan.Verbesselt at csiro.au wrote:
> Dear all,
>
> I'm working with satellite images in R and plotting them via the code
> below.
> I was wondering whether coordinates (spatial["x"],
spatial["y"]) are
> used as centre coordinates of the pixels in the GRID? In this script;
> spatial["x"] & spatial["y"] are the centre
coordinates of the satellite
> image pixels. I'm asking this because some software packages use the
> upper left corner of a pixel as a reference.
>
> require(sp)
>   # --> Create GRID and image in R
>    S <- SpatialPoints(cbind(spatial["x"],
spatial["y"]), proj4string > proj)
>     SP <- SpatialPixels(S, proj4string = proj, tolerance = 5e-07)
>     GRID <- SpatialPixelsDataFrame(points = SP , tolerance = 5e-07, data
> = spatial[columnname])
>     proj4string(GRID) <- proj
>     gridded(GRID)
>     
>     nr <- 30
>     zaxis <- c(0.7, 1)
>     plot(as(GRID, "Spatial"), axes=T)
>     title(main = paste(" Day ",dy[i,1], " Year
",dy[i,2], sep=""))
>     image(GRID, zlim=zaxis, col=rainbow(nr,start=0, end=2/6),add=T)
>
> Thanks for your help,
> Regards,
> Jan
>
> Ps. Is this good approach to plot satellite images as spatial grids in
> R? is rgdal an better approach?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>   

-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone: 	+31302535773
Fax:	+31302531145
http://intamap.geo.uu.nl/~paul

...and in addition:

The r-sig-geo mailing list is a better forum for your kind of questions, 
you are much more likely to get good answers.

cheers,
Paul

Jan.Verbesselt at csiro.au wrote:
> Dear all,
>
> I'm working with satellite images in R and plotting them via the code
> below.
> I was wondering whether coordinates (spatial["x"],
spatial["y"]) are
> used as centre coordinates of the pixels in the GRID? In this script;
> spatial["x"] & spatial["y"] are the centre
coordinates of the satellite
> image pixels. I'm asking this because some software packages use the
> upper left corner of a pixel as a reference.
>
> require(sp)
>   # --> Create GRID and image in R
>    S <- SpatialPoints(cbind(spatial["x"],
spatial["y"]), proj4string > proj)
>     SP <- SpatialPixels(S, proj4string = proj, tolerance = 5e-07)
>     GRID <- SpatialPixelsDataFrame(points = SP , tolerance = 5e-07, data
> = spatial[columnname])
>     proj4string(GRID) <- proj
>     gridded(GRID)
>     
>     nr <- 30
>     zaxis <- c(0.7, 1)
>     plot(as(GRID, "Spatial"), axes=T)
>     title(main = paste(" Day ",dy[i,1], " Year
",dy[i,2], sep=""))
>     image(GRID, zlim=zaxis, col=rainbow(nr,start=0, end=2/6),add=T)
>
> Thanks for your help,
> Regards,
> Jan
>
> Ps. Is this good approach to plot satellite images as spatial grids in
> R? is rgdal an better approach?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>   

-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone: 	+31302535773
Fax:	+31302531145
http://intamap.geo.uu.nl/~paul