[R-sig-Geo] Import Part of a GeoTiff using readGDAL{rgdal}

[Michael Sumner]

With readGDAL you must figure out the impled coordinates, referenced
from the top left - it's confusing, but with some experimenting you
will get it.

But, far easier is to use the raster package, which will let you deal
with the entire Etopo1 if you want by accessing the file as needed,
and there are simpler methods to crop the big raster - and coerce to
SpatialGridDataFrame if need be.

E.g.
## all this works pretty fast as raster takes care of the memory
library(raster)
etopo1 <- raster("Etopo1.tif")
 e <- extent(-90, -32, -60, 15)
r <- crop(etopo1, e)
plot(r)

## if we want to enforce the full memory use as an sp object
x <- as(r, "SpatialGridDataFrame")
image(x)
summary(x)
Object of class SpatialGridDataFrame
Coordinates:
   min max
s1 -90 -32
s2 -60  15
Is projected: FALSE
proj4string :
[+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0]
Number of points: 2
Grid attributes:
   cellcentre.offset   cellsize cells.dim
s1         -89.99167 0.01666667      3480
s2         -59.99167 0.01666667      4500
Data attributes:
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
  -8086   -4188   -2583   -2025     148    6494



sessionInfo()
R version 2.11.1 (2010-05-31)
x86_64-pc-mingw32

locale:
[1] LC_COLLATE=English_Australia.1252  LC_CTYPE=English_Australia.1252
[3] LC_MONETARY=English_Australia.1252 LC_NUMERIC=C
[5] LC_TIME=English_Australia.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] rgdal_0.6-27 raster_1.2-6 sp_0.9-65

loaded via a namespace (and not attached):
[1] grid_2.11.1    lattice_0.18-8 tools_2.11.1

On Fri, Jul 23, 2010 at 1:14 AM, Rodrigo Aluizio <r.aluizio at gmail.com> wrote:
> Hi List.
>
> I’m trying to import only a part of interest from a raster image map (let
> say, ETOPO 1 Global Relief GeoTiff). I realize that, PROBABLY, the readGDAL
> function can deal with my issue through the arguments:
>
>
>
> offset - Number of rows and columns from the origin (usually the upper left
> corner) to begin reading from; presently ordered (y,x) - this may change
>
> region.dim - The number of rows and columns to read from the dataset;
> presently ordered (y,x) - this may change
>
>
>
> But, I don’t know exactly how to specify that I want the rows (and columns?)
> which show only South America, I do know the SA coordinate limits, but not
> the corresponding rows.
>
> The real problem is that I don’t know how to use the function resources
> properly. I’m trying to limit the region during import to avoid memory limit
> issues, once importing the ETOPO 1 Geotiff tries to generate a huge vector.
>
> Really Sorry About It, Thanks for Your Patience and Attention.
>
>
>
> -------------------------------------------------------------
>
> MSc.  <mailto:r.aluizio at gmail.com> Rodrigo Aluizio
>
> Centro de Estudos do Mar/UFPR
> Laboratório de Micropaleontologia
> Avenida Beira Mar s/n - CEP 83255-000
> Pontal do Paraná - PR - Brasil
>
>
>
>
>        [[alternative HTML version deleted]]
>
>
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>



-- 
Michael Sumner
Institute for Marine and Antarctic Studies, University of Tasmania
Hobart, Australia
e-mail: mdsumner at gmail.com