[R-sig-Geo] extracting information from a raster file

[Paul Hiemstra]

Hi Harry,

I think what you want can be achieved using the overlay function 
(package sp). Code example:

sgdf    #SpatialGridDataframe
sp       # SpatialPoints (can also be SpatialPointsDataFrame), put the 
coordinates
           # where you want to know the attributes of the grid in here
overlay(sp, sgdf) # Returns the indices in sgdf that match sp
sgdf[overlay(sp,sgdf),] # Returns the values using those indices

So I think that you don't have to write any code to get what you want :).

cheers,
Paul

Harry Kim wrote:
> Dear R-sig-geo users,
>
>     I have a question about extracting information from a raster file
> after I read it in using rgdal package, and I would be infinitely
> grateful if someone could help me.
>     The summary of raster file looks like this:
>
> #summary of raster file
>   
>> summary(xxx)
>>     
> Object of class SpatialGridDataFrame
> Coordinates:
>    min max
> x -180 180
> y  -90  90
> 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
> x           -179.10      1.8       200
> y            -89.55      0.9       200
> Data attributes:
>      Min.   1st Qu.    Median      Mean   3rd Qu.      Max.      NA's
>     0.000     0.000     0.000    47.010     3.264 13540.000 26563.000
>
> I would like to write a code that extracts the value of associated
> cell given a specific location in latitude and longitude. I've noticed
> that xxx at data contains the values as 40000 x 1 matrix. I've wrote the
> following code assuming that the values are stored as in C programing
> (column wise seperation):
>
>
> #initiate values
> x_inc=xxx at grid@cellsize[1]
> y_inc=xxx at grid@cellsize[2]
>
> x_start=xxx at grid@cellcentre.offset[1]
> y_start=xxx at grid@cellcentre.offset[2]
>
> x_dim=xxx at grid@cells.dim[1]
> y_dim=xxx at grid@cells.dim[2]
>
> #given coordinate
> x=26
> y=45
>
> #find the index
> index=floor((x-x_start)/x_inc)*y_dim + floor( (y-y_start)/y_inc )
> xxx at data[index,1]
>
> The result does not seem to match what I should get. Could anybody
> explain to me how the data is stored in SpatialGridDataFrame? Your
> help would be much much appreciated.
>
> Thank you and have a good night
> Harry
>
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>   


-- 
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