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

[Edzer Pebesma]

We (the developers from sp) tried hard to relieve you from writing such 
code. Please try the following code, which requires you to click on the 
grid map after locator(1):

library(sp)
data(meuse.grid)
coordinates(meuse.grid)=~x+y
gridded(meuse.grid)=TRUE
fullgrid(meuse.grid) = TRUE
image(meuse.grid["dist"])
pt = locator(1)
pt
pt.sp = SpatialPoints(matrix(c(pt[[1]], pt[[2]]), 1, 2))
pt.sp
overlay(meuse.grid, pt.sp)

Besides being equally efficient, this code should remain working even if 
some internal representations of the sp classes were to change.

While looking at your code below, I have the feeling you forget that R 
arrays start with index 1, whereas C arrays start with index 0. You can 
check the sp sources to be 100% certain, but I also believe that the sp 
organization lets vary x first, then y.
--
Edzer

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
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>   

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de