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 [EMAIL PROTECTED] 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
[EMAIL PROTECTED]@cellsize[1]
[EMAIL PROTECTED]@cellsize[2]
[EMAIL PROTECTED]@cellcentre.offset[1]
[EMAIL PROTECTED]@cellcentre.offset[2]
[EMAIL PROTECTED]@cells.dim[1]
[EMAIL PROTECTED]@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 )
[EMAIL PROTECTED],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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--
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 [EMAIL PROTECTED]
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