Hi Maxime,
You need the grid.locator() function from the grid package in
combination with the trellis.focus() command from the lattice pacakge.
The following function I wrote has a syntax similar to spplot but
allow the user to click screen. The output of the function is either a
SpatialPoints object if attach_data is FALSE or a
SpatialPointsDataFrame otherwise, the data is extracted from
spatial_object using overlay(). This function only works if
spatial_object is a grid or a polygon. An example script plus the code
for the function:
library(sp)
data(meuse.grid)
gridded(meuse.grid) = ~x+y
# Be sure to paste the function into the session before using this
command
CreateSpatialPointsFromGraphicDevice(meuse.grid, "dist")
CreateSpatialPointsFromGraphicDevice = function(spatial_object, ...,
proj4string = NA, attach_data = TRUE) {
require(grid)
require(lattice)
cat("Exit with right mouse-click...\n")
print(spplot(spatial_object, ...))
trellis.focus()
x_list = vector()
y_list = vector()
data = data.frame()
while(TRUE) {
dum = grid.locator()
if(is.null(dum)) break
ptdf = data.frame(x = as.numeric(dum$x), y = as.numeric(dum$y))
x_list = c(x_list, as.numeric(dum$x))
y_list = c(y_list, as.numeric(dum$y))
lpoints(ptdf, pch = 3)
if(attach_data) {
ovr = overlay(spatial_object, SpatialPoints(ptdf))
if(is(spatial_object, "SpatialPolygonsDataFrame")) {
data = rbind(data, ovr)
}
if(is(spatial_object, "SpatialGridDataFrame") || is(spatial_object,
"SpatialPixelsDataFrame")) {
data = rbind(data, spatial_object at data[ovr,])
}
#if(length(data) == 0) stop("No data could be extract, input object
does not contain data")
}
}
trellis.unfocus()
#print(data)
if(length(data) == 0) {
warning("No data could be extracted, reverting to a SpatialPoints
object")
attach_data = FALSE
}
if(attach_data) {
return(SpatialPointsDataFrame(data.frame(x = x_list, y = y_list),
data = data, proj4string = CRS(as.character(proj4string))))
} else {
return(SpatialPoints(data.frame(x = x_list, y = y_list), proj4string =
CRS(as.character(proj4string))))
}
}
cheers and hth,
Paul
Maxime Pauwels wrote:
Dear all,
I think there is a function in the sp package to get the coordinates
of a point on a sp plot (~equivalent to "locator").
Could someone remember me its name?
Many thanks,
max