AKIMA: z values at a set coordinate
Hi Anonymous, (Maybe next time include your name)
There are data objects in R that are designed for spatial data, look at
the sp package. Casting them into this format gives you an enormous
increase in flexibility with analyzing spatial data. See the example
below using your example:
library("akima")
library(sp)
data(akima)
akima.li <- interp(akima$x, akima$y, akima$z)
# Change to sp object
# Note that we swap the x and y column [1]
y = rep(akima.li$x, each = length(akima.li$y))
x = rep(akima.li$y, length(akima.li$x))
z = as.numeric(akima.li$z)
akima.sp = data.frame(x, y, z)
# sp-function, which columns are the coordinates
coordinates(akima.sp) = ~x+y
# Tell sp that it is a grid
gridded(akima.sp) = TRUE
# Plot and compare
image (akima.li)
spplot(akima.sp)
# Use overlay from sp to get the value
# at a specific location
pt = data.frame(x = 11.25, y = 6.5)
coordinates(pt) = ~x+y
val = akima.sp at data[overlay(akima.sp, pt),]
val
# [1] 19.14752
Learning to use sp-objects is really worthwhile. See the spatial Task
view for more information, or check out the R-wiki [2]. With these kind
of geographic questions you might want to use the r-sig-geo mailing list
instead of R-help.
cheers,
Paul
[1] We do this because (from details section of Image):
Notice that ?image? interprets the ?z? matrix as a table of
?f(x[i], y[j])? values, so that the x axis corresponds to row
number and the y axis to column number, with column 1 at the
bottom, i.e. a 90 degree counter-clockwise rotation of the
conventional printed layout of a matrix.
[2] http://wiki.r-project.org/rwiki/doku.php?id=tips:spatial-data
Rhelp wanted wrote:
Dear all.
I am using the akima function to produce 3d contour plots using interp based
on irregular data.
using the eg in the akima manual
library("akima")
data(akima)
plot(y ~ x, data = akima, main = "akima example data")
with(akima, text(x, y, formatC(z,dig=2), adj = -0.1))
## linear interpolation
akima.li <- interp(akima$x, akima$y, akima$z)
image (akima.li, add=TRUE)
contour(akima.li, add=TRUE)
points (akima, pch = 3)
so with this in mind is there a way of obtaining the interpolated value at a
particular coordinate eg at (11.25,6.5) I can see that it as an orange and
should I look at the contour lines I can see what value it produces. However
Is there a way of saying function[11.25,6.5] which provides a value for that
coordinate.
Hope someone can help
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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: +3130 274 3113 Mon-Tue Phone: +3130 253 5773 Wed-Fri http://intamap.geo.uu.nl/~paul