convert x,y,z data into a grid

Michal,
thanks for the suggestions. If I try the wireframe I get a warning
message

"Warning message:
In nx * ny : NAs produced by integer overflow"

I haven't been able to figure out why that is, but could have something
to do with the 2.5mil * 2.5mil number of rows....

My data are not in a grid format, so the second option will not work. 
The data are simply UTM positions and depth values from a cruise
covering approximately an area of 10 square kilometers. In Grass there
is a function that reads in the dataframe and converts the data to a
grid which I think is easier to work with in many of the different
plotting routines.

Hope anyone can help.
Erik



-----Original Message-----
From: Michal Gallay [mailto:mgallay01 at qub.ac.uk] 
Sent: Wednesday, May 07, 2008 11:48
To: Meesters, Erik
Subject: Re: [R-sig-Geo] convert x,y,z data into a grid

Dear list members,
I'm looking for a function in R like the function r.in.xyz in GRASS

(vs.

6.2). I have x, y, z data frame (UTM) with z equals depth and would 
like to make a 3dperspective plot with rgl. My dataframe has 2.5 
milion rows, so I think it's necessary to convert the data to a grid 
with something like mean depth per grid cell. I know I've seen an 
answer to a similar question somewhere on one of the lists, but I just

don't seem to be able to find it again. Thanks for any help.
Erik Meesters
 

Hello Erik,

you could try something like the following code with lattice package.
You don't need to convert the dat to grid for this.

require(lattice)

mydata <- read.table(...)

wireframe(z ~ x * y, data = mydata,
          scales = list(arrows = FALSE),
          drape = TRUE, colorkey = TRUE,
          screen = list(z = 30, x = -60))

or with simple graphics function 'persp' but for this you need to
convert the data to matrix/grid. The following code presumes your x,y,z
coordinates are on a grid. It will not work if they are not or at least
I am not aware of any other approach.

require(sp)

coordinates(mydata) <- ~ x+y
gridded(mydata) <- TRUE
m <- as.matrix(mydata)
exag <- 3
spacing <- 5
theta_val <- 120
phi_val <- 35
ltheta_val <- 20
lphi_val <- 10
z <- exag * m      # Exaggerate the relief
x <- spacing * (1:dim(m)[1])
y <- spacing * (1:dim(m)[2])

par(c(0.5,0.5,0.5,0.5))
persp(x, y, z, theta = theta_val, phi = phi_val, col = "gray", scale =
FALSE, ltheta = ltheta_val, lphi=lphi_val, shade = 0.75, border = NA,
box = TRUE, r=600)

Both of them worked for my 2mil points data.

I hope this helps.

Michal

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--
Michal Gallay

Postgraduate Research Student
School of Geography, Archaeology and Palaeoecology Queen's University
Belfast BT7 1NN Northern Ireland

Tel: +44(0)2890 273929
Fax: +44(0)2890 973212
email: mgallay01 at qub.ac.uk
www: www.qub.ac.uk/geog