Convert SpatialPointsDataFrame to a SpatialGridDataFrame

Hi everyone,

I would like to know a good way in which to convert a
SpatialPointsDataFrame to a SpatialGridDataFrame.

The reason that I ask is that when I change the CRS of a
SpatialGridDataFrame, I inadvertently create a point data frame with the
new CRS.
Your input data are in geographical coordinates, and you are projecting 
(no datum transformation) to UTM. This inevitably means that while the 
input points are evenly spaced, the projected points are not, and do not 
constitute a regular grid (see any introduction to projections). To get a 
regular grid out, you will have to warp, that is interpolate, to a regular 
grid from the irregular points that you have generated.

Either use an interpolator such as idw() in gstat (see the task view for 
more), or use for example gdal_warp externally.

This is getting to be a FAQ, isn't it? Could someone put it in the R-wiki 
under spatial data?

Roger

To better illustrate my problem:

I have a grid file

grid90m <- readGDAL("edgeroi_dem.asc")

names(grid90m) <- "edgeroi_dem"

I give it a CRS

proj4string(grid90m)<- CRS("+init=epsg:4283")

Then I change it to a projected CRS

grid90.UTM <- spTransform(grid90m, CRS("+init=epsg:32755"))

Get a summary

summary(grid.UTM)

Object of class SpatialPointsDataFrame

Coordinates:

     min       max

x  738742  790718.3

y 6643860 6679923.8

Is projected: TRUE

proj4string :

[+init=epsg:32755 +proj=utm +zone=55 +south +ellps=WGS84 +datum=WGS84
+units=m +no_defs +towgs84=0,0,0]

Number of points: 220805

Data attributes:

  Min. 1st Qu.  Median    Mean 3rd Qu.    Max.

180.2   200.7   225.0   254.1   280.6   988.6

I have tried various ways to convert this points file to grid via a few
methods with resultant failure. Essentially I need to input some measure
of the pixel size.

My original thoughts were that the problem was to do with the
asymmetrical shape of my DEM layer. This may be true, but it would be
useful if people could provide some ideas on how to get around this.

Cheers,

Brendan Malone

PhD candidate

University of Sydney

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Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no

Convert SpatialPointsDataFrame to a SpatialGridDataFrame

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