Hi,
I'm adding a section on changing coordinate systems for Spatial objects
on the r tips wiki, including a section on reprojecting grids:
http://wiki.r-project.org/rwiki/doku.php?id=tips:spatial-data:change_crs
I'm still working on it right now, but everyone is welcome to add to it
later this day.
cheers,
Paul
Roger Bivand wrote:
On Wed, 23 Sep 2009, Brendan Malone wrote:
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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Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
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