[R-sig-Geo] Saving jpeg world files

[Tom Mulholland]

Doh! I had completely forgotten about rgdal. That's exactly the route I 
need to take.

Thanks

Roger Bivand wrote:
> On Wed, 29 Nov 2006, Mulholland, Tom wrote:
> 
>> I am beginning to get a little bit bog eyed with the myriad of options
>> there are within the various spatial packages. I am trying to find ways
>> of porting the results I get within R into a GIS environment and I can't
>> seem to see anything that would let me write a jpeg file with an
>> appropriate world file.
> 
> The most direct route is to use a SpatialGridDataFrame or equivalently 
> SpatialPixelDataFrame, and writeGDAL() in rgdal. That gives very good 
> support for GeoTiff files. The JPEG driver is present, but much more 
> limited than GeoTiff - it has to be either single-band or three-band, and 
> only Byte data. For released rgdal, you would write the jpeg from a single 
> variable by stretching it over 0-255, taking integers, writing as GeoTiff 
> to a temp file, opening the file as a GDAL data set, copying to JPEG, 
> and saving:
> 
> library(sp)
> library(rgdal)
> data(meuse.grid)
> coordinates(meuse.grid) <- c("x", "y")
> gridded(meuse.grid) <- TRUE
> # quantize
> rd <- range(meuse.grid$dist)
> meuse.grid$idist <- as.integer(255*(meuse.grid$dist-rd[1])/(rd[2]-rd[1]))
> # write temporary GTiff
> tf <- tempfile()
> writeGDAL(meuse.grid["idist"], tf, drivername="GTiff", type="Byte")
> # open temporary file as GDAL dataset and copy to JPEG
> x <- GDAL.open(tf)
> dx <- copyDataset(x, driver="JPEG", options="WORLDFILE=YES")
> # and write out
> td <- tempdir()
> tf1 <- paste(td, "output.jpg", sep="/")
> saveDataset(dx, tf1, options="WORLDFILE=YES")
> GDAL.close(x)
> GDAL.close(dx)
> # check to see if worldfile present and OK (*.wld use cell centre 
> # registration, the bounding box goes to the cell boundaries
> list.files(td)
> slot(meuse.grid, "grid")
> file.show(paste(td, "output.wld", sep="/"))
> 
> Had rgdal and the underlying GDAL library allowed jpeg files to be written 
> directly, this would be simpler. However, you may find that the software 
> that reads jpeg files also reads GeoTiff files, which are far more 
> flexible, taking many more data types, including floating point values.
> 
> For those interested in image overlays in Google Earth, this recipe lets 
> you make PNG files too (but so far I haven't got GE to read a worldfile to 
> place the image, and of course GE wants geographical coordinates). PNG 
> support an alpha channel too.
> 
> Hope this isn't too complicated. We probably need an interface between im 
> and sp classes so that getting from im to GDAL is simpler.
> 
> Roger
> 
> 
>> I have managed to create an ASCIIGrid but my first tests seem to
>> indicate that not all grids are created equally. My experience with some
>> of the "free" windows GIS applications is that jpg files seem to be more
>> universally supported, so I thought that might be a more fruitful way to
>> go. I will then have to worry less about making sure my output is the
>> type that can be imported.
>>
>> It looks to me as if the class "im" in spatstat is conceptually
>> straightforward to process. It looks as if all the attributes are there
>> to create a world file. Since my data is already projected it would seem
>> that I should be able to simply use the jpeg device for output (not that
>> I have any idea how I'll go about that) and create a text file to go
>> along with it.
>>
>> The first question is 'Am I missing something really simple?' and the
>> second is "Is there a straightforward solution that I have already
>> missed."
>>
>> Tom
>>
>> platform       i386-pc-mingw32             
>> arch           i386                        
>> os             mingw32                     
>> system         i386, mingw32               
>> status                                     
>> major          2                           
>> minor          4.0                         
>> year           2006                        
>> month          10                          
>> day            03                          
>> svn rev        39566                       
>> language       R                           
>> version.string R version 2.4.0 (2006-10-03) 
>>
>> Tom Mulholland
>> Senior Demographer
>> Applied Research and Modelling
>> State and Regional Policy
>> Department for Planning and Infrastructure
>> Perth, Western Australia
>> +61 (08) 9264 7936
>>
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>>
>