[R-sig-Geo] Including SAGA grid as GDAL-supported format

[Edzer Pebesma]

Tom, the idea is that you write (or find someone else to write) the
required driver for gdal, in C++, and contribute this to the gdal
project. If Frank Warmerdam then accepts it (which he usually will), the
next version of gdal will have the support, and calls like

meuse.grid <- readGDAL("meuse_soil.sgrd")

will work out of the box. In addition, all other things that rely upon
gdal will be able to read and write the grids as well. In essense, the
whole process has nothing to do with R, but will become available to R
(in due time, once gdal versions get released and updated), automatically.

Best regards,
--
Edzer

Tomislav Hengl wrote:
>
> Dear r-sig-geo,
>
> I would like to initiate the processes of registering the SAGA grid
> format under GDAL (the SAGA 2.0.4 source code is available for
> download here
> http://sourceforge.net/projects/saga-gis/files/SAGA%20-%202.0/SAGA%202.0.4/saga_2.0.4_src.zip/download).
>
>
> Do I have to follow some formal procedure, or do I have to prepare the
> GDAL driver myself (as explained at
> http://www.gdal.org/gdal_drivertut.html)? Any help/suggestions are
> welcome (apparently it should not be too complicated).
>
> SAGA grid consists of tree types of files:
> 1. "*.sgrd" - the header file with name, data format, XLL, YLL, rows
> columns, cell size, z-factor and no data value;
> 2. "*.sdat" - the raw data file;
> 3. "*.hgrd" - the history file;
>
> Here are some examples hot to read and write the SAGA grids to R:
>
>> library(gstat)
>> library(RSAGA)
>> library(spatstat)
>
>> data(meuse.grid)
>> coordinates(meuse.grid) <- ~x+y
>> gridded(meuse.grid) <- TRUE
>> proj4string(meuse.grid) = CRS("+init=epsg:28992")
>
> # write to SAGA grid format;
>> write.asciigrid(meuse.grid["soil"], "meuse_soil.asc")
>> rsaga.esri.to.sgrd(in.grids="meuse_soil.asc",
>> out.sgrd="meuse_soil.sgrd", in.path=getwd())
>
> # read SAGA grid format:
>> sgrd <- matrix((unlist(strsplit(readLines(file("meuse_soil.sgrd")),
>> split="\t= "))), ncol=2, byrow=T)
>> sgrd
>       [,1]              [,2]
>  [1,] "NAME"            "meuse_soil"
>  [2,] "DESCRIPTION"     "UNIT"
>  [3,] "DATAFILE_OFFSET" "0"
>  [4,] "DATAFORMAT"      "FLOAT"
>  [5,] "BYTEORDER_BIG"   "FALSE"
>  [6,] "POSITION_XMIN"   "178460.0000000000"
>  [7,] "POSITION_YMIN"   "329620.0000000000"
>  [8,] "CELLCOUNT_X"     "78"
>  [9,] "CELLCOUNT_Y"     "104"
> [10,] "CELLSIZE"        "40.0000000000"
> [11,] "Z_FACTOR"        "1.000000"
> [12,] "NODATA_VALUE"    "-9999.000000"
> [13,] "TOPTOBOTTOM"     "FALSE"
>
> # read the raw data: 4bit, numeric (FLOAT), byte order small;
>> sdat <- readBin("meuse_soil.sdat", what="numeric", size=4,
>> n=as.integer(sgrd[8,2])*as.integer(sgrd[9,2]))
>> sdat.sp <- as.im(list(x=seq(from=as.integer(sgrd[6,2]),
>> length.out=as.integer(sgrd[8,2]), by=as.integer(sgrd[10,2])),
>> y=seq(from=as.integer(sgrd[7,2]), length.out=as.integer(sgrd[9,2]),
>> by=as.integer(sgrd[10,2])), z=matrix(sdat,
>> nrow=as.integer(sgrd[8,2]), ncol=as.integer(sgrd[9,2]))))
>> sdat.sp <- as(sdat.sp, "SpatialGridDataFrame")
> # replace the mask value with NA's:
>> sdat.sp at data[[1]] <-
>> ifelse(sdat.sp at data[[1]]==as.integer(sgrd[12,2]), NA, sdat.sp at data[[1]])
>> spplot(sdat.sp)
>
>
> Of course, it would be much easier to have this in a single line:
>
>> meuse.grid <- readGDAL("meuse_soil.sgrd")
>
> or
>
>> writeGDAL(meuse.grid["soil"], "meuse_soil.sgrd", "SAGA")
>
>
>
> PS: A new version of SAGA has just been released few days ago.
>
>
> thank you,
>
> T. Hengl
> http://home.medewerker.uva.nl/t.hengl/
>
> Connected discussion:
> https://stat.ethz.ch/pipermail/r-sig-geo/attachments/20080130/cd5c3748/attachment.pl
>
>
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de