[GRASS-user] raster exchange between GRASS and R with nodata

[Johannes Radinger]

Hi all,

of course it is possible to load the raster maps directly via spgrass6.
However, we use this work
flow also to exchange some of the maps between different users (e.g. via
email) and to permanently
store single files (geotiffs that contain the proj information within the
file). So, I agree that using spgrass6 would be much more efficient, but
I'll stick to exporting to geotiffs and so I need to solve the issues with
NA's.

/Johannes


On Thu, Sep 4, 2014 at 2:31 PM, Thomas Adams <tea3rd at gmail.com> wrote:

> Johannes,
>
> If you want to read your file into R, there is no need to export your map
> from GRASS to do this. Simply install and use the R contributed package
> 'spgrass6' (spgrass6 has R dependencies that need to be installed first);
> it works wonderfully:
>
> Within GRASS, at the GRASS terminal prompt...
>
> > library(spgrass6)
> Loading required package: sp
> Loading required package: XML
> GRASS GIS interface loaded with GRASS version: GRASS 7.0.0beta3 (2014)
> and location: ohrfc_mpe
> > dat<-readRAST6("xmrg0101200306z")
> > image(dat)
>
> This is far more efficient.
>
> Tom
>
>
> On Thu, Sep 4, 2014 at 5:32 AM, Johannes Radinger <
> johannesradinger at gmail.com> wrote:
>
>> Hi,
>>
>> I want to export a raster map (FCELL) from GRASS70 to the geotiff format
>> using r.out.gdal and to import it later on in R. The map contains many no
>> data values.
>>
>> Here some details about the raster:
>> Type of Map:  raster               Number of Categories: 0
>> Data Type:    FCELL
>> Rows:         750
>> Columns:      750
>> Total Cells:  562500
>> total null and non-null cells: 15105636
>> total null cells: 15105047
>>
>> So when I export the map, r.out.gdal reports: "Input raster map contains
>> cells with NULL-value (no-data). The value -nan will be used to represent
>> no-data values in the input map. You can specify a nodata value with the
>> nodata option."
>>
>> When I subsequently try to import the geotiff into R (using the package
>> 'Raster') the nodata values are not recognised as NA's:
>>
>> a <- raster("*.tif")
>> summary(a)
>> Min.     0.5294496
>> 1st Qu.  0.7171210
>> Median   0.7871540
>> 3rd Qu.  1.1581826
>> Max.     1.5494517
>> NA's     0.0000000
>>
>> So I am wondering if I need to set any specific parameter during the
>> export (r.out.gdal) or import (raster()).
>>
>> As I am not only exporting FCELL (Float32) raster but also multiple
>> (N=500) other rasters to R I would be interested in a solution also for
>> DCELL (Float64). Of course I can export all of as Float64 as the file size
>> should not be a problem.
>>
>> Any suggestions or experiences of handling NA's during raster exchange
>> between GRASS and R?
>>
>> /Johannes
>>
>> _______________________________________________
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>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>>
>
>
>
>
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