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

[Thomas Adams]

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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