[R-sig-Geo] Writing an ASCII grid with nodata values

[Roger Bivand]

On Sat, 21 Nov 2009, Paul Hiemstra wrote:

> Hi Ned,
>
> Is it an option to switch to another GDAL format, GTiff for example? Check if 
> this exports the NA values correctly. Not really an answer to your question, 
> but maybe a helpful suggestion.
>
> cheers and good luck,
> Paul
>
> Ned Horning wrote:
>> I am trying to export a SpatialPixelsDataFrame object with floating point 
>> values to an ASCII grid file and want to specify a nodata value. I tried 
>> write.asciigrid from the sp package and I couldn't figure out how to output 
>> float values. The pixel values exported as integers.
>> 
>> For now I am using the following writeGDAL statement:
>> -- 
>> writeGDAL(sp_hsm_pard, out_pard, drivername="AAIGrid", type="Float32", 
>> options="DECIMAL_PRECISION=3", "NODATA=-9999")

Maybe reading:

> args(writeGDAL)
function (dataset, fname, drivername = "GTiff", type = "Float32",
     mvFlag = NA, options = NULL, copy_drivername = "GTiff")

would help?

You are getting:

dataset=sp_hsm_pard, fname=out_pard, drivername="AAIGrid", type="Float32", 
options="DECIMAL_PRECISION=3", mvFlag="NODATA=-9999"

by argument names and positions, I think, where internally 
as.double("NODATA=-9999") is being coerced to an NA (a warning may have 
been shown.

Perhaps try setting mvFlag=-9999? If you wanted to put it in the options, 
that may work, but as.double(mvFlag) is explicitly set as the no-data 
value on the band(s) being written in create2GDAL() inside writeGDAL().

Hope this helps,

Roger


>> 
>> -- 
>> This appears to output the image okay but it is not assigning the proper 
>> values to NODATA_value. The NODATA_value in the exported file is "1" 
>> instead of "-9999".
>> 
>> Can someone let me know how to output floating point data to an ESRI ASCII 
>> grid file and specify a nodata value.
>> 
>> Thanks,
>> 
>> Ned
>> 
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no