[gdal-dev] Setting NODATA to -nan

[Even Rouault]

On Wednesday 15 July 2015 10:21:54 Homme Zwaagstra wrote:
> Hello,
> 
> I've got the following raster exported from GRASS using `r.out.gdal`:
> 
> gdalinfo ppp-2015.tif
> Driver: GTiff/GeoTIFF
> Files: ppp-2015.tif
>         ppp-2015.tif.aux.xml
> Size is 432017, 216009
> Coordinate System is:
> GEOGCS["WGS 84",
>      DATUM["WGS_1984",
>          SPHEROID["WGS 84",6378137,298.257223563,
>              AUTHORITY["EPSG","7030"]],
>          AUTHORITY["EPSG","6326"]],
>      PRIMEM["Greenwich",0],
>      UNIT["degree",0.0174532925199433],
>      AUTHORITY["EPSG","4326"]]
> Origin = (-180.000000000000000,90.000000000000000)
> Pixel Size = (0.000833300541414,-0.000833298612558)
> Metadata:
>    AREA_OR_POINT=Area
> Image Structure Metadata:
>    COMPRESSION=DEFLATE
>    INTERLEAVE=BAND
> Corner Coordinates:
> Upper Left  (-180.0000000,  90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"N)
> Lower Left  (-180.0000000, -90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"S)
> Upper Right ( 180.0000000,  90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"N)
> Lower Right ( 180.0000000, -90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"S)
> Center      (   0.0000000,   0.0000000) (  0d 0' 0.01"E,  0d 0' 0.01"N)
> Band 1 Block=256x256 Type=Float32, ColorInterp=Gray
>    NoData Value=nan
>    Metadata:
>      COLOR_TABLE_RULES_COUNT=5
>      COLOR_TABLE_RULE_RGB_0=0.000000e+00 1.681282e+03 255 255 0 0 255 0
>      COLOR_TABLE_RULE_RGB_1=1.681282e+03 3.362563e+03 0 255 0 0 255 255
>      COLOR_TABLE_RULE_RGB_2=3.362563e+03 5.043845e+03 0 255 255 0 0 255
>      COLOR_TABLE_RULE_RGB_3=5.043845e+03 6.725127e+03 0 0 255 255 0 255
>      COLOR_TABLE_RULE_RGB_4=6.725127e+03 8.406408e+03 255 0 255 255 0 0
>      Generated_with=GRASS GIS 7.0.0
> 
> As you can see the NoData is set to `nan`.  However, within the data it is
> actually `-nan`:
> 
> gdallocationinfo ppp-2015.tif -wgs84 42.0776 30.9305
> Report:
>    Location: (266503P,70886L)
>    Band 1:
>      Value: -nan
> 
> I have tried to force the NoData to `-nan` as follows but with no joy:
> 
> gdal_translate -a_nodata '-nan' -of VRT ppp-2015.tif ppp-2015.vrt
> 
> gdalinfo ppp-2015.vrt
> Driver: VRT/Virtual Raster
> Files: ppp-2015.vrt
>         /data/data/ppp_v2c_2015/ppp-2015.tif
> Size is 432017, 216009
> Coordinate System is:
> GEOGCS["WGS 84",
>      DATUM["WGS_1984",
>          SPHEROID["WGS 84",6378137,298.257223563,
>              AUTHORITY["EPSG","7030"]],
>          AUTHORITY["EPSG","6326"]],
>      PRIMEM["Greenwich",0],
>      UNIT["degree",0.0174532925199433],
>      AUTHORITY["EPSG","4326"]]
> Origin = (-180.000000000000000,90.000000000000000)
> Pixel Size = (0.000833300541414,-0.000833298612558)
> Metadata:
>    AREA_OR_POINT=Area
> Image Structure Metadata:
>    INTERLEAVE=BAND
> Corner Coordinates:
> Upper Left  (-180.0000000,  90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"N)
> Lower Left  (-180.0000000, -90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"S)
> Upper Right ( 180.0000000,  90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"N)
> Lower Right ( 180.0000000, -90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"S)
> Center      (   0.0000000,   0.0000000) (  0d 0' 0.01"E,  0d 0' 0.01"N)
> Band 1 Block=128x128 Type=Float32, ColorInterp=Gray
>    NoData Value=nan
>    Metadata:
>      COLOR_TABLE_RULES_COUNT=5
>      COLOR_TABLE_RULE_RGB_0=0.000000e+00 1.681282e+03 255 255 0 0 255 0
>      COLOR_TABLE_RULE_RGB_1=1.681282e+03 3.362563e+03 0 255 0 0 255 255
>      COLOR_TABLE_RULE_RGB_2=3.362563e+03 5.043845e+03 0 255 255 0 0 255
>      COLOR_TABLE_RULE_RGB_3=5.043845e+03 6.725127e+03 0 0 255 255 0 255
>      COLOR_TABLE_RULE_RGB_4=6.725127e+03 8.406408e+03 255 0 255 255 0 0
>      Generated_with=GRASS GIS 7.0.0
> 
> Even editing the VRT to contain `<NoDataValue>-nan</NoDataValue>` yields a
> positive nan value in gdalinfo.
> 
> This looks like it might be a GDAL bug to me?  Is there a way I can specify
> `-nan` as the NODATA value for this dataset without regenerating it
> (it's quite
> large)?

Hi Homme,

I wasn't aware of this negative not a number variation. Could you create a 
ticket with a small extract of the dataset so as to inspect the hexadecimal 
representation of this -nan value ?

Even


> 
> Many thanks,
> 
> Homme
> 
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