[gdal-dev] Setting NODATA to -nan

[Homme Zwaagstra]

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)?

Many thanks,

Homme