[GRASS-user] Re: [GRASS-dev] gdal import an nodata cells

[Frank Warmerdam]

Michael Perdue wrote:
> Ops... Looks like Geotiff does allow for storing no-data values. But HFA 
> (not too sure which format IMA is) and others don't appear too.

Michael,

Note that GeoTIFF does not *generally* support nodata values, but there is
a GDAL specific tag used by GDAL and GDAL based software to keep track of
nodata values.

> Question?
> 
> is float(nan) equal to double(nan)?

They are not the same as the binary level of course.  I don't know if
they compare as equal in C or not.  Hopefully they do.

However, GDAL does *not* generally recognise NaN as a nodata value.

 >  If not, does gdal_translate
> understand this when it converts a file stored as float64 to float32? 
> gdal_translate allows one to define what the no-data value is, but how 
> do I tell it to change the input no-data value to a different output 
> no-data value? IE, the no-data value in my surfer file is a double 
> precision nan, but I want the no-data value in my output GeoTiff 
> (float32) to be -32767. -a_nodata changes the header no-data definition 
> but does not change the values of the pixels that represent no-data.

I'm not aware of a practical way of doing this. Sorry.
You are likely best off fixing this up in GRASS which is more
flexible.

(BTW, GDAL does support double precision geotiff files)

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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