[R-sig-Geo] readGDAL re-mapping data to 0-255

[Roger Bivand]

On Wed, 6 Mar 2013, Biuw, Martin wrote:

> Ah, that was the problem.
> The file with -9999 that works fine is Int16:
>
>       apparent band summary:
>         GDType Bmin Bmax     Bmean      Bsd hasNoDataValue NoDataValue
>       1  Int16  -26    0 -3.130923 3.832138           TRUE       -9999
>
> While the other one is Byte:
>
>      apparent band summary:
>         GDType Bmin Bmax     Bmean      Bsd hasNoDataValue NoDataValue
>       1   Byte  -26    0 -3.089263 3.864005           TRUE        -128
>
> So this sort of problem basically stems from the way ArcGIS saves the 
> geoTiff depending on the data range? Or is there a way to specify to 
> readGDAL how to deal with Byte or Int16 data?

If the input data are Byte, you may try the rgdal toSigned() function:

x <- c(1:10, 127:129, 240:256)
toSigned(x, 8)

which probably will not respect the -128, but you can then untangle that 
later with is.na(). You'll need to check the output as the function is not 
used often and may not do what you need. The function was added to the 
package in September 2009:

https://stat.ethz.ch/pipermail/r-sig-geo/2009-September/006387.html

Hope this helps,

Roger

> Thanks for the help!
> Martin
>
> -----Original Message-----
> From: Roger Bivand [mailto:Roger.Bivand at nhh.no]
> Sent: 06 March 2013 12:47
> To: Biuw, Martin
> Cc: r-sig-geo at r-project.org
> Subject: Re: [R-sig-Geo] readGDAL re-mapping data to 0-255
>
> On Wed, 6 Mar 2013, Biuw, Martin wrote:
>
>> Hi,
>> I'm using readGDAL to read data from a geoTiff file. The valid data
>> range of the single band is -26 to 0. I have two versions of the file,
>> one with a NoDataValue = -9999 and one with NoDataValue = -128. When
>> reading the version with -9999 everything works great, and I get the
>> data values in band1 mapped correctly on the original interval. But
>> when I use the version of the file with NoDataValue = -128, these
>> band1 data values get re-mapped to 0-255.
>
> This appears to indicate that the files do not only differ in NoDataValue, but also in storage form. Does examining the output of GDALinfo() on both tell you anything? Is one a signed 16-bit integer, the other Byte? Maybe paste some of the output into a follow-up?
>
> Roger
>
>> I remember reading a helpfile or R-sig post on why the NoDataValue has
>> such an effect, or what can be done to get around it. But can't for
>> the life of me find that message anymore.
>> The reason I need to use the file with NoDataValue = -128 is that this
>> is in UTM 35 while the other one is in UTM 33. Since it's a
>> SpatialGridDataFrames, I'd rather not use spTransform on it, since
>> this turns it into a SpatialPointsDataFrame. The files were originally
>> created (by someone else) in ArcGIS, so I could always ask them to
>> re-create them with correct NoDataValue. But maybe there's a way to
>> explicitly tell readGDAL how to treat the data values? I've
>> desperately been looking for any help on this, but must be looking in
>> the wrong place....
>> Thanks in advance,
>> Martin
>>
>> ________________________________________________________________
>> Martin Biuw (PhD)
>> Researcher
>>
>> Norwegian Institute for Nature Research - NINA Postal address:
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>> Phone: +47 77 75 04 17 * Cell: +47 40 72 96 15 * Fax: +47 77 75 04 01
>> * www.nina.no<http://www.nina.no/>
>> ________________________________________________________________
>>
>>
>> 	[[alternative HTML version deleted]]
>>
>>
>
> --
> Roger Bivand
> Department of Economics, NHH Norwegian School of Economics, 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
>
>

-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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