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.biv...@nhh.no]
Sent: 06 March 2013 12:47
To: Biuw, Martin
Cc: r-sig-geo@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:
Framsenteret, 9296 Troms?, NORWAY Delivery/Visiting address:
Framsenteret, Hjalmar Johansens gate 14, 9007 Troms?, NORWAY
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.biv...@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.biv...@nhh.no
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