[GRASS5] 0 != no data

[Bob Covill]

Justin Hickey wrote:
> 
> Hi all
> 
> Recently we experienced a problem with using aerial photographs as
> raster maps. The photographs are grayscale and thus range in value from
> 0 to 255. Here 0 is a valid CELL value, however, Grass 5 treated it as a
> "no data" value. So, displaying our maps, these white areas appeared
> (where they should have been black) that were displayed as "(Null) no
> data" in d.what.rast. Initially, we thought that the data had been
> corrupted somehow, but it was only Grass interpreting 0 as "no data".
> 
> For me, this is a show stopper for Grass5.0. Since Grass now has well
> defined values for Null, 0 should never be considered as "no data". This
> only leads to confusion to users who have no idea that in previous grass
> versions, 0 was associated with "no data". Like us, they will assume
> that their files have been corrupted somehow.
> 
> The fix for this was to run r.support and create a null file. This leads
> me to another problem, namely, r.support has the option to consider 0 as
> "no data". I think this option should be removed. Also, I don't see a
> reason for having a null file. The value in the raster file should
> indicate whether it is null or not. Thus, I think that the option to
> r.support to set 0 as valid data by creating a null file, should also be
> removed. Basically, Grass should simply read the raster file and if any
> values are equal to our defined Null values they are Null, straight
> forward and simple. Note that if some users have a reason for wanting 0
> to be "no data" then they can simply use r.mapcalc to convert 0 to Null.
> 
> Finally, d.what.rast currently reports the value 0 as "no data" even
> after the null file is created. I think this needs to be changed as
> well.
> 
> Unfortunately, I have no idea how to fix this problem. The ideal
> solution is to find the code that interprets 0 as "no data" and
> eliminate it. However, I don't even know how to search for this type of
> code. Does anyone have any ideas on how to solve this?
> 
> Thanks for listening.
> 
> --
> Sincerely,
> 
> Jazzman (a.k.a. Justin Hickey)  e-mail: jhickey at hpcc.nectec.or.th
> High Performance Computing Center
> National Electronics and Computer Technology Center (NECTEC)
> Bangkok, Thailand
> ==================================================================
> People who think they know everything are very irritating to those
> of us who do.  ---Anonymous
> 
> Jazz and Trek Rule!!!
> ==================================================================

Justin,

Zero should (hopefully) be imported as zero. I wonder if the problem is
actually with the import program. What program did you use to import the
images? Maybe as a test, try converting an image to another supported
format (eg. TIFF, SunRaster) and re-importing to see how the zeroes are
handled.

Hope this helps.

-- 
Bob Covill

Tekmap Consulting
P.O. Box 2016 Fall River, N.S.
B2T 1K6
Canada

E-Mail: bcovill at tekmap.ns.ca
Phone: 902-860-1496
Fax: 902-860-1498