[GRASS5] 0 != no data

[Justin Hickey]

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
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