[Qgis-user] 0, null values and color table GRASS/QGIS

[Patrick Giraudoux]

Hi,

I am using QGIS 1.7.0 Wroclaw (and GRASS 6.4.1 inside).

I have some troubles with map displays in QGIS with 0 and null values.

I just want to extract a particular habitat category from a Global Land 
Cover 2000 raster into a raster of presence/absence (and NULL value for 
ocean and waterbodies)

This makes the following:

r.mapcalc "V7_01=if(AsiaGLC2000==7,1,0)"

So far so good, I get a raster V7_01 with null values and 1 for pixel of 
category 7, 0 for the others and null values stay NULL. All is correctly 
displayed by QGIS.

Now, I would like to transform V7_01 into a floating point map, and then 
use r. neighbors for a neigborhood analysis.

r.mapcalc "V7=1.0*V7_01"

I get a raster named V7 BUT strangely for any color table, 0 value is 
considered NULL (transparent) when displayed by QGIS. However, the 
original NULL values are well kept and distinct in  this raster (e.g. 
NULL pixels are informed as NULL and 0 pixels are 0, respectively, which 
is correctly displayed within GRASS).

An indication about what happen is that the metadata inform that null 
values(nodata) are -2.14748e+09  in AsiaGLC2000 and V7_01, but become 
2.22507e-308 in V7 (so close to 0).

Any suggestion welcome, I am stuck...

Patrick