[GRASS-user] Make transparent null data.

[Nikos Alexandris]

Luis Miguel Royo Perez wrote:

> Hi everyone,

Hi Luis-Miguel!

> maybe the question is not difficult, but i couldn't find the way to make
> null data transparent, i've been reading the documentation, but still i
> can't found the answer, if anyone could help me i'll apreciate a lot.
> 
> Just indicate the name of algorythm that makes this and i'll study the
> documentation.

NULL cells (or call them No Data if you want) are by default "transparent" in 
GRASS GIS -- since they are empty cells, it is not possible to assign any 
value, hence it is not possible to assign a "color".

Note, all raster operations on a NULL cell, will result always, unless 
instructed by the user otherwise, in a NULL cell.

Check the following page -- search of course for the NULL string, as well as 
the MASK string which is a powerful concept in GRASS GIS):

<http://grass.osgeo.org/grass64/manuals/rasterintro.html>


If it is about drawing another map on top of another, then the default 
behaviour in GRASS 64 is to "erase" the underlying map, no matter if the drawn 
values are NULL or valid values, unless the user instructs the "-o" flag.

See also: <http://grass.osgeo.org/grass64/manuals/d.rast.html>.


In GRASS 70, the default drawing behaviour is inverted, i.e. the underlying 
map's cells are still visible wherever the map that is drawn on top holds NULL 
cells. The "-n" flag will make the NULL cells opaque.

See also: <http://grass.osgeo.org/grass70/manuals/d.rast.html>


Can you provide an example which or what is (still) confusing?
Best, Nikos
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