[GRASS-user] Null or 0?

GRASS GRASS user
1

Hello,

I have a technichal doubt. I used v.to.rast.constant to tranform a line vector layer to raster. This meant i have a raster with NULL values.
The output raster is to be used in map algebra operations.

So my question is: Is it advisable to set the Null cells to 0? I know r.calc works with NULL values, and I know that NULL is not the same as 0, however I never understood what difference does it make if any. Considering these two scenarios what do I win and what do I loose?

Any enlightening will be very aprecciated!

Thanks,

André Mano

Associação Leonel Trindade
SOCIEDADE DE HISTÓRIA NATURAL

Apartado 25 2564-909 Torres Vedras Portugal
Sede e Biblioteca: rua Cavaleiros da Espora Dourada, 27A 2560 Torres Vedras

Laboratório de Paleontologia e Paleoecologia: Polígono Industrial do Alto do Ameal 2565-641 Ramalhal
http://alt-shn.blogspot.com
www.alt-shn.org

2

Hi Andre,

On raster algebra, operations using null will return null while operations with 0 behaves like expected. Ex:
12 x null = null
12 x 0 = 0

Null is interesting to use on areas you don’t have / want to represent, like an ocean…

Hope it helps
Daniel

On Feb 5, 2012 12:03 AM, “ALT SHN” <i.geografica@alt-shn.org> wrote:

Hello,

I have a technichal doubt. I used v.to.rast.constant to tranform a line vector layer to raster. This meant i have a raster with NULL values.
The output raster is to be used in map algebra operations.

So my question is: Is it advisable to set the Null cells to 0? I know r.calc works with NULL values, and I know that NULL is not the same as 0, however I never understood what difference does it make if any. Considering these two scenarios what do I win and what do I loose?

Any enlightening will be very aprecciated!

Thanks,

André Mano

Associação Leonel Trindade
SOCIEDADE DE HISTÓRIA NATURAL

Apartado 25 2564-909 Torres Vedras Portugal
Sede e Biblioteca: rua Cavaleiros da Espora Dourada, 27A 2560 Torres Vedras

Laboratório de Paleontologia e Paleoecologia: Polígono Industrial do Alto do Ameal 2565-641 Ramalhal
http://alt-shn.blogspot.com
www.alt-shn.org

grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

3

On Sun, Feb 05, 2012 at 07:22:29AM -0200, Daniel Victoria wrote:

Hi Andre,

On raster algebra, operations using null will return null while operations
with 0 behaves like expected. Ex:
12 x null = null
12 x 0 = 0

Also means do not behave the same on NULL, for example when using r.univar
(according to my understanding of the documentation, haven't tested)
NULL cells are excluded from the computation, so 1 cell with a value 1
and 9 NULL will give a mean of 1 while 1 cell with a value 1 and 9 with
value 0 will give a mean of 0.1.

--
Pat

4

Patrice, Daniel,

Thank you for your commments - it shaded some light on the subject.

Regards,

André Mano

2012/2/5 Patrice Dumas <pertusus@free.fr>

On Sun, Feb 05, 2012 at 07:22:29AM -0200, Daniel Victoria wrote:

Hi Andre,

On raster algebra, operations using null will return null while operations
with 0 behaves like expected. Ex:
12 x null = null
12 x 0 = 0

Also means do not behave the same on NULL, for example when using r.univar
(according to my understanding of the documentation, haven’t tested)
NULL cells are excluded from the computation, so 1 cell with a value 1
and 9 NULL will give a mean of 1 while 1 cell with a value 1 and 9 with
value 0 will give a mean of 0.1.

–
Pat

grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

–

Associação Leonel Trindade
SOCIEDADE DE HISTÓRIA NATURAL

Apartado 25 2564-909 Torres Vedras Portugal
Sede e Biblioteca: rua Cavaleiros da Espora Dourada, 27A 2560 Torres Vedras

Laboratório de Paleontologia e Paleoecologia: Polígono Industrial do Alto do Ameal 2565-641 Ramalhal
http://alt-shn.blogspot.com
www.alt-shn.org