[GRASS-user] t.rast.mapcalc question

GRASS GRASS user
1

Dear GRASS users:

I’m experienced grass user… I use GRASS since I started my PhD research in landscape ecology back in 2009…

I’m trying to calculate the difference of NDVI between consecutive dates. I used to do it like:

i=0
for map in $(g.list rast pattern=MOD13Q1*01*); do
NDVI[i]=$map
echo "Adding: " $map
let i=i+1
done

for id in $(seq 0 $(echo ${#NDVI[@]})); do
echo $id
r.mapcalc “ndvi_dif2_$id = ${NDVI[$id+1]} - ${NDVI[id]}”
done

But i thing there must be an easier way using the grass 7 temporal framework. So i registred my dataset and tried the following…

t.rast.list input=ndvi@ts
MOD13Q1.A2009001.h12v12.005.2009019145228_01_4326|ts|2009-01-01 00:00:00|2009-01-17 00:00:00
MOD13Q1.A2009017.h12v12.005.2009035110230_01_4326|ts|2009-01-17 00:00:00|2009-02-02 00:00:00
…
MOD13Q1.A2010337.h12v12.005.2010355100650_01_4326|ts|2010-12-03 00:00:00|2010-12-19 00:00:00
MOD13Q1.A2010353.h12v12.005.2011005202724_01_4326|ts|2010-12-19 00:00:00|2011-01-04 00:00:00

t.rast.mapcalc input=ndvi expression=“ndvi[1] - ndvi[0]” output=ndvi_a basename=ndvi_a

But the output is not good (see attached file: left is OK, rigth is wrong)

Am I doing something wrong?

Thanks!

(attachments)

send.png

···

Dr. Leonardo A. Hardtke
Laboratorio de Teledetección y S.I.G.
Centro Nacional Patagónico (CONICET)
Bvd. Brown 2825, 9120
Puerto Madryn, Chubut, Argentina

2

Hola Leo!

You should use t.rast.algebra (https://grass.osgeo.org/grass71/manuals/t.rast.algebra.html) from GRASS GIS 7.1 for that kind of temporal algebra operation. The command should be like:

t.rast.algebra expression=“ndvi_a = ndvi[1] - ndvi[0]” basename=ndvi_a

For more examples on temporal modules, you can see:
https://grasswiki.osgeo.org/wiki/Temporal_data_processing

HTH,
Vero

···

2015-08-27 9:52 GMT-03:00 Leonardo Hardtke <leohardtke@gmail.com>:

Dear GRASS users:

I’m experienced grass user… I use GRASS since I started my PhD research in landscape ecology back in 2009…

I’m trying to calculate the difference of NDVI between consecutive dates. I used to do it like:

i=0
for map in $(g.list rast pattern=MOD13Q1*01*); do
NDVI[i]=$map
echo "Adding: " $map
let i=i+1
done

for id in $(seq 0 $(echo ${#NDVI[@]})); do
echo $id
r.mapcalc “ndvi_dif2_$id = ${NDVI[$id+1]} - ${NDVI[id]}”
done

But i thing there must be an easier way using the grass 7 temporal framework. So i registred my dataset and tried the following…

t.rast.list input=ndvi@ts
MOD13Q1.A2009001.h12v12.005.2009019145228_01_4326|ts|2009-01-01 00:00:00|2009-01-17 00:00:00
MOD13Q1.A2009017.h12v12.005.2009035110230_01_4326|ts|2009-01-17 00:00:00|2009-02-02 00:00:00
…
MOD13Q1.A2010337.h12v12.005.2010355100650_01_4326|ts|2010-12-03 00:00:00|2010-12-19 00:00:00
MOD13Q1.A2010353.h12v12.005.2011005202724_01_4326|ts|2010-12-19 00:00:00|2011-01-04 00:00:00

t.rast.mapcalc input=ndvi expression=“ndvi[1] - ndvi[0]” output=ndvi_a basename=ndvi_a

But the output is not good (see attached file: left is OK, rigth is wrong)

Am I doing something wrong?

Thanks!

Dr. Leonardo A. Hardtke
Laboratorio de Teledetección y S.I.G.
Centro Nacional Patagónico (CONICET)
Bvd. Brown 2825, 9120
Puerto Madryn, Chubut, Argentina

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

3

Oh! Thanks!
I was using GRASS 7.0.1, which has no t.rast.algebra module.
I didn’t noticed there where a t.rast.mapcalc and a t.rast.algebra in newer versions…

I installed grass71 and as you said,

t.rast.algebra expression=“ndvi_diff = ndvi[0] - ndvi[1]” basename=ndvi_diff

worked perfect! Much easy than the old mapcalc and for loop version!
I’m really enjoying the grass 7 temporal framework!

Thanks for the fast (and accurate) answer!

···

2015-08-27 10:04 GMT-03:00 Veronica Andreo <veroandreo@gmail.com>:

Hola Leo!

You should use t.rast.algebra (https://grass.osgeo.org/grass71/manuals/t.rast.algebra.html) from GRASS GIS 7.1 for that kind of temporal algebra operation. The command should be like:

t.rast.algebra expression=“ndvi_a = ndvi[1] - ndvi[0]” basename=ndvi_a

For more examples on temporal modules, you can see:
https://grasswiki.osgeo.org/wiki/Temporal_data_processing

HTH,
Vero

–

2015-08-27 9:52 GMT-03:00 Leonardo Hardtke <leohardtke@gmail.com>:

Dear GRASS users:

I’m experienced grass user… I use GRASS since I started my PhD research in landscape ecology back in 2009…

I’m trying to calculate the difference of NDVI between consecutive dates. I used to do it like:

i=0
for map in $(g.list rast pattern=MOD13Q1*01*); do
NDVI[i]=$map
echo "Adding: " $map
let i=i+1
done

for id in $(seq 0 $(echo ${#NDVI[@]})); do
echo $id
r.mapcalc “ndvi_dif2_$id = ${NDVI[$id+1]} - ${NDVI[id]}”
done

But i thing there must be an easier way using the grass 7 temporal framework. So i registred my dataset and tried the following…

t.rast.list input=ndvi@ts
MOD13Q1.A2009001.h12v12.005.2009019145228_01_4326|ts|2009-01-01 00:00:00|2009-01-17 00:00:00
MOD13Q1.A2009017.h12v12.005.2009035110230_01_4326|ts|2009-01-17 00:00:00|2009-02-02 00:00:00
…
MOD13Q1.A2010337.h12v12.005.2010355100650_01_4326|ts|2010-12-03 00:00:00|2010-12-19 00:00:00
MOD13Q1.A2010353.h12v12.005.2011005202724_01_4326|ts|2010-12-19 00:00:00|2011-01-04 00:00:00

t.rast.mapcalc input=ndvi expression=“ndvi[1] - ndvi[0]” output=ndvi_a basename=ndvi_a

But the output is not good (see attached file: left is OK, rigth is wrong)

Am I doing something wrong?

Thanks!

Dr. Leonardo A. Hardtke
Laboratorio de Teledetección y S.I.G.
Centro Nacional Patagónico (CONICET)
Bvd. Brown 2825, 9120
Puerto Madryn, Chubut, Argentina

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

Dr. Leonardo A. Hardtke
Laboratorio de Teledetección y S.I.G.
Centro Nacional Patagónico (CONICET)
Bvd. Brown 2825, 9120
Puerto Madryn, Chubut, Argentina