[GRASS-user] Problem with g.region in lat-lon projection

GRASS GRASS user
,
1

Hi,

I am trying to set up a grid with a specified resolution using g.region. The information about the grid that came with the data that I want to plot is:
  X grid origin: 0.2812500
  Y grid origin: -89.81250
  X grid size: 640
  Y grid size: 480
  X grid resolution: 0.5625000
  Y grid resolution: 0.3750000

I set up the grid with the following command:

GRASS 6.4.1 (global_ll_wgs84):~/NAME/G2011 > g.region w=0.2812500 s=-89.81250 cols=640 rows=480 ewres=0.562500 nsres=0.3750000

I would expect this grid to cover the whole globe (e.g. 640*0.5625=360), but instead it just generates a thin strip, and the ewres is too small.

GRASS 6.4.1 (global_ll_wgs84):~/NAME/G2011 > g.region -p
projection: 3 (Latitude-Longitude)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 74N
south: 89:48:45S
west: 0:16:52.5E
east: 24E
nsres: 0:20:28.59375
ewres: 0:02:13.417969
rows: 480
cols: 640
cells: 307200

Where am I going wrong?

John

--
Dr John A Stevenson
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(co-funded by Marie Curie Actions)
School of GeoSciences
The University of Edinburgh
Grant Institute
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Scotland
PHONE: (+44) 131 650 7526
FAX: (+44) 131 668 3184
email: john.stevenson@ed.ac.uk
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The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

2

On 28/11/11 13:23, John A Stevenson wrote:

Hi,

I am trying to set up a grid with a specified resolution using
g.region. The information about the grid that came with the data that
I want to plot is: X grid origin: 0.2812500 Y grid origin: -89.81250
X grid size: 640 Y grid size: 480 X grid resolution: 0.5625000 Y grid
resolution: 0.3750000

I set up the grid with the following command:

GRASS 6.4.1 (global_ll_wgs84):~/NAME/G2011 > g.region w=0.2812500
s=-89.81250 cols=640 rows=480 ewres=0.562500 nsres=0.3750000

I would expect this grid to cover the whole globe (e.g.
640*0.5625=360), but instead it just generates a thin strip, and the
ewres is too small.

If you have existing settings for e and n, it will keep those. You have
to use g.region -d to reset to default values (assuming your default values are -180 - 180 & -90 - 90).

However, I think that g.region maxes out at 180E and 90N. When I use
your settings, I get the following:

GRASS 6.4.1 (LLWGS84):~ > g.region -d w=0.2812500 s=-89.81250 cols=640
rows=480 ewres=0.562500 nsres=0.3750000 -p
projection: 3 (Latitude-Longitude)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 90N
south: 89:48:45S
west: 0:16:52.5E
east: 180E
nsres: 0:22:28.59375
ewres: 0:16:50.917969
rows: 480
cols: 640
cells: 307200

or by aligning to resolution:

GRASS 6.4.1 (LLWGS84):~ > g.region -d w=0.2812500 s=-89.81250 cols=640
rows=480 ewres=0.562500 nsres=0.3750000 -pa
projection: 3 (Latitude-Longitude)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 90N
south: 90S
west: 0
east: 180E
nsres: 0:22:30
ewres: 0:16:52.5
rows: 480
cols: 640
cells: 307200

i.e. it adapts the resolution in order to get 640 columns between 0W and 180E.

Moritz

3

John wrote:

I am trying to set up a grid with a specified
resolution using g.region. The information about
the grid that came with the data that I want to
plot is:
X grid origin:
0.2812500
Y grid origin:
-89.81250
X grid size:
640
Y grid size:
480
X grid resolution:
0.5625000
Y grid resolution:
0.3750000

I set up the grid with the following command:

...

dumb question: does this work?

r.info your_imported_map
g.region rast=your_imported_map

do you know if the original data used the cell-
center registered or grid line-confluence registered
convention?

Hamish

4

John A Stevenson wrote:

Hi,

I am trying to set up a grid with a specified resolution using g.region.
The information about the grid that came with the data that I want to plot
is:
X grid origin: 0.2812500
Y grid origin: -89.81250
X grid size: 640
Y grid size: 480
X grid resolution: 0.5625000
Y grid resolution: 0.3750000

I set up the grid with the following command:

GRASS 6.4.1 (global_ll_wgs84):~/NAME/G2011 > g.region w=0.2812500
s=-89.81250 cols=640 rows=480 ewres=0.562500 nsres=0.3750000

I would expect this grid to cover the whole globe (e.g. 640*0.5625=360), but
instead it just generates a thin strip, and the ewres is too small.

GRASS 6.4.1 (global_ll_wgs84):~/NAME/G2011 > g.region -p
projection: 3 (Latitude-Longitude)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 74N
south: 89:48:45S
west: 0:16:52.5E
east: 24E
nsres: 0:20:28.59375
ewres: 0:02:13.417969
rows: 480
cols: 640
cells: 307200

Where am I going wrong?

I think GRASS prefers to have all the extents n,s,e,w and either
rows/cols or resolution to calculate a region.

With the information available,
North = -89.81250 + 480 * 0.375 = 90.1875
Interestingly, the North deviation from 90N and the South deviation
from 90S is 0.1875 = 0.375 / 2, half a cell

East = 0.2812500 + 640 * 0.5625 = 360.28125
Interestingly, the East deviation from 360E and the West deviation
from 0 is 0.28125 = 0.5625 / 2, half a cell

It is therefore possible that the true region extents are n=90 s=-90
w=0 e=360 and that X and Y grid origin refer to the center, not the
corner of the lower left cell.

This should work:
g.region n=90 s=-90 w=0 e=360 rows=480 cols=640 -p
and
r.region map=<my_map> n=90 s=-90 w=0 e=360

Markus M