[GRASSLIST:403] how to refer to cell's ,longitude latitude coordinates?

GRASS GRASS user
1

Hi all,

I have a raster map, lat/long, 0.5 degrees resolution. I want to develop solar radiation and related maps maps using r.mapcalc. This requires refering to each cell’s latitude and longitude, i.e. the equations will include the cell’s latitude and longitude. How do i do this in GRASS? Is there like an intenral variable that indexes each cell’s coordinates?

thanks,
vishal

2

Vishal: any reason you aren't using r.sun?

--j

--

Jonathan A. Greenberg, PhD
NRC Research Associate
NASA Ames Research Center
MS 242-4
Moffett Field, CA 94035-1000
Phone: 415-794-5043
AIM: jgrn3007
MSN: jgrn3007@hotmail.com

  _____

From: owner-GRASSLIST@baylor.edu [mailto:owner-GRASSLIST@baylor.edu] On
Behalf Of Vishal Mehta
Sent: Monday, March 27, 2006 12:59 AM
To: GRASS Users list
Subject: [GRASSLIST:403] how to refer to cell's ,longitude latitude
coordinates?

Hi all,

I have a raster map, lat/long, 0.5 degrees resolution. I want to develop
solar radiation and related maps maps using r.mapcalc. This requires
refering to each cell's latitude and longitude, i.e. the equations will
include the cell's latitude and longitude. How do i do this in GRASS? Is
there like an intenral variable that indexes each cell's coordinates?

thanks,
vishal

3

Vishal Mehta wrote:

I have a raster map, lat/long, 0.5 degrees resolution. I want to develop
solar radiation and related maps maps using r.mapcalc. This requires
refering to each cell's latitude and longitude, i.e. the equations will
include the cell's latitude and longitude. How do i do this in GRASS? Is
there like an intenral variable that indexes each cell's coordinates?

In r.mapcalc, the x() and y() functions return the coordinates of the
centre of the current cell.

--
Glynn Clements <glynn@gclements.plus.com>

4

thanks everyone,

i’ll try r.sun, but i thought i’d like the freedom of running my own algorithms- i’m not sure i’ll have access to the papers that are referred to in the r.sun help page, to check out the algorithm.

meanwhile, here is what i’d like to do, using either r.sun or my own algorithms. If you can suggest some general steps, that would speed up my attempts:

I want to create a monthly map of extraterrestrial radiation, and net radiation, on a 0.5 degree grid for extents 60 -100.5E and 5 to 45.5N. I am not particular about corrections for aspect and slope at this point. So particular questions i have now are:
a) I already have a location with the above coordinates and cell size. Can i use r.sun on an empty map or do i absolutely need an elevations, slope and aspetc map?

b) At this point i can only derive monthly radiation maps by summing the daily estimates in r.sun- right?

thanks all!
vishal

On 3/27/06, Glynn Clements <glynn@gclements.plus.com> wrote:

Vishal Mehta wrote:

I have a raster map, lat/long, 0.5 degrees resolution. I want to develop
solar radiation and related maps maps using r.mapcalc. This requires
refering to each cell’s latitude and longitude, i.e. the equations will
include the cell’s latitude and longitude. How do i do this in GRASS? Is
there like an intenral variable that indexes each cell’s coordinates?

In r.mapcalc, the x() and y() functions return the coordinates of the
centre of the current cell.

–
Glynn Clements < glynn@gclements.plus.com>