(cc dev list)

On Fri, Aug 25, 2006 at 04:55:34PM +0530, Vishal Mehta wrote:

Thanks everyone,

a few thoughts-

1. Neither the r.sun manual, nor the r.slope.aspect manual, states that

the x,y and elevation units should be the same. In fact both manuals say

the elevation should be in meters! r.sun talks about how if the maps are

projected then it does'nt matter- but it does not talk of use with

lat-long system(nor does r.slope.aspect).do these manuals then just assume that we are using a projection where the

x,y are in meters??

...

I still cant get over the idea that GRASS cant handle correctly slope

generation in lat-long coordinates with elevation in meters!

No, this isn't fully the case:

r.slope.aspect help

...

zfactor Multiplicative factor to convert elevation units to meters

But for r.sun we need to find a solution.

Markus

thanks again,

vishalOn 8/25/06, Hamish <[1]hamish_nospam@yahoo.com > wrote:

> But is it really possible that the r.sun module (i'm using Grass 6.0.1

> on Ubuntu Breezy) would treat elevation in degrees??Yes, it is possible. Maybe even likely.

> And can you please tell me where the 1852*60 comes from?

There are 1852 meters in a nautical mile. There are 60 nautical miles in

a degree of latitude. elev_in_meters / (1850.0 * 60) = elevation in degThe width of a deg longitude is equal to width_lat*cos(lat).

If rescaling the elevation into degress be sure to /(1852.0*60). This

ensures that the result will be a floating point value (as opposed to

/(1852*60) ).> I guess i would have to creat a new location in UTM coordinates and

> then do the projecting back and forth you suggest..I think this is the saner solution, but I expect that a single UTM zone

is not an appropriate projection for working with an area the size of

India.Markus wrote:

> This should be a projection covering the entire subcontinent

> in a reasonable way - what are the official projection(s) for

> that area?If you have problems finding one, you might ask for help on the PROJ.4

mailing list.Hamish

References

Visible links

1. mailto:hamish_nospam@yahoo.com

_______________________________________________

grassuser mailing list

grassuser@grass.itc.it

http://grass.itc.it/mailman/listinfo/grassuser

--

Markus Neteler <neteler itc it> http://mpa.itc.it/markus/

ITC-irst - Centro per la Ricerca Scientifica e Tecnologica

MPBA - Predictive Models for Biol. & Environ. Data Analysis

Via Sommarive, 18 - 38050 Povo (Trento), Italy