Hamish,<br>
<br>
You may be right that elevation in meters is being used as degrees (my
region is lat long, elevation in meters)...because since my last email
I have run the r.sun module using a new region based on the finer
GTOPO30 resolution. With the shadow option on, i am getting even more
zeros- almost all zeros!!<br>
<br>
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?? <br><br>
And can you please tell me where the 1852*60 comes from?<br>
I guess i would have to creat a new location in UTM coordinates and then do the projecting back and forth you suggest..<br>
<br>
Jose, I am not using a particular model for PET, rather I am using a
bunch of equations mostly from FAO-56 (just google it, the book is
online); met. data from the Climate Research Unit, UK.<br>
And there's no easy way to do it, at least based on my research. To give just 2 examples,<br>
1. r.sun module gives solar radiation(clear sky), and you need observed
spatio-temporal data (difficult to get) on either clear sky index or
the coeffd, coefb parameters to get real sky radiation;<br>
2. The FAO 56 manual does have some simple options. but these involve
working with extra terrestrial radiation- i dont think r.sun gives that
(its internally calculated i believe).<br>
<br>
anyway..it might take me 2-3 months to produce some final output..<br>
<br>
cheers,<br>
vishal<br><div><span class="gmail_quote">On 8/24/06, <b class="gmail_sendername">Hamish</b> <<a href="mailto:hamish_nospam@yahoo.com">hamish_nospam@yahoo.com</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<br>I think you are correct, elevation in meters is probably being treaded<br>as degrees, so you get an unwanted 1852*60 vertical exaggeration.<br>Sounds plausible at least.<br><br>Solutions:<br><br>a) reproject your DEM into a meter-based projection (I couldn't tell you
<br>which one), then reproject the results back to lat/lon.<br><br>b) rescale your DEM into "degrees". e.g.:<br><br>r.mapcalc elevmap_scaled="elevmap / 1852*60"<br> [maybe figure in cos(mean_lat)]<br>
<br>That will probably mess up the linke turbidity functions which I believe<br>takes physical elevation into account. (but I could be wrong)<br><br><br>Hamish<br></blockquote></div><br>