[GRASS-user] Difference in sunset/sunrise time between r.sun and r.sunmask

[thomas.huld at jrc.ec.europa.eu]

Hello,

Sorry for the late reply, I tried to send this once but something must have 
gone wrong.

By default r.sun calculates times as true solar time, whereby solar noon is 
always exactly 12 o'clock everywhere. Depending on where you are in your time 
zone, this may cause differences of up to an hour, in some cases (like Western 
Spain) even more. On top of this, the offset varies during the year according 
to the Equation of Time.

You can use the option civiltime=<timezone_offset> in r.sun to make it use 
wall clock time. For Central Europe the timezone offset is +1, +2 when 
daylight saving time is in effect.

I'm not completely sure about this, but if you use civiltime you may have to 
supply the longitude as a raster with the "longin" option. With latlon 
projection you can make a longitude raster simply with

r.mapcalc lon_raster='x()'

Hope this helps.

Thomas




On Monday 02 January 2012 12:57:27 pm Büro Seling wrote:
> Dear Group!
> 
> Happy New Year @all!
> 
> I work on a Linux GRASS 6.4.2 RC1. The area of interest is in central
> Europe and
> the projection is WGS84(EPSG:4326).
> 
> 
> When I use the r.sun module and let it show the sunset/sunrise time and
> compare
> it with the sunset/sunrise from a map created with r.sunmask I see great
> differences (up to 50 minutes).
> 
> 
> It is not the timezone (which is set correctly) in r.sunmask.
> 
> The results from r.sunmask seem to be, according to other sources, seem
> to be
> right, but the results from r.sun are fare off.
> 
> Please give a suggestions or help!
> 
> Stefan
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> 

-- 
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Thomas Huld   
Joint Research Centre of the European Commission
T.P. 450
I-21027 Ispra, Italy
phone: +39 0332785273
e-mail: Thomas.Huld at jrc.ec.europa.eu
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