[R-sig-Geo] rsaga, radiation, rsaga.pisr

[Alexander Brenning]

Dear Melita,

I will try to answer your questions, but you will likely get better 
feedback from the SAGA pros if you post them to the SAGA user forum on 
sourceforge, see
http://sourceforge.net/projects/saga-gis/support

> Dear saga and rsaga users and developers (Olaf, Alex, Victor..)<br><br>
> I ran a small piece of  code, testing the rsaga.pisr function for
> calculation of potential solar radiation.<br><br>
> <ul>
> <li>Question is: if I calculate the rsaga.pisr for April 1th till April
> 4th, day by day, sum those and compare it with the rsaga.pisr calculated
> for the range of days April 1-4th, there is an unexpected difference
> ranging from (4.821417, 6.996113)kWh/m2. Can you help me to fix or
> explain this?

Solar radiation was likely only calculated until April 3rd. I haven't 
tried this out with rsaga.pisr, but some time ago in an earlier version 
I found this in rsaga.solar.radiation, i.e. a different SAGA modules 
that takes similar arguments:

> In SAGA 2.0.2, solar radiation sums calculated for a range of days, say days=c(a,b) actually calculate radiation only for days a,...,b-1 (in steps of day.step - I used day.step=1 in this example). The setting a=b however gives the same result as b=a+1, and indeed b=a+2 gives twice the radiation sums and potential sunshine duration that a=b and b=a+1 both give.

This might explain your problem, but if you want to be sure you'd better 
examine this in more detail by looking at different time spans, 
especially one versus two days (does radiation increase by a factor of 2?)

> <li>Second question is a technical one: when the R script calls
> rsaga.pisr I can not have the SAGA gui opened since it crashes down. Is
> that expected?

no it isn't, and I haven't experienced this problem with SAGA 2.0.7 on 
Windows.

> <li>Considering the latitude=user defined, why do we need it since the
> latitude grid is defined?

not all the arguments are mandatory. you would specify either latitude 
or in.latitude.grid

> <li>And finally, there is a comment concerning the units in the
> rsaga.pisr when you chose kJ/m2. If I'm not mistaken, the resulting grid
> is actually in MJ/m2.

sorry I cannot confirm this, but it should be possible to determine that 
by comparing average hourly PISR with the solar constant times 1 hour, 
which will be higher but of the same order of magnitude as PISR if in 
the same units. If that doesn't help, please follow up in the SAGA GIS 
forums to find out if that's an error and issue a bug report if necessary.

I hope this helps
   Alex


> </ul>The R script and input grids are provided in
> <a href="http://radar.dhz.hr/~melita" eudora="autourl">
> http://radar.dhz.hr/~melita<br><br>
> <br>
> </a>Thank you for the help and a nice tools that we can all use and
> benefit from it.<br><br>
> Regards,<br><br>
> Melita Percec Tadic<br><br>



Date: Fri, 25 Nov 2011 16:52:57 +0100
From: Melita Percec Tadic <melita at cirus.dhz.hr>
To: r-sig-geo at r-project.org
Subject: [R-sig-Geo] rsaga, radiation, rsaga.pisr
Message-ID: <6.2.1.2.2.20111125163122.081c4ac0 at cirus.dhz.hr>
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-- 
Alexander Brenning
brenning at uwaterloo.ca - T +1-519-888-4567 ext 35783
Department of Geography and Environmental Management
University of Waterloo
200 University Ave. W - Waterloo, ON - Canada N2L 3G1
http://www.environment.uwaterloo.ca/geography/faculty/brenning/