[GRASS-user] Combining r.sun and r.horizon: Seeking clarity on conflicting statements and best practice recommendations :)

[Daniel Lee]

Dear list,

I'm trying to plan some scripts that will perform r.sun iteratively for
every day in a year. I've found some info that's confused me a bit though,
perhaps somebody could tell me what's right?

- http://grass.osgeo.org/grass64/manuals/html64_user/r.sun.html says that
r.sun is a lot faster with r.horizon
- http://www.osgeo.org/pipermail/grass-user/2009-November/053313.html says
that r.sun without r.horizon uses a lot less RAM
- http://www.osgeo.org/pipermail/grass-user/2012-January/063254.html says
that slope and aspect maps aren't required any more
- https://trac.osgeo.org/grass/ticket/498#comment:18 makes it look like not
using slope and aspect maps could be pretty disastrous.

My experience with using r.sun with slope and aspect maps is positive, has
this bug been resolved or am I perhaps wasting computing time by making the
maps beforehand? The bug report makes me want to be very cautious about not
making them.

If I remember correctly, r.sun used to not be able to partition maps
without r.horizon maps, which is the original reason that I wanted to use
r.horizon. However, using 6.4.2 I took a look in the GUI and noticed that
it doesn't seem to require a horizon map to partition now. Does that mean
that I can partition without the horizon maps? And if I do so, will I have
shadowing errors at the edges of partitions? I can imagine that if the
partitions overlap (which would make sense to avoid such artefacts) that it
could eventually mean using MORE memory, or am I mistaken? Does anyone have
any experience with that?

In addition, if I DO use r.horizon (which I intuitively think must be
faster than using only r.sun), I notice that I can enter horizon steps and
a horizon prefix. What if I'm only wanting to use the horizon maps as an
input for r.sun? Do I have to compute all horizons? Or could I write a
script that only makes the maps that I'm interested in? Concretely, from
what direction are the maps made (from north like many programs, or from
east like r.sun, etc.), and in which order (clockwise or counterclockwise)?
How does r.sun know which horizon map to use? Does it get it out of the
metadata or from the filename (it asks for a map prefix)? Here's kind of
what's really confusing me, from the r.horizon manual:

The *horizonstep* parameter gives the angle step (in degrees) between
successive azimuthal directions for the calculation of the horizon. Thus, a
value of 5 for the *horizonstep* will give a total of 360/5=72 directions
(72 rasters if used in the raster mode).

The *direction* parameter gives the initial direction of the first output.
This parameter acts as an direction angle offset. For example, if you want
to get horizon angles for directions 45 and 225 degrees, the
*direction*should be set to 45 and
*horizonstep* to 180. If you only want one single direction, use this
parameter to specify desired direction of horizon angle, and set the *
horizonstep* size to 0 degrees.
Alright, I understand that, but what would I do if I wanted all horizons
from 45°-225° in e.g. 5° steps? Or does that not work at all? In any case,
I'd really like to know what direction r.horizon is producing the rasters
from.

And, last but not least, what is the low memory version of r.sun? Do I have
any disadvantages by using that (precision, speed, etc.)?

Lots of questions, but I'm grateful for all answers, even if it's only for
parts of the email. Thanks a bunch!

Best,
Daniel

--

B.Sc. Daniel Lee
Geschäftsführung für Forschung und Entwicklung
ISIS - International Solar Information Solutions GbR
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