[GRASS-user] Introducing myself and my first questions

Daniel Lee lee at isi-solutions.org
Thu Dec 27 06:23:55 PST 2012

Hi Paul,

Welcome to the list :) Here are a couple of pointers:

2012/12/27 Paul Meems <bontepaarden at gmail.com>

> The 'Input options' tab already raises questions for me. For aspect and
> slope map in ArcMap FROM_DEM is used so I select my clipped tiff again. My
> area is located in The Netherlands so I use 90 for aspect. I don't see
> something in ArcMap that looks like linkein so I leave this blank. The same
> for albedo. I'm not sure what to use for longin and latin. My tiff is
> projected (Amersfoort/RD_New). I'm not sure either what to use for coefbh,
> coefdh, horizon or horizonstep.

For slope and aspect, you need to use maps of the slope and aspect in your
study area. They can both be calculated with r.slope.aspect. Using 90 for
aspect because you're in the Netherlands doesn't make sense since the
aspect is the orientation of each sloped pixel - a hill in the Netherlands
has north, east, south and west sides just like anywhere else.

I've heard that it's possible to just leave these options blank and have
r.sun calculate slope and aspect on the fly for you, but I've seen mixed
reviews of that and have no personal experience in doing so since my
company's programs were written at a time when those options were required
and - well, if I can, I don't change a running system unless I've got a
good reason to :)

The projection of your tiff itself shouldn't matter because GRASS doesn't
use the projection of single maps - you work inside a location and the
location has a single location. This forces you to use one single
coordinate system for your entire project, removing the temptation to do
on-the-fly reprojection, which is a really, really good idea. If your
projection doesn't match that of your map, you didn't import it right (you
can check the projection using g.proj).

coefbh, coefdh, horizon and horizonstep can be left blank. coefbh and
coefdh just help you make more realistic assessments - I'm not sure how
realistic you need to be - and horizon and horizonstep are similar. You can
read about them in the manual but if you're not sure what to enter, I
wouldn't touch it.

> On the 'Output options' tab I'm completely puzzled. I would have expected
> to set file names but all I can do is select my clipped tif.

GRASS doesn't work with external files, it creates maps that it manages in
an internal database. You just enter the names of the maps you want. If you
need them as files later, you can export them.

> The 'Optional' tab. I want the calculation to use nearby houses, trees and
> chimneys so I check 's'. 'step' I keep as 0.5, assuming this is the same as
> ArcMap's 'hour interval'. I don't know what declin or dist should be. For
> time Mode 1 is mentioned. What are the different modes?

These are described pretty well in the manual. If you're working with a
headless Ubuntu server, just google it. All the GRASS manuals are available
online. r.sun's manual is located at

> For ArcMap I need to pass the Latitude. I can't find that for r.sun.

You don't need to in GRASS. ArcMap assumes the same latitude for the entire
map (bad). GRASS computes the solar zenith and azimuth dependent on each
pixel's latitude (good).

Hope that helps!

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