[GRASS-user] Introducing myself and my first questions

Paul Meems bontepaarden at gmail.com
Fri Dec 28 00:13:28 PST 2012

Thanks Daniel,

This really helps. I understand it more, not all ;)

I started creating the slope and aspect files first using:
r.slope.aspect elevation="w001001 at temp" slope="slope" aspect="aspect"
format="degrees" prec="float" zfactor=1.0 min_slp_allowed=0.0
But this didn't work. It took 0 seconds and I got empty data files.
I can view my input file so I assume that it is good.
Reading the manual doesn't give me any clues what I'm doing wrong.

My input file contains several houses so slope and aspect can be calculated.

Any new pointers are much appreciated.


 *Paul Meems *
Release manager, configuration manager
and forum moderator of MapWindow GIS.

Owner of MapWindow.nl - Support for
Dutch speaking users.


2012/12/27 Daniel Lee <lee at isi-solutions.org>

> 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
> http://grass.osgeo.org/grass64/manuals/r.sun.html.
>> 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!
> Daniel
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