[GRASS-user] Introducing myself and my first questions

[Paul Meems]

Doing some more searching using Google and changing my keywords I found the
answer to my own question below.
I need to call g.region rast=your_DEM to match my computational region
first.
I did that and now I have slope and aspect files. It is still very fast (1
second) but the files look OK.

I will now continue with the rest.
Sorry for the noise. I'll use Google more before posting ;)



Paul

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

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

*
*


2012/12/28 Paul Meems <bontepaarden at gmail.com>

> 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
>
>  *Paul Meems *
> Release manager, configuration manager
> and forum moderator of MapWindow GIS.
> www.mapwindow.org
>
> Owner of MapWindow.nl - Support for
> Dutch speaking users.
> www.mapwindow.nl
>
> *
> *
>
>
> 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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