[GRASS-user] re: streams under r.watershed

[Kurt Springs]

Thanks Annalisa,

That does help immensely.  Hopefully, it will tell me something useful  
about how the location for these monuments was constructed.

Kurt
On Oct 14, 2008, at 4:25 AM, Annalisa Minelli wrote:

> Hi Kurt!
> I work/study Hydrology and I use GRASS in my work... so, for my  
> short experinece, I can tell you:
> the "stream map", generated by r.watershed (or r.watershed.fast) is  
> the map of the reaches, so the colors you can see and the numbers  
> (categories) associated with each reach (you can see it if you query  
> the map) is simply an "id" of the reach (it is useful to distinguish  
> the reaches...).
> r.watershed extracts these stream segments from the DEM basing his  
> analysis on the elevation data... so, depending on the "threshold"  
> value you give him in input, the module will return a different  
> result (so it happens that if you give a threshold value greater you  
> will see in your map only principal reaches, or if you give a lesser  
> threshold value you will obtain more and more reaches - more dense  
> network -).
>
> A way to isolate some reaches -remainig in raster format- could be  
> using r.reclass:
>
> type "r.reclass" in the shell: it requires you the name of the map  
> to reclassify, the name of a new map (reclassified) and the values  
> of the categories to change (e.g. I want category 30 to become  
> category 1... so I type: 30 = 1).
>
> To do this you can query the map for the reaches you interest and  
> read the actual category (e.g. 30,32,34) then type:
> > r.reclass
> name of the new map: OPTION:   Raster map to be reclassified
>      key: input
>   format: name
> required: YES
>
> Enter the name of an existing raster file
> Enter 'list' for a list of existing raster files
> Hit RETURN to cancel request
> > streams    #from r.watershed
> <streams>
>
> OPTION:   Name for output raster map
>      key: output
>   format: name
> required: YES
>
> Enter a new raster file name
> Enter 'list' for a list of existing raster files
> Hit RETURN to cancel request
> > WhatYouWant   #the name of the new map I'm going to create
> <WhatYouWant>
>
> OPTION:   File containing reclass rules
>      key: rules
>   format: name
> required: NO
>
> Enter the name of an existing input file
> Hit RETURN to cancel request
> >                        #I have not a file containig the rules...
> <>
>
> OPTION:   Title for the resulting raster map
>      key: title
> required: NO
> enter option >  #I choose not to give a title to the map I'm going  
> to create
> Enter rule(s), "end" when done, "help" if you need it
> Data range is 2 to 66
> > 30 = 1           #...I prefer entering the rules by myself
> > 32 = 2
> > 34 = 3
> > end
>
> so you can open your "WhatYouWant" raster map where you will see  
> only the reaches you selected by category.
>
> Hoping I've been helpful...
>
> Annalisa
>
>
>
>
> 2008/10/14 Kurt Springs <ferret_bard at mac.com>
> Thanks to Hamish and Markus Metz,
>
> What I am doing is comparing two or three (possibly four) types of  
> megaliths in my study area and trying to understand the relation  
> ship between both contemporary monuments and later monuments.  While  
> I was looking at old GIS maps, I was looking at the stream and basin  
> maps and had overlaid the monuments on these maps.
>
> First off, the vector river and stream maps, representing the  
> current water ways are a bit different then the stream map.  The  
> physical streams do seem to follow the stream maps, but there are  
> many more branches on the stream map than there are actual streams.
>
> Some of the megalithic tombs types seem to fall into the web of the  
> streams, while others don't.  I think this is important.  I am,  
> unfortunately, an archaeologist, not a hydrologist, so I am not up  
> on watershed lingo.
>
> What I need to know is what information on these maps may tell me  
> something important.  For example, are the stream systems shown on  
> the watershed stream maps just important of them selves, or might  
> the values the colors represent tell me something?  Is there a way  
> to isolate them?
>
> I guess what I am hoping for is a hydrologist who uses grass and can  
> explain what I am looking at in the real world.
>
> Kurt Springs_______________________________________________
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>

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