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

[Annalisa Minelli]

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_______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user
>
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