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
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
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@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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