[GRASS-user] Stream flow direction map

[Nikos Alexandris]

(
The following question(s) might be already answered and I just miss the  
hydrological background to understand "why is not possible to...". If it is 
the case, scratch-out this post entirely
)

Nikos:
> > Ideally, I imagine a module like:
> >  "v.stream.directions elevation=HighResolutionDEM streams=StreamNetwork 
> >  output=StreamNetwork_WITH_directions"

Maybe this idea was wrong. The "problem to be solved" that I have in mind is 
the following:

Let's say the user (as it is the case of my friend) has digitised from scratch 
highly accurate (concerning the position) and topologically clean (vector) 
streams and  wants to order them (according to the usual suspect systems... 
Strahler, etc.).


1. As far as I can remember, there are ordering systems that are based on the 
length of segments (or let's call them branches) of a stream   or   on the 
number of "children" branches of that (Strahler), right? If this is correct 
(for some ordering systems), why is it required to use an elevation and a flow 
direction map just to order an available/existing (vector) stream network 
based on a ordering system that do not care about the flow direction?


2. Having a look at the current status of the hydro-modules, there is no way 
to quickly order a user-provided (i.e. on-display digitised) stream network 
(which shoud be [in most/any case(s)?] far more positionally accurate than the 
automatically extracted streams by using an elevation model.

The available solutions (like carving, as suggested in previous posts in this 
thread) can use the existing highly-accurate vector stream network to 
"improve" the elevation model in order to "improve" the stream extraction 
process, right? But they do not use directly a user-provided stream network, 
right?


*. The idea is a "v.stream.direction" module. It will output a vector stream 
network. In fact the same (user-provided) stream network that will be used as 
input but with the addition (as extra attribute(s) or extra map) of the 
_correct_ direction of each segment to satisfy (together with an elevation 
model) the requirements of "r.stream.order". Something like:

  - input(s): elevation, (vector) streams
  - output: streams (vector, with the correct flow direction for each 
segment/stream)   _or_   just the directions map (?)

The output could be then fed directly to "r.stream.order" for example to order 
the streams. Of course the question arises "what is the correct direction". 
But isn't this answered by the algorithms that are already used in 
r.stream.extract?


--%<---
# as a side-note: why does for example:
"r.stream.extract elevation=elevation_10m direction=TEST_direction 
threshold=100"

fail to run the error message:

"ERROR: Sorry, you must choose at least one output map."

Isn't the direction map an output on its own (enough?). A bug?
---%<--

Nikos