[GRASSLIST:2783] continued problems with v.net.alloc

[Christopher Fonnesbeck]

I am trying to extract a continuous stream layer from a fragmented 
layer using v.net.alloc and v.extract, following the 5.7 tutorial, but 
have yet to replicate the results properly. I have  a vector layer 
derived from a terraflow accumulation raster. In trying to extract the 
largest contiguous network, I digitized a point "outlet" near the 
outlet of the watershed, then derived a vector to connect the point to 
the network using v.distance. The stream layer, outlet point and 
connecting vector were the joined using v.patch:

v.patch input=streams_clean,outlet_to_streams,outlet out=streams_outlet

I then ran v.net.alloc to try and allocate subnets:

v.net.alloc in=streams_outlet out=streams_alloc nfield=1 ccats=1

However, when I then try to extract the largest subnet with v.extract, 
it does not work. That is, I get only 2 subnets, the connecting vector 
used in v.patch (cat=0) and the entire stream network (cat=1), 
including the disconnected fragments! I thought v.net.alloc was 
supposed to allocate connected regions to subnets, yet this is not what 
is happening (apparently). What am I doing wrong?

Thanks,
C.
--
Christopher J. Fonnesbeck ( c h r i s @ f o n n e s b e c k . o r g )
Georgia Cooperative Fish & Wildlife Research Unit, University of Georgia