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.
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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