Aspect map generation

[N R A]

I know this isn't a good time to ask the question, what with everyone off 
for Christmas (Happy Christmas everyone!), but anyway :-

We've been using the 'r.water.outlet' program to generate catchments. The 
aspect/drainage map for this program was generated by 'r.watershed'. 
Rather than keep regenerating the aspect map each time, I decided to do 
it for our whole region. I created a shell script to create a 10 km 
squares and then 'stitched' these together to create the single big map. 
I was concerned over any 'edge' effects, so made the 10 km squares 
slightly bigger (10 km + 100m for a 50 m resolution DTM) and then cut out 
the centre 10 km.

Everything seemed OK, derived catchments look fine, but now find that 
there are some weird effects which seem to be on the boundaries of the 10 
km squares. Basically the derived catchment didn't include some rivers, 
which it should have.

If I zoom in to cover the catchment, generate another aspect and then the 
catchment, the missing rivers get included. I've done a 'r.infer' and 
found the differences - along the 10 km grid line.

Question is - does anyone know how the aspect part of 'r.watershed' 
works. Primarily is there an 'edge' effect and how far does it extend - 
on the test above I could see difference around most of the edge. Some 
extended upto maybe 10 cells into the map.

Or better yet how can I generate a single aspect map from our DTM map in 
one go ? - now that would be really useful  8-)

Thanks,

Steve Culshaw
NRA North West
e-mail : nra at cix.compulink.co.uk (use this for general usage)
or         sculshaw at cix.compulink.co.uk (Private - only checked at best 
biweekly)