Thanks everyone for the quick responses. I've managed to get what I needed.
I've been trying to improve upon the stochastic modelling of watershed
boundaries, as described in the r.stream.basins manual and Metz and
Jasiewicz (2011, p. 1167). Instead of using an area as the catchment
outlet, I wanted to use the closest point in the extracted streams to
where I thought the outlet should be (a known point). Finding the
minimum in a raster allowed me to do the distance calculation needed for
this.
The finished product – lines are median and 95% confidence intervals for
DEM-derived catchment boundary:
http://snorf.net/pub/middleburn/basins_sto.png
Reference:
Jasiewicz, J., Metz, M., (2011). A new GRASS GIS toolkit for Hortonian
analysis of drainage networks. Computers & Geosciences. 37. 1162–1173.
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