[GRASSLIST:4063] Re: direction to nearest...?

[Glynn Clements]

Alex Thorn wrote:

> What I'd like to be able to do is to calculate the
> direction from a point to the nearest cell that has a
> given raster value (raster not important here, I just
> assume that it is the best way to implement this). 
> I've been looking over the available GRASS commands,
> but haven't been able to find something that will do
> quite what I want.

Is r.distance not suitable?

>  The commands dealing with
> calculating flow seem related, although they call for
> a continuous coverage (e.g. elevation), rather than
> discrete data about a particular feature being present
> or absent in a given cell.  
> 
> It seems that I ought to be able to get something
> along the lines of the flow/drainage models to work if
> I can generate a coverage in which each cell is
> defined by its distance from a cell with the
> appropriate raster value (I seem to remember reading
> about something like this in one of the tutorials).

That should be achievable with r.cost, with a constant cost map and a
start_rast map where the only non-null cells are those with the chosen
value (e.g. generated by r.reclass).

However, this won't give you the exact Euclidian distance, but an
approximation where "circles" are actually polygons (16-sided with the
-k flag, 8-sided without).

-- 
Glynn Clements <glynn.clements at virgin.net>