[GRASS-user] Different distances to outlet for r.stream.order and r.stream.distance

[Mira Kattwinkel]

Dear List

r.stream.order gives, among other output, the distance of current the 
stream init from the outlet of the catchment ('out_dist'). So for the 
most downstream segment this is identical to the length of the segment. 
r.stream.distance calculates the distance to the outlet and results in a 
raster file. Both are based on a stream raster and a direction raster.

When I compare the results, they are slightly different, although based 
on the same input files (see the attached Fig1). The raster value is 
212.33 at the junction, while the out_dist of the line is 237.69. The 
difference is one cellsize (25.023 in may example), probably due to the 
horizontal bit of the line at the end (upper right of the figure). 
However, this difference is not fixed. In the middle of the network 
(Fig2) it is larger (81028.9 - 81002.89 = 27).

What is the reason for this difference? Is this a bug or a wanted behaviour?

I want to extract the position of points on the network (i.e. upstream 
of a junction on the lines). To this end, I planned to use the upstream 
distance of the points and the out_dist of the segments (length(segment) 
- (out_dist(segment) - upDist(point)) = position(point)).
Has anybody another idea how to accomplish this?

Thanks a lot,
Mira

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