[GRASS-user] r.stream.extract: lat-long vs projected location

Markus Metz markus.metz.giswork at gmail.com
Fri May 3 13:07:59 PDT 2019

On Fri, May 3, 2019 at 4:17 PM Moritz Lennert <mlennert at club.worldonline.be>
> Hi,
> Does anyone have any experience, or theoretical knowledge based on the
> code, on whether the results of r.stream.extract (and also r.watershed)
> will be influenced by working in a lat-long location instead of a
> projected location ?

The A* search in both r.stream.extract and r.watershed distinguishes only
between direct and diagonal neighbor cells. This determines flow D8
direction. MFD flow direcition also
distinguishes only between direct and diagonal neighbor cells. Flow
accumulation is reported as number of cells, i.e. independent of he CRS. In
r.watershed. slope length and slope steepness use map units for horizontal
length and units of the elevation map for vertical units, which will cause
nonsense output for slope steepness. TCI and SPI use distance in meters for
contour length in ll, but they use the average nsres and ewres for the
current region in ll which is not accurate for areas away from the equator.

In both modules, threshold refers to the unit of flow accumulation (usually
number of cells). r.stream.extract accepts any flow accumulation of any
units as input, the threshold option must match the units of flow
accumulation. Montgomery adjustment distinguishes only between direct and
diagonal neighbors.


Markus M
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