[GRASS-user] r.stream.extract: lat-long vs projected location
mlennert at club.worldonline.be
Mon May 6 00:56:42 PDT 2019
On 3/05/19 22:07, Markus Metz wrote:
> On Fri, May 3, 2019 at 4:17 PM Moritz Lennert
> <mlennert at club.worldonline.be <mailto:mlennert at club.worldonline.be>> wrote:
> > 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.
Thanks for the clarification !
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