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

[Moritz Lennert]

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 !

Moritz