[GRASS-user] Stream power Index calculation?

[Helmut Kudrnovsky]

>> in the book SPI is defined as:
>>
>>     SPI = A x tan(beta)
>>
>> where A is the specific catchment area and beta is the local slope angle.
>>
>>in link [1] there is the SAGA GIS formula.
>>
>>
>> sometimes SPI is also defined as:
>>
>>     SPI = ln(A x tan(beta))
>
>This is also known as Topographic Convergence Index (TCI) or
>Topographic Wetness Index (TWI). You can calculate this index with
>r.watershed and r.terraflow. If you need the non-logarithmic form, you
>can use r.mapcalc "SPI = exp(TCI)". 

AFAIU from the manual:

http://grass.osgeo.org/grass71/manuals/r.watershed.html

TCI is definded as: Topographic index ln(a / tan(b))

divison vs. multiplication



-----
best regards
Helmut
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