>> 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 -- View this message in context: http://osgeo-org.1560.x6.nabble.com/Stream-power-Index-calculation-tp5190756p5191377.html Sent from the Grass - Users mailing list archive at Nabble.com. _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user