from DEM to CTI

[TaiseerHarb]

Dear all,

I have a dem raster for a certain area, and I need to have the compound topographic index of the same area.

Can anyone help me and write the steps (really the steps in details) how can I go from dem to this index.

thanks
Taiseer

[JeffreyEvans]

The implementation of CTI can be shown as: CTI = ln(a / (tan (beta))
where: a = Upland contributing area [(flow accumulation + 1 ) * (cellsize)] and beta is the slope expressed in radians.

Here are the raster algebra steps to calculate CTI:
cellsize = 30
fd = flowdirection(dem)
sca = flowaccumulation(fd)
slope = ( slope(dem) * 1.570796 ) / 90
tan_slp = con( slope > 0, tan(slope), 0.001 )
sca_scaled = ( sca + 1 ) * cellsize
cti = ln ( sca_scaled / tan_slp )

[CodieWilson]

Thanks for making that so easy! Thanks Jeff!!!

[WilliamLidberg]

I have a question about the cellsize.

My pixels are 2 by 2 meters (4m^2) should i put 2 or 4 as cellzise?