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Compound Topographic Index
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Apr 22 2004
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CTI is a steady state wetness index. The CTI is a function of both the slope and the upstream contributing area per unit width orthigonal to the flow direction. CTI was desigined for hillslope catenas. Accumulation numbers in flat areas will be very large and CTI will not be a relevant variable. CTI is highly correlated with several soil attributes such as horizon depth(r=0.55), silt percentage(r=0.61), organic matter content(r=0.57), and phosphorus(r=0.53) (Moore et al. 1993).
The implementation of CTI can be shown as:
CTI = ln (As / (tan(beta)) where As = Area Value calculated as (flow accumulation + 1 ) *(pixel area m2) and beta is the slope expressed in
Gessler, P.E., I.D. Moore, N.J. McKenzie, and P.J. Ryan. 1995.
Soil-landscape modeling and spatial prediction of soil attributes.
International Journal of GIS. Vol 9, No 4, 421-432.
Moore, ID., Gessler, P.E., Nielsen, G.A., and Petersen, G.A. 1993
Terrain attributes: estimation methods and scale effects.
In Modeling Change in Environmental Systems, edited by A.J. Jakeman
M.B. Beck and M. McAleer (London: Wiley), pp. 189 - 214.
06/05/2002 Updated AML
Optional flow direction input.
The ArcInfo approach to calculating Flow Direction uses the D8 algorithm
producing very unrealistic results. Several other methods are available
for calculating flow directions. One of the more robust approaches is the
D infinity algorithm (Tarboton 1997). There is a freeware download
and documentation for TARDEM at http://www.engineering.usu.edu/dtarb/
or TAUDEM http://moose.cee.usu.edu/taudem/taudem.html
The deritive of the FLOWDIRECTION caculation from either of these two programs
can be used in the CTI AML or you can caculate FLOWDIRECTION in GRID.
NOTE: If you use CTI without the FLOWDIRECTION input all caculations
will be conducted in ArcInfo - GRID