topic Re: Topographic Wetness Index in ArcGIS Spatial Analyst Questions

Topographic Wetness Index

StefanieSteinbach — Tue, 29 Oct 2013 15:44:48 GMT

Hi all,
I am trying to calculate the topographic wetness index for an area and know the formula, but somehow get an error everytime I try. There are so many different approaches on the internet that I am not sure which to use and why it does not work for my project.
Can anyone tell me exactly how to use my layers with the formula?
Your help is very much appreciated.

Re: Topographic Wetness Index

DanPatterson_Retired — Tue, 29 Oct 2013 16:58:09 GMT

give a code example of what you have tried

Re: Topographic Wetness Index

StefanieSteinbach — Tue, 29 Oct 2013 17:05:09 GMT

I used this one below. My cell size is 30 m (an ASTER DEM). Slope is in degrees, I have also tried the formula with Tan("SLOPE")*�?�/180 as seen in many examples.

Ln((�??FLOWACC�?�*900) / Tan(�??SLOPE�?�))

Re: Topographic Wetness Index

DanPatterson_Retired — Tue, 29 Oct 2013 17:23:22 GMT

is slope supposed to be in degrees or radians? are you getting a specific error message?
did you go through these posts?
http://forums.arcgis.com/search.php?searchid=1784175

Re: Topographic Wetness Index

StefanieSteinbach — Tue, 29 Oct 2013 17:35:53 GMT

I attached the error message. Slope is in degrees, but I have read it is supposed to be in radian so I used slope * 0.0175
I checked the forums but couldn't find anything useful for my problem. If it is something simple I apologize. I am relatively new to this.

Re: Topographic Wetness Index

JeffreyEvans — Tue, 29 Oct 2013 22:14:40 GMT

It may be as simple as a missing space around "*" in your syntax.
Try: Ln((???FLOWACC??? * 900) / Tan(???SLOPE???))

You are missing a few critical steps. I would recommend looking at this thread
http://forums.arcgis.com/threads/32896-from-DEM-to-CTI?highlight=CTI

I also have this model available in our gradient toolbox.
http://evansmurphy.wix.com/evansspatial#!arcgis-gradient-metrics-toolbox/crro

Re: Topographic Wetness Index

StefanieSteinbach — Wed, 30 Oct 2013 14:55:45 GMT

Thanks a million, your method seems to work out fine for my project.
Why do you use *cellsize for the upper contributing area whereas I found *cellsize^2 in other threads?

Re: Topographic Wetness Index

JeffreyEvans — Thu, 31 Oct 2013 01:20:10 GMT

Here is a nice explanation provided by Tom Vanwalleghem to a similar thread on the GIS Stack Exchange.

"The units of upslope contributing area are m2/m or m (in fact A is As or specific contributing area, contributing area per unit contour width) So that's why you multiply by cell resolution = 30 m and not by cell area (30*30) A more correct way to calculate this would be to take into account the flow direction (if it is diagonal or perpendicular), but for most applications, the approximated calculus given here is enough."

Re: Topographic Wetness Index

toddsams — Wed, 15 Jan 2014 18:37:54 GMT

Here is a nice explanation provided by Tom Vanwalleghem to a similar thread on the GIS Stack Exchange.

"The units of upslope contributing area are m2/m or m (in fact A is As or specific contributing area, contributing area per unit contour width) So that's why you multiply by cell resolution = 30 m and not by cell area (30*30) A more correct way to calculate this would be to take into account the flow direction (if it is diagonal or perpendicular), but for most applications, the approximated calculus given here is enough."

I am not understanding why the fac grid would be multiplied by 30 rather than 900. Why would upslope contributing "area" be in units of m2/m or m, which are units of "length"?